Question

Magnesium metal (0.100 mol) and a volume of aqueous hydrochloric acid that contains 0.500 mol of HCl are combined and react to completion. How many liters of hydrogen gas, measured at STP, can be produced? `"Mg"(s) + 2"HCl"(aq) -> "MgCl"_2(aq) + "H"_2(g)`

Answer 1

`2.24 "L"`

Explanation:

Step 1: determine the reaction

The chemical equation is already given.

`"Mg" + 2 "HCl" -> "MgCl"_2 + "H"_2`

Step 2: determine which reactant is limiting

0.100 mol of Mg reacts with 0.500 mol of HCl, which of them will run out first?

From the reaction, the ratio of `"Mg"` to `"HCl"` used is `1:2`. This means that if the reaction goes to completion, the `0.100 "mol"` of `"Mg"` will react with `0.100 "mol" xx 2 = 0.200 "mol"` of `"HCl"`. This leaves `0.500 "mol" - 0.200 "mol" = 0.300 "mol"` of `"HCl"`.

Step 3: determine how much `"H"_2` is produced.

`0.100 "mol"` of `"Mg"` and `0.200 "mol"` of `"HCl"` will produce `0.100 "mol"` of `"H"_2`.

Step 4: determine the volume at STP

Each mole of ideal gas occupies `22.4 "L"` at STP. Therefore, `0.100 "mol"` of `"H"_2` will occupy `2.24 "L"` of space.