# Why coefficient doesn't change amount of substance

$$\ce{3Mg + 2As -> Mg3As2}$$ ($$10\ \mathrm g$$ 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 $$\ce{Mg}$$

• The stochiometric coefficient only helps you realise the ratio of moles of reactant or products. 3Mg in the equation does not imply anything about the number of moles you have in your reaction vessel, but rather only tells you that you will need 3 moles of Mg to completely react with 2 moles of As. – William R. Ebenezer Apr 4 at 12:04
• – A.K. Apr 4 at 12:27
• Welcome to Chemistry.SE! Take the tour to get familiar with this site. Mathematical expressions and equations can be formatted using $\LaTeX$ syntax. Please note that the proper term for "number of moles" is amount of substance. The former would be the same as referring to the mass as "number of kilograms". – Martin - マーチン Apr 4 at 14:40

Even if you were to use 10g Mg in a reaction such as $$\ce{ 2Mg + O2 -> 2MgO}$$, you would still have 0.411 moles of Mg as it is still equivalent to 10g. Nothing changes on the basis of the 'coefficient'.