If you start with 4.50 grams of Magnesium (Mg), how many grams of Hydrogen gas (H2) will you theoretically produce? If you actually produce 0.25 grams, what is your percent yield?

Question

Mg (s) + 2HCl (aq) → MgCl2 (aq) + H2 (g)

Answer 1

Hello, Maddie,

The balanced equation tells us that we'll produce 1 mole of H₂ for every 1 mole of Mg metal. So let's find out how many moles are in 4.5 grams of Mg. Divide the mass of Mg by it's molar mass (24.3 g/mole). That is 0.185 moles of Mg. Since the molar ratio is 1, we can set a conversion formula:

(1 mole H₂)/(1 mole Mg)

So we should obtain 0.1875 moles of H₂, assuming that there is sufficient hydrochloric acid (HCl). The theoretical yield of H₂, in grams, can be found by multiplying the 0.1875 moles by the molar mass of H₂ (2 grams/mole). We would expect, and love to find, 0.37g in our possession. Doesn't sound like much, but it makes a nice bang, when mixed with oxygen. But we only got 0.25 grams, a (0.25g/0.37g) 67.5% yield.

Blame it on the boss. I hope this helps.

Bob