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

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  • $\begingroup$ 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. $\endgroup$ – William R. Ebenezer Apr 4 at 12:04
  • $\begingroup$ related: chemistry.stackexchange.com/q/99809/23561 $\endgroup$ – A.K. Apr 4 at 12:27
  • $\begingroup$ 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". $\endgroup$ – Martin - マーチン♦ Apr 4 at 14:40
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When you say number of moles, it is a number to express the number of constituent species (atoms, in this case) in the given substance. It does not have any correlation to where that substance will be used and what will be its 'coefficient' in that reaction.

When you say 10g Mg, you mean 0.411 moles of Mg (and vice versa) irrespective of whatever you are going to do with those 10 grams.

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'.

You can further try to look up more information to get a better understanding about 'The Mole Concept' because that is where I think your problem lies.

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    $\begingroup$ 'The mole concept' appears to be such a loose invention of a particular brand of books. The concept/theory itself should be called stoichiometry. And I think calling it by the wrong name is where the problem actually starts. 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". And the base unit of the amount of substance is called mole. $\endgroup$ – Martin - マーチン♦ Apr 4 at 15:28
  • $\begingroup$ Yes I agree, and I think I tried to make the same clear in my answer, but apparently it wasn't, so please feel free to suggest an edit if you want. $\endgroup$ – Arush Sharma Apr 5 at 7:08

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