$22400\mathrm{cc}\text{ of gas at STP has }6\times {10}^{23}\text{ molecules }$

$\begin{array}{l}\therefore 1.12\times {10}^{-7}\text{ of gas at STP has }\frac{6\times {10}^{23}\times 1.12\times {10}^{-7}}{22400}\\ =0.03\times {10}^{14}=3\times {10}^{12}.\end{array}$