According to the given equation, if 2.0 moles of Zn completely reacts with 109.5 grams of HCl, what is the number of moles of ZnCl₂ produced? Zn plus 2HCl react to form ZnCl₂ plus H₂. A) 1.0 moles, B) 1.5 moles, C) 2.0 moles, D) 3.0 moles, or E) 4.0 moles.
We’re dealing here with the reaction of a metal, zinc, with an acid, hydrochloric acid, producing a salt, zinc chloride, and hydrogen gas. And we’ve been told the amount in moles of zinc and the mass in grams of hydrochloric acid that we’re reacting together. And what we need to work out is the amount in moles of zinc chloride we’d produce.
On the surface, this question looks quite easy. We generate one unit of zinc chloride per zinc atom we put in. So surely, if we’re putting two moles of zinc in, we’re getting two moles of zinc chloride out. But let’s take a closer look.
The question tells us that the zinc and the hydrochloric acid react completely, but that doesn’t necessarily mean that we don’t have an excess of one or the other. So, let’s first work out whether we’ve got enough HCl to use up the entirety of the two moles of zinc. To work out the amount of HCl in moles, we’ll need to take the mass in grams and divide it by the molar mass.
Hydrogen has an atomic mass of one unified atomic mass unit and chlorine has an atomic mass of 35.5 unified atomic mass units. Therefore, the molecular mass of HCl is 36.5 unified atomic mass units, and the molar mass of HCl is 36.5 grams per mole. Now, we can plug that into our equation and multiply the mass of HCl, 109.5 grams, by one mole of HCl per 36.5 grams. This gives us three moles of HCl.
The next thing we have to work out is how much zinc we use up if we consume all the HCl. And we do that by multiplying our three moles of HCl by one mole of zinc per two moles of HCl that we gathered from the chemical equation. This gives us only 1.5 moles of zinc.
So, even though we started with two moles, we only have enough HCl to consume 1.5 moles, leaving 0.5 moles behind. Now that we know that the hydrochloric acid is the limiting reagent. We can work out the amount of zinc chloride by multiplying the amount of zinc we used by one mole of zinc chloride per mole of zinc. Giving us 1.5 moles of zinc chloride.