How do we measure concentration, find an unknown concentration by titration, and work out percentage yield?
Concentration in g per dm cubed and mol per dm cubed, using titration results to find an unknown concentration, and calculating percentage yield.
A focused CCEA GCSE Double Award Science (Chemistry Unit C2) answer on quantitative chemistry, covering concentration in g per dm cubed and mol per dm cubed, using titration results to find an unknown concentration, and calculating percentage yield.
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What this dot point is asking
CCEA Double Award wants concentration in two units, how to find an unknown concentration from a titration, and how to calculate percentage yield. These are pure calculation marks, so practise the steps until they are automatic.
Concentration
Remember the volume conversion: 1000 cm cubed = 1 dm cubed, so divide a volume in cm cubed by 1000 to get dm cubed.
Titration
The method: use a pipette to measure a known volume of alkali into a conical flask and add a few drops of an indicator; fill a burette with the acid and record the start reading; add the acid slowly, swirling, until the indicator just changes colour (the end point); record the final reading and work out the volume of acid added. Repeat until you get concordant (close) results and take a mean of those. From the volumes and one known concentration, you can calculate the unknown concentration.
A titration is much more accurate than judging a colour change by eye in a single rough run, which is why a rough titration is done first and then careful repeats. Using a pipette and burette, which measure to a small fraction of a cm cubed, is what makes the result reliable enough to find an unknown concentration.
Percentage yield
The theoretical yield is the maximum mass that could be made from the amounts of reactants. The actual yield is what you really get, which is usually less.
Examples in context
Example 1. Checking the strength of vinegar. A titration of vinegar against a known alkali finds the concentration of the acetic acid, so manufacturers can check the product is the right strength. This is titration used for quality control.
Example 2. Why industry cares about yield. A reaction with a low percentage yield wastes reactants and money, so chemists try to improve yields by reducing losses and choosing better conditions. The percentage yield is a direct measure of how efficient the process is.
Try this
Q1. How many dm cubed is 250 cm cubed? [1 mark]
- Cue. 0.25 dm cubed.
Q2. A reaction gives an actual yield of 9 g from a theoretical 12 g. What is the percentage yield? [1 mark]
- Cue. 9 divided by 12, times 100 = 75 percent.
Exam-style practice questions
Practice questions written in the style of CCEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
CCEA-style3 marksA solution contains 20 g of sodium hydroxide in 500 cm cubed of solution. Calculate its concentration in g per dm cubed.Show worked answer →
Convert the volume and divide for three marks.
500 cm cubed is 0.5 dm cubed (divide by 1000).
Concentration equals mass divided by volume in dm cubed: 20 divided by 0.5.
That gives 40 g per dm cubed. Markers reward converting the volume to dm cubed and the correct concentration.
CCEA-style4 marksIn a reaction the expected (theoretical) mass of product was 8 g, but only 6 g was obtained. Calculate the percentage yield and suggest one reason it is below 100 percent.Show worked answer →
Calculate and explain for four marks.
Percentage yield equals actual mass divided by theoretical mass, times 100.
That is 6 divided by 8, times 100, which equals 75 percent.
A reason for less than 100 percent is that some product was lost in transferring or filtering, or the reaction did not fully complete, or there were side reactions. Markers reward the correct calculation and a sensible reason.
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Sources & how we know this
- CCEA GCSE Science Double Award specification — CCEA (2017)