# Balancing & stoichiometry lectures

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## Balancing questions » limiting reagent

Mixture of powdered zinc and sulfur is sometimes used as a rocket propellent. 10.0 g of Zn powder was mixed with 5.00 g of sulfur and ignited. What was the mass on zinc sulfide produced?

Balanced equation is probably the simplest imaginable:

Zn + S -> ZnS

But it seems like we don't know which reagent will run out first. So it is limiting reagent question. Obvious way of doing the question will be to calculate how much sulfur will react with 10.0 g of zinc and how much zinc will react with 5.00 g of sulfur - which will tell us which reagent is the limiting one, then we will use this information to calculate ZnS mass.

Do we have to do calculations three times? No - it is enough to calculate masss of ZnS for both reactants and check which mass of product is lower. Effect is exactly the same, but we will save some time.

So, first, what is the mass of ZnS that can be produced from 10.0 g of Zn?

 Zn + S -> ZnS 65.41 g : 97.47 g 10.00 g : x g

So proportion is

65.41 g : 97.47 g = 10.00 g : x g

and x = 97.47/65.41×10.0 = 14.90 g

What is the mass of ZnS that can be produced from 5.00 g of S?

 Zn + S -> ZnS 32.07 g : 97.47 g 5.00 g : x g

So proportion is

32.07 g : 97.47 g = 5.00 g : x g

and x = 97.47/32.07×5.00 = 15.20 g

And that's all - the lower of these two is what we are looking for, 14.90 g of ZnS will be produced. (We also know at this moment that Zn is a limiting reagent - but there is no need it include this information in the answer, as we were not asked about the limiting reagent but only about the mass of the product).

(try to do this question using EBAS - mass/number of moles of sulfur are displayed in red, which means S is the limiting reagent).