Answer
$2.69\times10^{25}molecules/m^3$.
Work Step by Step
At STP, 1 mole of ideal gas occupies 22.4 L.
$$\frac{1mole}{22.4L}\frac{6.02\times10^{23}molecules}{mole}\frac{1L}{10^{-3}m^3}=2.69\times10^{25}molecules/m^3$$
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