Q50 / 150 · MHT CET 2025 (22 Apr Shift 2) · Chemistry

Find the number of water molecules in 1 mL of water vapours at STP?

  • A
    $1.69 \times 10^{19}$
  • B
    $2.00 \times 10^{21}$
  • C
    $1.05 \times 10^{21}$
  • Correct answer
    D
    $2.69 \times 10^{19}$

Solution

Calculating water molecules in 1 mL vapor at STP

Standard Temperature and Pressure conditions define molar gas volume as 22.4 L/mol. For water vapor occupying $1 \text{ mL} = 1 \times 10^{-3} \text{ L}$, the number of moles is $n = \frac{1 \times 10^{-3}}{22.4} = \frac{1}{22400} \text{ mol}$.

Using Avogadro's number $N_A = 6.022 \times 10^{23} \text{ molecules/mol}$, the molecular count becomes $n \times N_A = \frac{1}{22400} \times 6.022 \times 10^{23}$.

This simplifies to approximately $2.69 \times 10^{19}$ molecules, corresponding to option D.

Hence, the correct answer is option: 'D'.

Relevant Questions