Question

From $320$ mg of $O_2$, $6.023\times {10}^{20}$ molecules are removed. The number of moles remaining is ___________.

• A. $9\times {10}^{-3}$
• B. $9\times {10}^{-2}$
• C. $0$
• D. $3\times {10}^{-3}$

Answer 1

The correct option is A $9\times {10}^{-3}$

$320$ mg (or $0.320$ g) of oxygen corresponds to $0.01$ mole or $6.023\times {10}^{21}$ molecules.

this, $6.023\times {10}^{20}$ molecules are removed.

The number of molecules remaining is ($6.023\times {10}^{21}$ - $6.023\times {10}^{20}$) $=5.4\times {10}^{21}$.

$1$ mole of oxygen contains $6.023\times {10}^{23}$ molecules. Thus, $5.4\times {10}^{21}$ molecules of oxygen correspond to $0.009$ moles.