Question

Air contains 21% of oxygen by volume. The number of moles of $O_2$ present in 5 L of air at STP conditions is:

• A. ${2.23\times 10}^{-1}$
• B. ${4.68\times 10}^{-4}$
• C. ${4.68\times 10}^{-2}$
• D. $0.0234$

Answer 1

The correct option is C ${4.68\times 10}^{-2}$

$1$ L of air contains $21$ % of oxygen i.e; $0.21L$ of oxygen.

At STP 1 mole $=22.4L$

Therefore,

No. of moles of $o_2$ at $0.21L$

$\to \frac{0.21}{22.4}=0.0094$ moles

Therefore;

$5L$ of air contains, $5\ast 0.0094$ moles

$\to 0.0462\to 4.68\ast {10}^{-2}$ moles