10.04.1 Percentage Yield

Syllabus

- Even though no atoms are gained or lost in a chemical reaction, it is not always possible to obtain the calculated amount of a product because:

1. the reaction may not go to completion because it is reversible
2. some of the product may be lost when it is separated from the reaction mixture
3. some of the reactants may react in ways different to the expected reaction.

- The amount of a product obtained is known as the yield.
- When compared with the maximum theoretical amount as a percentage, it is called the percentage yield.

% Yield = (Mass of product actually made / Maximum theoretical mass of product) × 100

- Students should be able to:

1. calculate the percentage yield of a product from the actual yield of a reaction

(HT only) calculate the theoretical mass of a product from a given mass of reactant and the balanced equation for the reaction.

What does this mean?

What does Yield mean?

It is all very well to work out how much "stuff" you can make in a reaction.

This assumes that you live in an ideal world where everything happens exactly as you planned it.

But in the real world things go wrong.

Substances evaporate, or get stuck in filters, or in glassware, or unexpected the side-reactions occur.

So, the amount of "stuff" we make is always less than the amount we should have been able to make.

We can measure how efficiently the reaction we're using makes what we're trying to make by using the formula:

%Yield = (Actual Yield/Theoretical Yield) x 100

In other words, (what you really made divided by what you should have made) x 100

The higher the better (although if it's greater than 100% you have done the calculation wrong.)

Example Questions

Example 1

NaOH(aq) + HCl (aq)--> NaCl (aq) + H2O (l)

If a student has 20 cm3 of Sodium Hydroxide (NaOH) with a concentration of 0.5 mol/dm3, she makes 0.008 mols of NaCl.

What is her percentage yield?

1.

Mols of NaOH used

Mols = Conc. x Vol/1000

= 0.5 x 20/1000

= 0.01

2.

Mols of NaCl expected

Ratio NaOH : NaCl

1 : 1

0.01 : 0.01

3.

Percentage yield

% yield = (actual / expected) x 100

= (0.008 / 0.01) x 100

= 80%

Example 2

Mg(as) + 2 HCl (aq)--> MgCl2 (aq) + H2O (l)

If a student has 60 cm3 of Hydrochloric acid (HCl) with a concentration of 0.4 mol/dm3, she makes 1.05 g of MgCl2.

What is his percentage yield?

1.

Mols of HCl used

= Conc. x Vol/1000

= 0.4 x 60/1000

= 0.024

2.

Mols of MgCl2 expected

Ratio HCl : MgCl2

2 : 1

0.024 : 0.012

3.

Mass of MgCl2 expected

Mols = Mass / Mr

Mass = Mols x Mr

= 0.012 x 95 =1.14g

4.