The number of chlorine atoms in 20 mL of chlorine gas at STP is ______ $\times 10^{21}$ (Round off to the nearest integer). [Assume chlorine is an ideal gas at STP \[ \left.\mathrm{R}=0.083 \mathrm{~L} \text { bar } \mathrm{mol}^{-1} \mathrm{~K}^{-1}, \mathrm{~N}_{\mathrm{A}}=6.023 \times 10^{23}\right] \]

JEE Main · 2021 · 17 Mar 2021 · Shift 2 · Some Basic Concepts of Chemistry

The number of chlorine atoms in 20 mL of chlorine gas at STP is ______

\times 1 0^{21}

(Round off to the nearest integer).
[Assume chlorine is an ideal gas at STP

R = 0.083 L bar mol^{- 1} K^{- 1}, N_{A} = 6.023 \times 1 0^{23}]

Answer: 1

At STP the pressure is

1

bar and the temperature

273 K

, so the amount of chlorine gas in

20 mL

n = \frac{P V}{R T} = \frac{1 \times 0.020}{0.083 \times 273} = 8.83 \times 1 0^{- 4} mol

Chlorine is diatomic, so each mole of gas carries two moles of atoms:

N = 2 n N_{A} = 2 \times 8.83 \times 1 0^{- 4} \times 6.023 \times 1 0^{23} = 1.06 \times 1 0^{21}

To the nearest integer the number of chlorine atoms is

1 \times 1 0^{21}

JEE Main · 2021 · 17 Mar 2021 · Shift 2 · Integer answer