Question

Number of water molecules in one litre of pure water is :

• A. $6.023\times {10}^{23}$
• B. $3.45\times {10}^{25}$
• C. $55.4\times {10}^{23}$
• D. $18\times {10}^{23}$

Answer 1

Answer: (B)

$3.45\times {10}^{25}$

One mole of water has a mass of $18$ grams and a volume of $18$ mL (as the density of water is 1 g per cm${}^3$).

Therefore, number of moles of water in $1$ litre $=\frac{1\times 1000}{18}$.

Number of molecules $=$ Number of moles $\times N_A$

$=\frac{1\times 1000}{18}\times 6.023\times {10}^{23}$

$=3.45\times {10}^{25}$ molecules.

Hence, the correct answer is option $\text{B}$.