Question

How many liters of hydrogen gas are produced at STP from the decomposition of `"70.0 g"` of hydrogen chloride?

Answer 1

The answer is `"21.5 L"`.

So, start with the balanced chemical equation for the decomposition of hydrochloric acid

`2"HCl" -> "H"_2 + "Cl"_2`

Notice that you have a `"2:1"` mole ratio between `"HCl"` and `"Cl"_2`, which means that every 2 moles of the former will produce 1 mole of the latter. The number of moles of `"HCl"` you have is

`"70.0 g" * ("1 mole")/("36.5 g") = 1.92` `"moles"`

As a result, the number of `"Cl"_2` moles is

`n_(Cl_2) = n_(HCl)/2 = 1.92/2 = 0.96` `"moles"`

At STP, 1 mole of any ideal gas occupies 22.4 L, which means that the volume of `"Cl"_2` produced will be

`n = V/V_("molar") => V = n * V_("molar") = 0.96` `"moles" * 22.4` `L/("mole") = 21.5` `"L"`