States of Matter 1 Question 88
88. Oxygen is present in one litre flask at a pressure of $7.6 \times 10^{-10}$ $\mathrm{mm} \mathrm{Hg}$. Calculate the number of oxygen molecules in the flask at $0^{\circ} \mathrm{C}$.
(1983, 2M)
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Solution:
- Number of moles $(n)=\frac{p V}{R T}$
$$ \text { and } \quad \begin{aligned} n & =\frac{N(\text { Number of molecules })}{N_{A}(\text { Avogadro number })} \ \Rightarrow \quad N & =n N_{A}=\left(\frac{p V}{R T}\right) N_{A} \ & =\left(\frac{7.6 \times 10^{-10}}{760} \times \frac{1}{0.082 \times 273}\right) \times 6.023 \times 10^{23} \ & =2.7 \times 10^{10} \text { molecules } \end{aligned} $$
