Question

The mass of one mole of a substance in gram is called its:
A. Molecular mass
B. Molar mass
C. Avogadro’s mass
D. Formula mass

Answer 1

Mass of a substance can be expressed in different ways based on the quantity of the substance under consideration. The mass associated with a mole of any substance is to be considered here.

Step by step answer: In chemistry, we have quantitative as well as qualitative analysis and both are inter-related. For example, we can deduce the formula of a compound from its composition expressed as the mass percentage of each constituent element. We have different methods of expressing a given quantity such as heat capacity and specific heat capacity of a substance. Both measures of the same property are inter-related but are different. Similarly, we have different ways of expressing mass of a substance which is based on how much of the substance has been taken under consideration. Let’s have a look at some commonly used ones:
-Atomic mass: For a given substance, it represents the mass of a single atom of that substance. For example, for hydrogen, atomic mass is $1.00784\;{\rm{u}}$.
Molecular mass: It is the mass of one molecule of a substance. For example, for hydrogen gas which has diatomic molecules, molecular mass is $2.01568\;{\rm{u}}$.
-Formula mass: We don’t have molecules in case of ionic compounds as they are made of ions surrounded and held by oppositely charged ions. So, in such cases, we consider a formula unit in place of molecules and use formula mass. For example, in ionic compounds such as ${\rm{NaCl}}$ or ${\rm{KCl}}$.
-Molar mass: Now, this one is an important one as it is widely used across various fields of not just chemistry but other areas as well. For a given substance, it represents the mass of $1{\rm{ mole}}$ of that substance. It is expressed in ${\rm{g}} \cdot {\rm{mo}}{{\rm{l}}^{ - 1}}$ For example, for hydrogen atoms, molar mass is $1.00784\;{\rm{g}} \cdot {\rm{mo}}{{\rm{l}}^{ - 1}}$.
As we can see that the one which represents mass of one mole of a substance in gram is molar mass.

Hence, the correct option is B.

Note: Numerically, molar mass can be equal to atomic mass, molecular mass or formula mass depending on if we are talking about $1{\rm{ mole}}$ of atoms, molecules or formula units respectively. It is also related to Avogadro’s number as the amount in moles is related to Avogadro’s number only.