Question

3 litre aqueous solution is prepared by dissolving 4.9 grams of ${{H}_{2}}S{{O}_{4}}$ ​ in water. To calculate pH of this solution, there is a need to calculate moles ${{H}_{2}}S{{O}_{4}}$​. Which of the following formulas would you use?
a. Mole of ${{H}_{2}}S{{O}_{4}}$​= $\dfrac{Molar~mass}{litre}$ ​
b. Mole of ${{H}_{2}}S{{O}_{4}}$​= $\dfrac{Weight~of~substance}{Molar~mass}$ ​
c. Mole of ${{H}_{2}}S{{O}_{4}}$​= $\dfrac{Molar~mass}{Weight~of~substance}$ ​
d. Mole of ${{H}_{2}}S{{O}_{4}}$​= $\dfrac{Weight~of~substance}{litre}$

Answer 1

Hint: A mole is a unit defined for the amount of substance which is in the International System of Units (SI) and the short form of which is “mol” and it is defined as the mass of a substance that contains the same number of fundamental units such as atoms in 12.0 grams of C.

Complete Step-by-Step answer:
Before trying to calculate the number of moles of ${{H}_{2}}S{{O}_{4}}$, let us first try to understand what the mole concept really is.
The mole defined is the standard unit of measurement present in the International System of Units (SI) for the amount of substance. It is chemically defined as the amount of a chemical substance that contains as many basic chemical units, e.g., atoms, molecules, ions or electrons, as are present atoms in 12 grams of carbon-12, which is the isotope of carbon with relative atomic mass 12. This number is experimentally found and is expressed as the Avogadro constant, which has a value of $6.022140857\times {{10}^{23}}$mol−1. The mole is one of the base units of the SI, and has the unit symbol mol.
Think of moles as a "chemist's dozen". Just as 12 eggs is a dozen eggs, $6.022140857\times {{10}^{23}}$ eggs constitute a mole of eggs. $6.022140857\times {{10}^{23}}$molecules of oxygen constitute a mole of oxygen.
Now, let us look into what the molar mass of a substance really is.
The molar mass of a substance is defined as the mass of as many particles as there are in 12 grams of C-12. As we have already established, this number is experimentally found and is expressed as the Avogadro constant and is $6.022140857\times {{10}^{23}}$. With these ideas in mind, we find that the number of moles of ${{H}_{2}}S{{O}_{4}}$​ is found out by dividing weight of substance by molar mass.
Mole of ${{H}_{2}}S{{O}_{4}}$​= $\dfrac{Weight~of~substance}{Molar~mass}$ ​
Thus, we can safely conclude that the answer is b.

Note:
Be very careful of the differences between the molecular mass and the molar mass of a substance since the molecular mass is the mass of the molecule relative to the 1/12 of the mass of the C-12 atom(relative to the mass of a proton.). It is a number with no unit and is numerically equal to the molar mass of the same thing expressed in grams.