Concentration & Titration
How concentrated is a solution, and how do we measure it? Learn concentration in g/dm3 and mol/dm3, the cm3-to-dm3-to-moles chain, and the titration technique that finds an unknown concentration - with the calculations to match.
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How concentrated is it?
A concentrated squash has lots of solute in the water; a dilute one has little. Chemists need to say exactly how much - and to measure it accurately. This module covers concentration (in g/dm3 and mol/dm3), the calculations that link mass, moles and volume, and titration - the technique for finding an unknown concentration.
Concentration words
Four terms underpin the topic:
Match each piece of apparatus to its job
- Burette
- Pipette
- Conical flask
- Indicator
- Delivers a measured, variable volume of acid
- Measures a fixed volume of alkali accurately
- Holds the solution being titrated
- Shows the endpoint by changing colour
Units of concentration
Which of these is a unit of concentration?
- mol/dm3
- g (grams)
- cm3
- degrees Celsius
The concentration formula
Concentration links the amount of solute to the volume of solution: - concentration (g/dm3) = mass (g) / volume (dm3) - concentration (mol/dm3) = moles / volume (dm3) Watch the units: volume must be in dm3, and 1 dm3 = 1000 cm3, so divide a cm3 volume by 1000 first.
Convert the volume
A titration uses 250 cm3 of solution. Since 1 dm3 = 1000 cm3, how many dm3 is 250 cm3? (Divide by 1000.)
True of titration
Select the TWO statements that are true about carrying out a titration.
- An indicator shows when the reaction is complete, at the endpoint
- A burette measures the variable volume of acid added
- Titration is used to measure temperature
- You add all the acid at once, as fast as possible
Concentration in g/dm3
A solution contains 20 g of solute dissolved to make 0.5 dm3 of solution. What is its concentration in g/dm3? (Use concentration = mass / volume.)
Two ways to give concentration
The same solution can be described in grams or in moles per dm3.
Find the moles
A solution has a concentration of 2 mol/dm3 and a volume of 0.5 dm3. How many moles of solute does it contain? (Use moles = concentration x volume.)
Put it together
A _____ measures a fixed volume of alkali into a flask. Acid is added from a _____ until an _____ shows the _____. Concentration in mol/dm3 equals moles divided by the _____ in dm3.
The titration method
Put the steps of a titration into the correct order.
- Use a pipette to add a fixed volume of alkali to the flask
- Add a few drops of indicator
- Fill the burette with acid and record the start reading
- Add acid slowly, swirling, until the indicator just changes colour
- Record the final burette reading and work out the volume added
Run the titration
Three decisions during a titration. Pick the best choice each time.
- You are near the endpoint and the colour flickers when you add acid. What should you do?
- Why do you repeat the titration several times?
- You know the moles of acid and its volume in dm3. How do you find its concentration in mol/dm3?
The grade-9 chain
Top answers chain the calculation cleanly: convert the volume (cm3 to dm3 by dividing by 1000), find the moles if needed (mass / Mr), then divide by the volume for the concentration. Link the technique to the numbers: the pipette fixes the volume, the burette measures the acid added, the indicator marks the endpoint, and concordant repeats make the result reliable. Method plus calculation, with the right units - top band.