How do you get the mass of Zn? I've been staring at this question for a solid 15 minutes now...

Question

Solving numerical problems involving empirical formulae and molecular formulae:

What is the mass of zinc required to combine with 0.5 mol of chlorine to produce zinc chloride, ZnCl₂?

Answer 1

Hi Nichole L.,

So quit staring already. These questions all yield to the following protocol:

1) Write the balanced equation for the reaction you are considering

here that's Zn + Cl₂ = ZnCl₂

2) Convert any masses data you are given to moles basis. That's already been done for you. When the problem says "chlorine", you may assume the standard state of the element, which is the diatomic gas. Hence, 0.5 mol of Cl₂ gas.

3) Look for limiting reactants (here, the Cl₂ is effectively limiting since youa re given only so much of it).

4) Work the reaction to use just that amount of the limiting reactant. That means, use the proportions as given by the coefficients in the balanced equation. Here, that means taking 0.5 mol Zn.

5) If requested, convert back reactants and/or products to a mass basis, using elemental masses, molar masses, etc. Here you just need the elemental mass of Zn * 0.5 .

NOTE: you only work reactions on a molecular or a molar basis. Molecules, (or their "big brothers"=moles) is how matter recognizes other matter and reacts with it. Mass is just how we measure things out, since we can't see molecules or generally manipulate them individually!