Percentage Yield

Question 1

In a lot of reactions, not all reactants react to form products which can be due to several factors:

Answer 1

Other reactions take place simultaneously

The reaction does not go to completion

Reactants or products are lost to the atmosphere

Question 2

The percentage yield shows

Answer 2

how much of a particular product you get from the reactants compared to the maximum theoretical amount that you can get:

Question 3

percentage yield equation

Answer 3

percentage yield = actual yield / predicted yield x 100

The actual yield is the number of moles or mass of product obtained experimentally

The predicted yield is the number of moles or mass obtained by calculation

Question 4

You will often have to use the following equation to work out the reacting masses, to calculate the predicted yield

the moles, mass and molar mass (mr equation)

Answer 4

moles x mr = mass

mass= grams
mr= g/mole -1

Question 5

It is important to be clear about the type of particle you are referring to when dealing with moles

Eg. 1 mole of CaF2 contains one mole of CaF2 formula units, but one mole of Ca2+ and two moles of F- ions

Question 6

In an experiment to displace copper from copper sulfate, 6.5 g of zinc was added to an excess of copper (II) sulfate solution.

The copper was filtered off, washed and dried.

The mass of copper obtained was 4.8 g.

Calculate the percentage yield of copper

Answer 6

Step 1: The symbol equation is:

Zn (s) + CuSO4 (aq) → ZnSO4 (aq) + Cu (s)

Step 2: Calculate the amount of zinc reacted in moles

Mole Calculations Worked Example 2 equation 1

Step 3: Calculate the maximum amount of copper that could be formed from the molar ratio:

Since the ratio of Zn(s) to Cu(s) is 1:1 a maximum of 0.10 moles can be produced

Step 4: Calculate the maximum mass of copper that could be formed (theoretical yield)

mass = mol x Mr

mass = 0.10 mol x 64 g mol-1

mass = 6.4 g
Step 5: Calculate the percentage yield of copper

Question 7

Limiting & Excess reagents
Sometimes, there is an excess of one or more of the reactants (excess reagent)

The reactant which is not in excess is called the limiting reagent
To determine which reactant is limiting:

Answer 7

The number of moles of each reactant should be calculated

The ratio of the reactants shown in the equation should be taken into account e.g.

Question 8

2Na + S → Na2S

what is the ratio

Answer 8

Here, the ratio of Na : S is 2 : 1, and this should be taken into account when doing calculations

Once all of one reactant has been used up, the reaction will stop, even if there are moles of the other reactant(s) leftover

The reactant leftover is in excess, the reactant which causes the reaction to stop once it is used up is the limiting reagent

Question 9

PRACTICE QUESTIONS