To define the quantity of any substance in the field of chemistry, we use the term "mole". It is defined as the quantity comprising 6.022 × 10²³ elementary units. They can be atoms, molecules, ions, or electrons. The value mentioned is defined by the name as Avogadro's number, after the famous Italian scientist, Amedeo Avogadro. This number helps chemists calculate entities during a chemical reaction and relate the mass of the substance to the number of particles it contains. For instance, one mole of carbon would weigh 12 grams, which would have 6.022 × 10²³ atoms of carbon.
where m = Wt. of substance in grams. and M = Molar mass of substance in G.m.m.
Here G.m.m = Gram molecular mass or molar mass, which is the mass of 1 mole of any substance
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A mole of any substance (like O2) stands for:
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Recommended topic video on (Mole Concept Basic )
Que 1: Number of atoms in 558.5 gram Fe (at. wt. of Fe = 55.85 g mol-1) is:
1) twice that in 60 g carbon
2) 6.023 × 1022
3) half that in 8 g He
4) 558.5 × 6.023 × 1023
Solution
Number of moles of Fe atom =558.555.85=10Mole
Total number of atom
=10×6.022×1023=6.022×1024=10×NA
12 g of carbon means Avogadro's number,
therefore, 60 g of carbon means, 5 moles of carbon and thus contains 5×NA atoms, twice of this equals 10NA atoms of carbon.
Hence, the answer is the option (1).
Que 2: If we consider that 1/6, in place of 1/12, the mass of the carbon atom is taken to be the relative atomic mass unit, the mass of one mole of a substance will
1) decrease twice
2) increase two-fold
3) remain unchanged
4) be a function of the molecular mass of the substance.
Solution
When you consider 1/6 in place of 1/12 since the mass of the carbon atom is still the same, new amu1 = 2amu ( where amu1 is the new amu unit and amu is the old amu unit). So now Na1 (Avogadro's number) becomes half ( Na/2) since the weight of carbon corresponding to atomic weight becomes 6gm in place of 12. ( where Na1 is the new Avogadro's number and Na is the old Avogadro's number). And we know that the mass of one mole of a substance is = Na x amu. So new atomic mass= Na1 x amu1 = Na/2 x 2amu = Na x amu = old atomic weight.
Hence, the answer is the option (3).
Que 3: What is the total number of moles if we have a mixture of 34g of Ammonia, 78g of Benzene, and 10g of Hydrogen?
1) 8
2) 9
3) 7
4) 10
Solution
Number of moles of ammonia = mass of ammonia molar mass of ammonia =3417=2
Number of moles of benzene =7878=1
Number of moles of hydrogen =102=5
Total number of moles = 2 + 1 + 5 = 8
Hence, the answer is (8).
Que 4: We have a substance x. What is the molar mass (in g) of x if 2 moles of it weigh 196g?
1) 147
2) 196
3) 98
4) 49
Solution
Molar mass of x=1962g=98g
Hence, the answer is (98g).
Que 5. 14 g of Nitrogen gas and 22 g of CO2 gas are mixed together. Find the volume of the gaseous mixture at STP.
(Response should be up to only one decimal digit like 78.7).
1) 22.4
2) 11.2
3) 44.8
4) 52.8
Solution
Number of Moles of a gas at STP -
no. of moles of a gas at STP = given volume of gas / 22.4 liter
Moles of N2=0.5
moles of CO2=0.5
So total moles 0.5+0.5=1
So vol. at STP =1×22.4=22.4lit
Hence, the answer is the option (1).
The number 6.022 x 1023 is called the Avogadro number or Avogadro constant. It is denoted by NA in honor of nineteenth-century Italian scientist, Amedeo Avogadro. In other words, a mole is an Avogadro number of particles. The mass of one mole of any substance will be its molar mass and it contains 6.022 x 1023 particles of that substance.
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The mole concept is used to convert between the mass of a substance and the number of particles it contains, facilitating stoichiometric calculations in chemical reactions.By using molar mass, one can calculate the number of moles from a given mass and vice versa, allowing for accurate predictions of reactant and product quantities.
The relationship is defined by the formula: concentration (C) = moles of solute (n) / volume of solution (V), where concentration is typically expressed in moles per liter (M). This formula allows for the determination of how much solute is present in a given volume of solution.
Avogadro's law states that equal volumes of gases, at the same temperature and pressure, contain an equal number of moles. This principle supports the mole concept, demonstrating how the mole measures the number of atoms in a given volume of gas.
The value of one mole is 6.022 $\times$ 1023
The number of atoms in a mixture can be calculated by first determining the number of moles of each component in the mixture using the molar masses of the components. The number of atoms of each element in the mixture can then be calculated by multiplying the number of moles of each component by the subscript of that element in the chemical formula for that component. The total number of atoms of each element in the mixture can then be calculated by adding the number of atoms from each component.
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