Limiting Reagents and Percent Yield

Limiting ReagentIf you are given one dozen loaves of bread, a

gallon of mustard and three pieces of salami, how many salami sandwiches can you make.

The limiting reagent is the reactant you run out of first.

The excess reagent is the one you have left over.

The limiting reagent determines how much product you can make

How do you find out?Do two stoichiometry problems.

The one that makes the least product is the limiting reagent.

For exampleCopper reacts with sulfur to form copper

( I ) sulfide. If 10.6 g of copper reacts with 3.83 g S how much product will be formed?

If 10.6 g of copper reacts with 3.83 g S. How many grams of product will be formed?

2Cu + S Cu2S

10.6 g Cu 63.55g Cu 1 mol Cu

2 mol Cu 1 mol Cu2S

1 mol Cu2S159.16 g Cu2S

= 13.3 g Cu2S

3.83 g S 32.06g S 1 mol S

1 mol S 1 mol Cu2S

1 mol Cu2S159.16 g Cu2S

= 19.0 g Cu2S

= 13.3 g Cu2S

Cu is Limiting Reagent

OR…Convert both grams to molesUse the mole ratio to determine if you have

enough of one reactant to use up the other.Whatever you don’t have enough of is the

limiting reactant

10.6 g Cu 63.55g Cu 1 mol Cu

3.83 g S 32.06g S 1 mol S = 0.12mol S

= 0.17mol Cu

Ratio of Cu to S is 2:1

0.17 mol Cu 2 mol Cu 1 mol S = .08mol S needed

.12mol S 1 mol S

2mol Cu = .24mol Cu needed

We have enough S

We do not have enough Cu

Your turnIf 10.1 g of magnesium and 4.7g of HCl are

reacted, which would be the limiting reactant?

Your Turn IIIf 10.3 g of aluminum are reacted with 51.7

g of CuSO4, which would be the limiting reactant?

Percent YieldMeasures the effectiveness of a chemical

reactionThere are three types:

Actual yield- what you get in the lab when the chemicals are mixed

Theoretical yield- what the balanced equation tells you you should make.

Percent yield = Actual x 100 TheoreticalThe closer to 100%, the more effective the

reaction is.Cannot exceed 100%- why?

Example6.78 g of copper is produced when 3.92 g of Al

are reacted with excess copper (II) sulfate.What is the actual yield? What is the theoretical yield?

What is the percent yield?

2Al + 3 CuSO4 Al2(SO4)3 + 3Cu

3.92 g Al 26.98g Al 1 mol Al

2 mol Al 3 mol Cu

1 mol Cu63.55 g Cu

= 13.9 g Cu

6.78 x 100 = 48.7%13.9

6.78 g

Your TurnMethanol, CH3OH, can be produced by the

reaction of carbon monoxide and hydrogen. If 75.0g of CO reacts to produce 68.4g CH3OH, what is the percent yield of CH3OH?

Your Turn IIWhen calcium carbonate, CaCO3, is heated, it

decomposes to form calcium oxide and carbon dioxide. When 2000.g of CaCO3 are heated, the actual yield of CaO is 1050.g. What is the percent yield?