Question

How do you calculate molar mass of compounds?

Answer 1

Hint: The atomic mass or the molar mass is the sum of the protons and neutrons present in the atom i.e. the weight of the nucleus. Add up the atomic mass of all the atoms present in the compound.

Complete answer:
So in the question, it is asked that how we can calculate the molar mass of the compound, to calculate the molar mass of the compounds at least we should know the molecular formulae of the compound or the name of the compound should be given so that we can derive the molecular formulae for the compound and from the molecular formulae we get to know the different atoms present in the compound and also about how many atoms of each kind are present.
And then if we get the molecular formulae we can sort each atom and calculate each number of atoms and multiply the atomic mass of each atom, then add up all these masses obtained for each kind of atoms which will finally yield the molar mass of the compound.
Let’s discuss a few problems so that the doubts get clarified and the concept becomes strong.
So let’s begin with a simple compound NaCl, here the atomic mass of Na is 23 and Cl is 35.5. Since the compound does not have any subscripts just add up these atomic masses to get the molar mass of NaCl.
$\text{Atomic mass of NaCl = 23 + 35}\text{.5 = 58}\text{.5g/mol}$
Now let’s solve for compounds which have subscript, carbon dioxide. The molecular formula is$C{{O}_{2}}$.
Here there are two oxygen atoms and one carbon atom and the atomic mass of C is 12 and for O is 16.The mass of O should be added twice to get the correct atomic mass.
And the atomic mass of $C{{O}_{2}}$is calculated as,
$\text{Atomic mass of C}{{\text{O}}_{\text{2}}}\text{= 12 + 2 }\!\!\times\!\!\text{ }\left( \text{16} \right)\text{ = 44g/mol}$

Note: For compounds with more number of polyatomic species like in Vanadium (V) hydrogen phosphate, the polyatomic species should be written in parenthesis and the subscript given is applicable for all the atoms inside the parentheses i.e. the formula for Vanadium (V) hydrogen phosphate is ${{V}_{2}}{{\left( HP{{O}_{4}} \right)}_{5}}$.
Now let’s calculate the molar mass of this compound, here there are 2 vanadium atoms, 5 H atoms, 5 P atoms and 20 oxygen atoms.
Now calculate the mass of these atoms,
$\text{Atomic mass of }{{\text{V}}_{\text{2}}}{{\left( \text{HP}{{\text{O}}_{\text{4}}} \right)}_{\text{5}}}\text{ = 2(50}\text{.94) + 5}\left( \text{1}\text{.008} \right)\text{ + 5}\left( \text{30}\text{.97} \right)\text{ + 20}\left( \text{16} \right)$
$\text{Atomic mass of }{{\text{V}}_{\text{2}}}{{\left( \text{HP}{{\text{O}}_{\text{4}}} \right)}_{\text{5}}}\text{ = 101}\text{.88 + 5}\text{.04 + 154}\text{.85 + 320 = 581}\text{.77g/mol}$