How many grams of magnesium chloride is produced when .500 moles of magnesium reacts with an excess of hydrochloric acid?

Question

aaronq · Answer

First write and balance an equation for the reaction. Then use the stoichiometric coefficients to find moles produced. Set up a ratio using the species of interest, like so:

e.g. for a general reaction:

                      $\color{red}{a}A + \color{blue}{b}B$ $\rightleftharpoons$ $ \color{green}{c}C$

where upper case are the species (A,B,C), and lower case (a,b,c) are the coefficients ,

                     $\dfrac{n_A}{\color{red}{a}}=\dfrac{n_B}{\color{blue}{b}}=\dfrac{n_C}{\color{green}{c}}$

From here you can isolate what you need. 
For example: if you have 2 moles of B, how many moles of C can you produce?

solve algebraically:      $\dfrac{2}{\color{blue}{b}}=\dfrac{n_C}{\color{green}{c}}\rightarrow n_C=\dfrac{2*\color{green}{c}}{\color{blue}{b}}$
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To interconvert from mass to moles or moles to mass, use the relationship:

                             $n=\dfrac{m}{M}$

        where, M=molar mass, m=mass, and n= moles.

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anonymous · Answer

o-o

anonymous · Answer

Well you could just do $$n =\frac{ M }{ Mr }$$
That would give you N x Mr = M
0.500 x 94.3 (Molecular mass of MgCl2) = 47.15 Grams

aaronq · Answer

^while you're right it doesn't really show how you arrived at that answer, because you can only do that because the stoichiometric ratios are 1:1. If this person assumes that every question is solved like that, then they're not really learning correct the process.

anonymous · Answer

Of course ratios play a massive part in this but im just assuming that this reaction is all 1:1 like you said. Its good you pointed that out to them, I should have been a better chemist and mentioned that.