Given the reaction:
PbCl2(aq) + Na2CrO4(aq) PbCrO4(s) + 2
NaCl(aq)

What is the total number of moles of NaCl formed
when 2 moles of Na2CrO4 react completely?

4 moles of NaCl is produced from 2 moles of Na₂CrO₄.

Explanation:

Given reaction is

PbCl₂(aq) + Na₂CrO₄(aq)→ PbCrO₄(s) + 2 NaCl (aq)

It is the balanced equation which means that on both sides of the equation, number of atoms of each element are equal.

From the above balanced equation it says that molar ratio of Na₂CrO₄ to NaCl is 1 : 2.

That is 1 mole of Na₂CrO₄ produces 2 moles of NaCl, so the molar ratio is 1:2.

2 moles of Na₂CrO₄  produces 4 moles of NaCl.

So the molar ratio of Na₂CrO₄ to NaCl is 2: 4.

"record the time to complete a chemical reaction" would most likely be part of a lab procedure.

the answer would be a