The number of moles is calculated by dividing the mass of the substance by its molar mass.
For $\mathrm{H}_{2}$, the molar mass is 2 g/mol. So, the number of moles is $\frac{12.45 \, \mathrm{g}}{2 \, \mathrm{g/mol}} = 6.225 \, \mathrm{mol}$.
For
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