Question

The molecular mass of$CuS{O_4}.5{H_2}O$ is:
(Atomic mass of $Cu = 63.5,H = 1,O = 16,S = 32$)
A: $233.5gm/mol$
B:$249.5gm/mol$
C: $251.5gm/mol$
D: $247.5gm/mol$

Answer 1

Hint : Molecular mass of a compound is the sum of atomic mass of all the atoms present in that compound. It is the mass of one molecule of the compound. Gram molecular mass and molecular mass are not the same. Gram molecular mass is the mass of one mole of a substance.

Complete step by step solution :
We know the molecular mass of a substance is the sum of the atomic mass of all the atoms present in that substance. Given compound is $CuS{O_4}.5{H_2}O$ .In this given molecule there is one atom of copper $(Cu)$, one atom of sulphur$(S)$, 10 atoms of hydrogen $(H)$, $9$ atoms of oxygen $(O)$. We have also given atomic mass of all these elements. To find molecular mass we have to find the mass of all these elements.
Atomic mass of one atom of copper$= 63.5gm$
Atomic mass of one atom of sulphur$= 32gm$
Atomic mass of one atom of hydrogen$= 1gm$
So, atomic mass of $10$ atoms of hydrogen$= 1 \times 10 = 10gm$
Atomic mass of one atom of oxygen$= 16gm$
So, atomic mass of nine atoms of oxygen$= 16 \times 9 = 144gm$
Molecular mass of molecule=sum of mass of all atoms
$= 63.5 + 32 + 144$
$= 249.5gm$

So, the molecular mass of $CuS{O_4}.5{H_2}O$ is $249.5gm$ and the correct answer is option B that is $249.5gm/mol$.
Additional information: There are Avogadro numbers of molecules in one mole of a substance. Avogadro number is equal to $6.022 \times {10^{23}}$. The mass which is given of the atoms is the mass per mole of that atom. That is it is the mass of $6.022 \times {10^{23}}$atoms.

Note : Molecular mass is the mass of one molecule of a compound and molar mass is mass of one mole molecules i.e. mass of $6.022 \times {10^{23}}$molecules. Formula mass is the sum of atomic masses of ions present in one formula unit of ionic compounds.