$\ce{3Mg + 2As -> Mg3As2}$ ($10\ \mathrm g$ of $\ce{Mg}$ and $10\ \mathrm g$ of $\ce{As}$)
I don't understand why coefficient doesn't change the amount of substance, here's what I did:
$n_{\ce{Mg}}=3\cdot\frac{10\ \mathrm g}{24.31\ \mathrm{g/mol}}=1.233\ \mathrm{mol}$ of $\ce{Mg}$
But the correct solution is:
$n_{\ce{Mg}}=\frac{10\ \mathrm g}{24.31\ \mathrm{g/mol}}=0.411\ \mathrm{mol}$ of $\ce{Mg}$