Question

How many molecules of $O_2$ are present in 1 L air containing 80% volume of $O_2$ at STP?

Answer 1

Amount of oxygen in $1$ litre air is $=80$% ($1000$ml)

$=\frac{80}{100}\times 1000=800$ ml

$1$ mole of any gas at $STP$, occupies $22.4$ litres

$22400$ ml of $O_2$ gas contains $6.022\times {10}^{23}$ molecules

$1$ ml of oxygen gas contains $=\frac{6.022\times {10}^{23}}{22400}$

$800$ ml contains $=\frac{6.022\times {10}^{23}}{22400}\times 800=2.15\times {10}^{22}$ molecules.

$\therefore$ molecules present in $1$ litre air containing $80$% volume of at $STP$ is $2.15\times {10}^{22}$ molecules.