stoichiometry?

Question

i have this stoichiometry question that ive completed but for some reason i dont think that i have the right answer

how many grams of magnesium metal will completely reasct with 50 g of phosporic acid?
Mg+H3PO4
M? 50g
24.31g/mol 98g/mol

n H3PO4= 50g * (1mol/98g) =0.5102mol
n MG= 0.5102mol *(1mol/1mol) = 0.5102mol
M MG= 0.5102 mol *(21.31g/1mol) = 12.43g

also if someone could help me understand molar ratios i would really appriciate it
thanks

if anyone else reads this.. if i change my eqation to 3Mg + 2H3PO4 ~> Mg3(PO4)2 + 3H2 like the person below me said would the g/ mol calculations that i did for magnesium and phosphoric acid change.

Answer 1

I like everything you wrote except for one thing. Like you said, you did not use your molar ratio correctly because you did not discover the balanced chemical equation for this reaction.

You wrote the reactants perfectly: Mg + H3PO4

However, you need your products. Since you have a metal reacting with an acid, you can predict a single replacement reaction, meaning the magnesium will combine with the phosphate and you will have hydrogen by itself in elemental form.

The full equation is Mg + H3PO4 ~> Mg3(PO4)2 + H2

Balancing it, we get 3Mg + 2H3PO4 ~> Mg3(PO4)2 + 3H2

Now we see that it takes 3 mol of Mg to react with 2 mol of H3PO4.

So, in your second step of the calculations, instead of a 1mol/1mol ratio, you need a 3mol / 2mol ratio.

Other than that, it was very well done!