Gravimetric Analysis: Definition, Formula, Questions and Examples

Edited By Shivani Poonia | Updated on Jul 02, 2025 07:40 PM IST

The gravimetric analysis technique is based on the measurement of mass, relying on precise and accurate weighing to determine the concentration of an analyte. At its core, gravimetric analysis involves the conversion of the analyte into a stable, pure compound that can be weighed. This often entails a series of carefully controlled chemical reactions, followed by filtration, washing, drying, or ignition of the precipitate to a constant mass.

This Story also Contains

Gravimetric Analysis
Some Solved Examples
Conclusion

Gravimetric Analysis: Definition, Formula, Questions and Examples

Gravimetric Analysis

It is an analytical technique based on the measurements of the mass of solid substances or the volume of the gaseous species. It is divided into three categories:

Mass-Mass (weight-weight) relation

This relationship relates to the mass of a reactant or product with the mass of another reactant or product.

Write down the balanced equation to represent the chemical change.
Write the number of moles below the formula of reactants and products.
Finally, apply the unitary method to calculate the unknown factor.

Mass-volume relation

This relationship relates the mass of a reactant or product with the volume of another gaseous reactant or product involved in a chemical reaction.

Write down the relevant balanced chemical equation.
Write the weights of various solid reactants and products.
Gases are normally expressed in terms of volume. In case the volume of the gas is measured at room temperature and pressure, convert it into N.T.P. by applying gas laws.
The volume of a gas at any temperature and pressure can be converted into its weight and vice versa by using the relation,
PV = (g/M) x RT
Here g is the weight of the gas, M is the molecular weight of gas and R is the gas constant.
Finally, calculate the unknown factor (n or s) by unitary method.

Volume-Volume relation

This relationship relates the volume of a gaseous reactant or product with the volume of another gaseous reactant or product involved in a chemical reaction.

First, write down the relevant balanced chemical equation.
Now write down the volume of the reactants and the products below the formula to each reactant and product using the fact that one gram molecules of every gaseous substance occupies 22.4 liters at N.T.P. (22.7 Litre at STP)
If the volume of the gas is measured at a particular or room temperature, convert it to N.T.P. with the help of the ideal gas equation.
Now use Avogadro’s hypothesis that gases under similar conditions of temperature and pressure contain the same number of molecules. Thus under similar conditions of temperature and pressure, the number of moles of the gases in the balanced equation.

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Some Solved Examples

Que 1. What is the mass (in kg) of the solution when the mass of solvent is 3.5 kg and that of solute is 0.5 kg?

1) 1.25

2) 7

3) 3

4) 4

Solution

Mass of solution = mass of solute + mass of solvent

= 3.5 + 0.5

= 4 kg

Hence, the answer is (4 kg).

Que 2. What volume of O₂ is obtained at STP, when 12.25 g of KClO₃is decomposed according to the reaction (Molar mass of KClO₃= 122.5 g/mol)

$\mathrm{KClO}_3 \xrightarrow{\Delta} \mathrm{KCl}+\frac{3}{2} \mathrm{O}_2$

1) 22.4 L

2) 11.2 L

3) 3.36 L

4) 44.8 L

Solution

1 mole of KClO₃ produces 1.5 moles of O₂

So, 0.1 mole of KClO₃ will produce 0.15 moles of O₂

Thus, volume = 22.4 X 0.15 = 3.36L

Hence, the answer is the option (3).

Que 3. 6 g of ethane (C₂H₆) is burnt in excess of O₂. The moles of H₂O formed would be

1) 0.2

2) 0.4

3) 0.6

4) 0.8

Solution

The reaction occurs as follows:

$2 \mathrm{C}_2 \mathrm{H}_6+7 \mathrm{O}_2 \rightarrow 4 \mathrm{CO}_2+6 \mathrm{H}_2 \mathrm{O}$

Molar Mass of Ethane = 30 g/mole.
The molar mass of water = 18 g/mole.
From the Equation,
∵ 2 x 30 grams of Ethane produces 6 x 18 grams of Water.
∴ 1 gram of the Ethane produces (9/5) grams of Water.
∴ 6 grams of Ethane produces 10.8 grams of Water.
Now, Using the Formula,
No. of Moles of Water = Mass/Molar Mass
= 10.8/18
= 0.6 moles.

Hence, the answer is the option (3).

Que 4. At 300 K and 1 atmospheric pressure, 10 mL of a hydrocarbon required 55 mL of O₂ for complete combustion, and 40 mL of CO₂ is formed. The formula of the hydrocarbon is:

1) C₄H₁₀

2) $\mathrm{C}_4 \mathrm{H}_6$

3) $\mathrm{C}_4 \mathrm{H}_7 \mathrm{Cl}$

4) $\mathrm{C}_4 \mathrm{H}_8$

Solution

The equation of hydrocarbon can be written as :

$\mathrm{C}_x \mathrm{H}_y+\left(x+\frac{y}{4}\right) \mathrm{O}_2 \rightarrow x \mathrm{CO}_2+\frac{y}{2} \mathrm{H}_2 \mathrm{O}$

Since , 10mL of $C_x H_y$ produces 40mL of CO₂ &

1mL of $C_x H_y$ produces $x$ mL of CO₂

$\therefore x=\frac{40}{10}=4$

Now,

10mL of $C_x H_y$ requires 55mL of O₂

$\therefore$ 1mL of $C_x H_y$ requires $\left(\frac{55}{10}\right)$mL of O₂

$\therefore x+\frac{y}{4}=\frac{55}{10}$

=> $\frac{y}{4}=\frac{55}{10}-x$

=> $y=4(5.5-x)$

=>$y=4(5.5-4)$

=>$y=6$

$\therefore$ Hydrocarbon $=C_4 H_6$

Hence, the answer is the option (2).

Que 5. 25g of an unknown hydrocarbon upon burning produces 88 g of $\mathrm{CO}_2$ and 9 g of H₂O. This unknown hydrocarbon contains:

1) 20 g of carbon and 5 g of hydrogen

2) 22 g of carbon and 3 g of hydrogen

3) 24 g of carbon and 1 g of hydrogen

4) 18 g of carbon and 7 g of hydrogen

Solution

$\mathrm{C}_x \mathrm{H}_y+\mathrm{O}_2 \rightarrow x \mathrm{CO}_2+y \mathrm{H}_2 \mathrm{O}$

$\eta_{\mathrm{co}_2}=\frac{88}{44}=2 \mathrm{~mole}$

$\eta_c=2 \mathrm{~mole}$

weight of carbon $W_c=2 \times 12=24 g$

Mole of water $\eta_{H_2} O=\frac{9}{18}=\frac{1}{2}$

weight of water $W_{\mathrm{H}_2 \mathrm{O}}=\frac{1}{2} \times 18=9 g$

Thus, the weight of Hydrogen $W_g=1 g$

Therefore, the hydrocarbon contains 24g of carbon and 1g of hydrogen

Hence, the answer is the option (3).

Conclusion

The importance of gravimetric analysis spans various fields, from environmental science, where it is used to determine pollutant levels in air and water, to pharmaceuticals, ensuring the purity and dosage of active ingredients. Its application in materials science and metallurgy further underscores its versatility and robustness.

Frequently Asked Questions (FAQs)

1. What is gravimetric analysis?

Gravimetric analysis is a quantitative analytical method used to determine the amount of a substance by measuring its mass. It involves separating the analyte from a sample through precipitation, filtration, or volatilization, and then weighing the isolated substance to calculate its concentration in the original sample.

2. How does gravimetric analysis differ from volumetric analysis?

Gravimetric analysis measures the mass of an analyte, while volumetric analysis measures the volume of a solution needed to react with the analyte. Gravimetric methods often involve precipitation and filtration, whereas volumetric methods typically use titration techniques.

3. What are the key steps in a typical gravimetric analysis procedure?

The key steps in gravimetric analysis are: 1) Sample preparation, 2) Precipitation of the analyte, 3) Filtration and washing of the precipitate, 4) Drying or ignition of the precipitate, 5) Weighing the final product, and 6) Calculation of the analyte's concentration in the original sample.

4. Why is the choice of precipitating agent important in gravimetric analysis?

The precipitating agent is crucial because it must form a highly insoluble compound with the analyte, be selective for the analyte, and produce a precipitate that can be easily filtered and purified. The wrong choice can lead to incomplete precipitation or co-precipitation of interfering substances.

5. What is the gravimetric factor, and how is it used in calculations?

The gravimetric factor is the ratio of the formula weight of the analyte to the formula weight of the weighed precipitate. It's used to convert the mass of the precipitate to the mass of the analyte in the original sample, accounting for any changes in composition during the analysis.

6. How does particle size affect the accuracy of gravimetric analysis?

Particle size is crucial in gravimetric analysis. Smaller particles have a larger surface area, which can lead to increased adsorption of impurities. However, very large particles may trap mother liquor, leading to inaccurate results. An optimal particle size ensures efficient filtration and washing while minimizing errors.

7. What is co-precipitation, and how does it impact gravimetric analysis?

Co-precipitation occurs when substances other than the analyte are carried down with the precipitate. This can lead to overestimation of the analyte's concentration. It's often caused by adsorption, occlusion, or inclusion of impurities in the crystal structure of the precipitate.

8. How does temperature affect solubility in gravimetric analysis?

Temperature generally increases the solubility of most substances. In gravimetric analysis, precipitation is often carried out at elevated temperatures to promote the formation of larger, more easily filterable crystals. However, the solution must be cooled before filtration to ensure complete precipitation.

9. What is the role of digestion in gravimetric analysis?

Digestion is the process of heating a precipitate in its mother liquor. It promotes the growth of larger crystals through Ostwald ripening, where smaller particles dissolve and redeposit on larger ones. This results in a purer precipitate that's easier to filter and wash.

10. How does pH affect precipitation in gravimetric analysis?

pH can significantly influence precipitation by affecting the solubility of the analyte and potential interfering substances. Controlling pH helps ensure complete precipitation of the analyte while minimizing co-precipitation of impurities. It's often crucial for selective precipitation in complex samples.

11. What is the significance of washing the precipitate in gravimetric analysis?

Washing the precipitate removes adsorbed impurities and traces of the mother liquor. It's crucial for obtaining accurate results. The wash solution should be chosen carefully to minimize dissolution of the precipitate while effectively removing contaminants.

12. How does the purity of reagents affect gravimetric analysis?

Reagent purity is critical in gravimetric analysis. Impurities can lead to co-precipitation, interfere with the formation of the desired precipitate, or contribute to the mass of the final product, causing systematic errors in the results.

13. What is the difference between ignition and drying in gravimetric analysis?

Drying involves removing moisture from the precipitate at relatively low temperatures (often using an oven), while ignition involves heating the precipitate to high temperatures in a muffle furnace. Ignition is used to convert the precipitate to a more stable form or to decompose it to a known composition.

14. How does the concept of solubility product (Ksp) relate to gravimetric analysis?

The solubility product (Ksp) determines the extent of precipitation in gravimetric analysis. A lower Ksp indicates lower solubility and more complete precipitation. Understanding Ksp helps in predicting whether precipitation will occur and in calculating the minimum concentration needed for precipitation.

15. What are some common sources of error in gravimetric analysis?

Common sources of error include incomplete precipitation, co-precipitation of impurities, loss of precipitate during filtration or transfer, incomplete washing, hygroscopic behavior of the precipitate, and errors in weighing. Careful technique and attention to detail are crucial to minimize these errors.

16. How does the concept of supersaturation relate to crystal formation in gravimetric analysis?

Supersaturation occurs when a solution contains more dissolved solute than it can theoretically hold at equilibrium. In gravimetric analysis, controlled supersaturation is crucial for forming large, pure crystals. Rapid precipitation can lead to small, impure crystals that are difficult to filter and wash.

17. What is the importance of using a desiccator in gravimetric analysis?

A desiccator is used to cool heated samples in a dry environment before weighing. This prevents the absorption of atmospheric moisture, which could add to the mass of the sample and lead to inaccurate results. It's especially important for hygroscopic substances.

18. How does the choice of filter paper affect gravimetric analysis?

The choice of filter paper is crucial for efficient filtration and retention of the precipitate. Factors to consider include pore size, ash content, and chemical resistance. Quantitative filter papers with very small pore sizes and low ash content are often used for precise gravimetric work.

19. What is the role of a weighing bottle in gravimetric analysis?

A weighing bottle is used to accurately determine the mass of samples or products in gravimetric analysis. It provides a clean, dry environment for the sample, prevents absorption of atmospheric moisture, and allows for easy transfer of the sample without loss.

20. How does the concept of relative supersaturation affect crystal growth in gravimetric analysis?

Relative supersaturation influences the rate of crystal growth and the size of crystals formed. Higher relative supersaturation leads to rapid nucleation and smaller crystals, while lower relative supersaturation promotes slower growth of larger, more perfect crystals. Controlling this parameter is crucial for obtaining easily filterable precipitates.

21. What is the significance of the common ion effect in gravimetric analysis?

The common ion effect reduces the solubility of a precipitate by increasing the concentration of one of its ions in solution. In gravimetric analysis, this effect can be used to enhance precipitation and reduce solubility losses, improving the accuracy of the method.

22. How does organic precipitation differ from inorganic precipitation in gravimetric analysis?

Organic precipitation often involves the formation of coordination compounds or ion associates, while inorganic precipitation typically results in ionic lattices. Organic precipitates are often more selective but can be less stable and more prone to contamination than inorganic precipitates.

23. What is the role of complexing agents in gravimetric analysis?

Complexing agents can be used to mask interfering ions, preventing their precipitation. They can also be used to selectively precipitate the analyte by forming a stable, insoluble complex. This enhances the selectivity of the gravimetric method.

24. How does the volatilization method in gravimetric analysis differ from precipitation methods?

The volatilization method involves converting the analyte into a volatile form that can be collected and weighed, or measuring the mass loss after volatilization. Unlike precipitation methods, it doesn't involve forming an insoluble compound and can be useful for elements that don't form suitable precipitates.

25. What is the importance of stoichiometry in gravimetric calculations?

Stoichiometry is crucial in gravimetric analysis for relating the mass of the precipitate to the amount of analyte in the original sample. It's used to determine the gravimetric factor and to account for any changes in composition during the analysis process.

26. How does the presence of colloids affect gravimetric analysis?

Colloids can interfere with gravimetric analysis by preventing the formation of large, filterable crystals. They can lead to gelatinous precipitates that are difficult to filter and wash. Techniques like digestion or adding electrolytes may be necessary to coagulate colloidal particles.

27. What is the significance of the filtrate in gravimetric analysis?

The filtrate is important for checking the completeness of precipitation. It can be tested for the presence of the analyte to ensure that precipitation was complete. In some cases, the filtrate may be used for further analysis or to recover valuable materials.

28. How does the concept of homogeneous precipitation apply to gravimetric analysis?

Homogeneous precipitation involves slowly generating the precipitating agent throughout the solution, often through a chemical reaction. This technique promotes the formation of larger, purer crystals by maintaining a low level of supersaturation throughout the precipitation process.

29. What is the role of surfactants in some gravimetric procedures?

Surfactants can be used in gravimetric analysis to modify crystal growth, prevent agglomeration of particles, or aid in the filtration of fine precipitates. They can help produce more uniform and easily filterable precipitates, improving the accuracy and precision of the analysis.

30. How does the concept of coprecipitation differ from post-precipitation in gravimetric analysis?

Coprecipitation occurs when impurities are incorporated into the crystal structure or adsorbed onto the surface of the precipitate as it forms. Post-precipitation happens after the main precipitation is complete, often during digestion or filtration. Both can lead to inaccuracies, but they occur at different stages of the analysis.

31. What is the significance of the crystal habit in gravimetric analysis?

Crystal habit refers to the external shape of crystals. In gravimetric analysis, the crystal habit affects filtration efficiency, washing effectiveness, and the tendency to retain impurities. Certain crystal habits (e.g., large, well-formed crystals) are preferred for easier handling and more accurate results.

32. How does the presence of peptizing agents affect gravimetric analysis?

Peptizing agents can disperse precipitates into colloidal particles, making filtration difficult. This can lead to loss of analyte and inaccurate results. Understanding and controlling the effects of peptizing agents is crucial for successful gravimetric analysis of certain systems.

33. What is the role of aging in gravimetric analysis?

Aging involves allowing a precipitate to stand in contact with its mother liquor for a period of time. This process can improve the purity and filterability of the precipitate by promoting recrystallization and the growth of larger particles. However, excessive aging can sometimes lead to increased solubility or contamination.

34. How does the concept of induced precipitation apply to gravimetric analysis?

Induced precipitation occurs when a substance that normally wouldn't precipitate is carried down with another precipitate. This can be used advantageously to quantify trace amounts of an analyte by co-precipitating it with a carrier. However, it can also be a source of error if unintended.

35. What is the significance of the precipitation sequence in gravimetric analysis of complex samples?

The precipitation sequence is crucial when analyzing samples with multiple analytes. It determines the order in which different species are precipitated and separated. Understanding and controlling this sequence is essential for selective precipitation and avoiding interference between different analytes.

36. How does the presence of complexing agents in the sample matrix affect gravimetric analysis?

Complexing agents in the sample can interfere with precipitation by forming soluble complexes with the analyte. This can lead to incomplete precipitation and underestimation of the analyte concentration. Identifying and accounting for such interferences is crucial for accurate analysis.

37. What is the role of seed crystals in gravimetric precipitation?

Seed crystals can be added to initiate and control precipitation. They provide nucleation sites, promoting the growth of larger, more uniform crystals. This can improve the purity and filterability of the precipitate, especially in cases where spontaneous nucleation is slow or leads to fine, difficult-to-filter particles.

38. How does the concept of fractional precipitation apply to gravimetric analysis?

Fractional precipitation involves selectively precipitating different components of a mixture based on their solubilities. It can be used to separate and quantify multiple analytes from a single sample. Understanding the solubility differences and controlling precipitation conditions are key to successful fractional precipitation.

39. What is the significance of the precipitate's particle size distribution in gravimetric analysis?

The particle size distribution affects filtration efficiency, washing effectiveness, and the tendency to retain impurities. A narrow distribution of relatively large particles is generally preferred for easier handling and more accurate results. Techniques like digestion and controlled precipitation conditions can help achieve optimal particle size distribution.

40. How does the presence of volatile components in the sample affect gravimetric analysis?

Volatile components can be lost during drying or ignition steps, leading to inaccurate results. Special care must be taken when analyzing samples with volatile constituents, such as using lower temperatures or alternative drying methods. In some cases, the loss of volatile components can be used analytically in volatilization methods.

41. What is the role of buffer solutions in some gravimetric procedures?

Buffer solutions help maintain a constant pH during precipitation, which is crucial for controlling the solubility and selectivity of the precipitate. They can prevent unwanted side reactions, ensure complete precipitation of the analyte, and minimize co-precipitation of interfering substances.

42. How does the concept of salting out relate to gravimetric analysis?

Salting out involves adding a high concentration of an electrolyte to decrease the solubility of the analyte, promoting more complete precipitation. This technique can be useful for precipitating substances with relatively high solubility or for enhancing the separation of analytes with similar solubilities.

43. What is the significance of the precipitate's crystallinity in gravimetric analysis?

The crystallinity of the precipitate affects its purity, filterability, and stability. Highly crystalline precipitates are generally preferred as they tend to be purer, easier to filter, and less prone to contamination. Amorphous precipitates can be more challenging to handle and may require special techniques for accurate analysis.

44. How does the presence of surfactants in the sample matrix affect gravimetric analysis?

Surfactants in the sample can interfere with precipitation by stabilizing colloidal particles or altering surface properties of the precipitate. This can lead to difficulties in filtration and washing. Identifying and removing or counteracting surfactants may be necessary for accurate gravimetric analysis of such samples.

45. What is the role of masking agents in gravimetric analysis?

Masking agents are used to prevent interference from certain ions by forming stable, soluble complexes with them. This allows for selective precipitation of the analyte in the presence of potentially interfering species. Proper selection and use of masking agents can significantly enhance the selectivity of gravimetric methods.

46. How does the concept of ostwald ripening apply to crystal growth in gravimetric analysis?

Ostwald ripening is the process where smaller particles dissolve and redeposit onto larger particles over time. In gravimetric analysis, this phenomenon can be exploited during digestion to produce larger, more easily filterable crystals. Understanding and controlling Ostwald ripening can improve the quality of the precipitate and the accuracy of the analysis.

47. What is the significance of the precipitate's surface area in gravimetric analysis?

The surface area of the precipitate affects its tendency to adsorb impurities and its ease of washing. A lower surface area (associated with larger particles) generally leads to less adsorption of contaminants and more efficient washing. However, extremely large particles may trap mother liquor, so an optimal surface area is desired for accurate results.

48. How does the presence of chelating agents in the sample affect gravimetric analysis?

Chelating agents can interfere with gravimetric analysis by forming stable, soluble complexes with the analyte, preventing its precipitation. This can lead to underestimation of the analyte concentration. Identifying and accounting for chelating agents, or using stronger precipitating agents, may be necessary for accurate analysis of such samples.

49. What is the role of counter-ions in gravimetric precipitation?

Counter-ions play a crucial role in forming the insoluble compound with the analyte. The choice of counter-ion affects the solubility, purity, and physical properties of the precipitate. Understanding the behavior of different counter-ions is essential for designing effective gravimetric procedures and interpreting results accurately.