Question

The number of water molecules present in 1 litre of water (at $4{}^{o}C$) is:

• A. 111.10 $N_A$
• B. 55. 55 $N_A$
• C. $N_A$
• D. 18 $N_A$

Answer 1

The correct option is B 55. 55 $N_A$

We know,
$d=\frac{M}{V}(d=density,M=mass,V=volume)$
Since d = 1 g/mL (for water),
Mass in gram = Volume in mL
$\therefore$ 18 g = 18 mL
$\text{Number of}{H}_{2}O\text{molecules in 18 mL}=\text{Number of}{H}_{2}O\text{molecules in 18 g}$
Molar mass of $H_{2}O$ = 18 g
$\therefore \text{Number of}{H}_{2}O\text{molecules in 1000 mL}=\frac{N_A}{18}\times 1000=55.55N_A$