Question

If 15.00 g of solid zinc reacts with 100.0 mL of 4.00 M hydrochloric acid, what volume of hydrogen gas is produced at 25°C and 1.00 atm?

Answer 1

Just under 4.5 litres

Explanation:

The reaction equation is:

`Zn(s) + 2HCl(aq) -> ZnCl_2(aq) + H_2(g)`

Atomic mass of zinc is 65.38 g/mol.

The limiting reagent here is the HCl. 15 g of zinc is 0.229 moles, and to react with all of this you will need 0.229 x 2 = 0.458 moles of HCl. But 100 ml of 4M HCl is only 0.400 moles (1 litre contains 4M, so 100 ml is 1/10 of a litre, so will contain 0.1 x 4 = 0.4 moles).

Therefore, because zinc and HCl react in the ratio 2:1, 0.2 moles of zinc will react, and this will generate 0.2 moles of hydrogen gas (`H_2`).

If we assume that hydrogen behaves as an ideal gas (which is not quite right but near enough for our purposes) then we can assume that 1 mole occupies 22.4 litres at 25 celcius and 1 atm, so 0.2 moles would occupy 4.48 litres.