Question

Find the number of water molecules in $1 \text{ mL}$ of water vapours at STP?

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

Hint:

Molecules in $V$ mL at STP $= \frac{V \times N_A}{22400}$.

Answer 1

The Correct Option is D

Step 1: Moles at STP
At STP, $1 \text{ mole} = 22400 \text{ mL}$.
Moles in $1 \text{ mL} = \frac{1}{22400} \text{ mol}$.
Step 2: Molecule Calculation
Molecules $= \text{moles} \times N_A$
Molecules $= \frac{1}{22400} \times 6.022 \times 10^{23}$
Step 3: Calculation
$\approx \frac{60.22}{2.24} \times 10^{18} \approx 26.88 \times 10^{18} = 2.688 \times 10^{19}$
Final Answer: (D)