Question

How many molecules of water of hydration are present in 630mg of oxalic acid (H2C2O4.2H2O)

Answer 1

H2C2O4.2H2O weight =126

126 grams of oxalic acid contains 2 molecules of H2O
630×10^-3 grams contain

126- 2
630×10^-3 -- ?

0.01

Answer 2

The gram molecular mass of oxalic acid or $\mathrm{H}_{2} \mathrm{C}_{2} \mathrm{O}_{4} .2 \mathrm{H}_{2} \mathrm{O}=126 \mathrm{gm}$.

We can calculate the molecules of water of hydration if we know the molar mass of oxalic acid. We already know that molar mass is the mass of 1 gram of one mole of any substance. Also the value of 1 mole is $6.023 \times 10^{23}$.

Water molecules present in 1 mole of oxalic acid = 2 mol

Or water molecules in 126gm of oxalic acid $=2 \times 6.023 \times 10^{23}$.

Number of water molecules present in 630 mg or 0.63 gm of oxalic acid

$\frac{2 \times 6.023 \times 10^{23}}{126}=3.01 \times 10^{21}$.