Question

Which compound has the maximum molar mass?
(A) $A{{l}_{2}}{{\left( S{{O}_{4}} \right)}_{3}}$
(B) $F{{e}_{2}}{{\left( S{{O}_{4}} \right)}_{3}}$
(C) ${{\left( N{{H}_{4}} \right)}_{2}}Cr{{O}_{4}}$
(D) $C{{a}_{3}}{{P}_{2}}$

Answer 1

Hint: Molar mass is the subtotal of the atomic masses multiplied by their respective subscripts. Atomic masses of atoms are empirical whereas, the molecular masses of a molecule depend on the number of atoms present within the molecule.

Complete step by step answer:
Firstly, let us know about the atomic and molecular masses in detail for better understanding.
- Atomic mass- The atomic mass is simply the mass of an atom present in a periodic table. Its SI unit is kilograms.
- Molecular mass- The molecular mass is the mass of a given molecule consisting of various atoms. Molecular mass or molar mass is used in stoichiometry calculations or vice versa.
As specified by Dalton, the molecular mass of one molecule is its mass expressed in unified atomic mass units. Mass of a molecule is based on 12 as the atomic weight of carbon-12.
In actual practice, it is calculated by summing up the atomic weights of atoms making up the substance’s molecular formula.

Illustration-
For calculation of molecular mass of any substance we should primarily know the atomic masses of atoms present in the substance.
If the atomic masses are unknown for the given problem then we should refer to the periodic table for specific atomic masses.
Given-
(A) Molar mass of $A{{l}_{2}}{{\left( S{{O}_{4}} \right)}_{3}}$ is,
Atomic mass of Al = 26.98 g/mol
Atomic mass of S = 32.06 g/mol
Atomic mass of O = 15.99 g/mol
Thus,
$A{{l}_{2}}{{\left( S{{O}_{4}} \right)}_{3}}$ = 342.15 g/mol

(B) Molar mass of $F{{e}_{2}}{{\left( S{{O}_{4}} \right)}_{3}}$ is,
Atomic mass of Fe = 55.84 g/mol
Atomic mass of S = 32.06 g/mol
Atomic mass of O = 15.99 g/mol
Thus,
$F{{e}_{2}}{{\left( S{{O}_{4}} \right)}_{3}}$ = 399.88 g/mol

(C) Molar mass of ${{\left( N{{H}_{4}} \right)}_{2}}Cr{{O}_{4}}$ is,
Atomic mass of N = 14.00 g/mol
Atomic mass of H = 1.00 g/mol
Atomic mass of Cr = 51.99 g/mol
Atomic mass of O = 15.99 g/mol
Thus,
${{\left( N{{H}_{4}} \right)}_{2}}Cr{{O}_{4}}$ = 152.07 g/mol

(D) Molar mass of $C{{a}_{3}}{{P}_{2}}$ is,
Atomic mass of Ca = 40.058 g/mol
Atomic mass of P = 30.97 g/mol
Thus,
$C{{a}_{3}}{{P}_{2}}$ = 182.18 g/mol.
So, the correct answer is “Option B”.

Note: Though the simplified definitions consist of SI units as kilogram still while calculating for masses we use mass of atom or molecule per mole i.e. gm/mole for easy and globally acceptable calculations.
Be specific while calculating the molecular masses as molecules and its subscripts are dignified by brackets.