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Understand Titration

In this worksheet, students will learn why titrations are useful, how to do them and how to use the results of titration to calculate the concentration of an unknown solution.

Worksheet Overview

QUESTION 1 of 10

Suppose we have some alkali which we need to neutralise. We want to add exactly the right amount of acid; not too much or not too little. This might be because we need to get rid of the alkali, or it might be because the neutralisation reaction makes a useful salt. We can also use titration to work out the concentration of an acid or alkali which isn't known.

This is the way to do a trial titration:

1.  Wear protective glasses all the time when doing the experiment.

2.  Use a measuring pipette to measure 25 cm3 of alkali and put the alkali in a conical flask.

3.  Add a few drops of indicator to the alkali. A good indicator to use is phenolphthalein. This is pink in alkali, and colourless in acid. Since it only has two colours, it changes sharply, so we know when neutralisation has happened.

4. Fill a burette with acid. A burette is a long cylinder with a tap at the bottom, and a measuring scale along it. Clamp the burette above the conical flask, and record the volume of acid in the burette at the start of the experiment. The setup should look like this:

5. Open the tap gently so that acid slowly goes into the flask. Swirl the flask as this happens, so that the solution mixes and reacts fully.

6.  The indicator will change colour suddenly. This is called the endpoint, because it is when the alkali is completely neutralised. When this happens, close the tap. Record the amount of acid left in the burette.

7.  Subtract the final volume of acid from the initial volume of acid. This difference tells you how much acid was needed to neutralise the alkali.

Once you have done a trial titration, you should do several accurate titrations. This is how to do an accurate titration.

Steps 1 to 5 are the same as before. Once you have added 1 cm3 less acid than the rough titration needed, stop.

6.  Add the rest of the acid, very slowly- drop by drop, swirling all the time. Just before you reach the end point, you will see pink and colourless regions in the flask at the same time, a bit like in this beaker:

7.  When the solution in the flask is all colourless, and stays colourless after swirling, stop adding acid. Record the amount of acid left in the burette.

8.  Subtract the final volume of acid from the initial volume of acid. This difference tells you how much acid was needed to neutralise the alkali.

9.  Repeat the accurate titration until you have three results which agree to within 0.1 cm3.

The titration method has lots of steps- the key to learning them is to remember what we are trying to do (how much acid does it take to neutralise this alkali?), that we need to measure this very accurately and precisely (so we add the acid very slowly), and that we need to be safe (so we wear goggles, and put the alkali in the flask). 

Using titration to work out an unknown concentration

Concentration depends on the amount of chemical dissolved and the volume of liquid it is dissolved in.

Amount of moles of acid (or alkali) = Concentration of acid (or alkali) x Volume of water.

Suppose we have some hydrochloric acid, with concentration 0.05 moles per litre, and some sodium hydroxide, whose concentration isn't known. By doing a titration, we can measure that 25.0 cm3 sodium hydroxide is neutralised by 19.6 cm3 hydrochloric acid. The equation of the reaction is NaOH (aq) + HCl (aq) → NaCl (aq) + H2O (l).

The amount of acid is (19.6 / 1000) x 0.05 = 0.000 98 mol. This exactly neutralises the alkali, so there must be 0.000 98 mol alkali as well. That amount of alkali is in 25.0 cm3 water, so

concentration of alkali = 0.000 98 ÷ (25.0 / 1000) = 0.0392 moles per litre.

The acid has a higher concentration than the alkali, which is why the volume of alkali is larger than that of the acid.

Both the titration method and the concentration calculation have got several steps. Remember to inculde all the steps in your thinking; if you try to jump over any, the methods won't work.

 

 

 

Which of these sentences about phenolphthalein are true?

Phenolphthalein is pink in acidic conditions

Phenolphthalein is pink in alkaline conditions

Phenolphthalein changes colour suddenly.

We use phenolphthalein when Universal Indicator is unavailable

Phenolphthalein is safer to use than Universal Indicator

Sort these steps for a rough titration into the right order.

Column A

Column B

Step 1
Pipette 25 cm3 alkali into a conical fl...
Step 2
Record the volume of acid left in the burette, cal...
Step 3
Put on safety glasses.
Step 4
Gradually add acid to the flask, until the indicat...
Step 5
Fill a burette with acid. Note the volume of acid.

Match these aspects of the titration experiment with their reasons.

Column A

Column B

We swirl the flask,
so that the acid and alkali mix well.
We add the last cm3 of acid drop-by-dro...
so that the volume of acid is very precise.
We repeat the experiment,
so we know the result is reliable.

Here are some results from a titration experiment:

Run initial volume (cm3) Final volume (cm3) Volume added (cm3)
1 49.6 27.8  
2 27.8 6.1  
3 48.9 28.6  
4 28.6 6.8  

 

Column A

Column B

We swirl the flask,
so that the acid and alkali mix well.
We add the last cm3 of acid drop-by-dro...
so that the volume of acid is very precise.
We repeat the experiment,
so we know the result is reliable.

Match these pieces of apparatus and chemicals with their purpose.

Column A

Column B

Pipette
Measurement of 25 cm3 alkali
Burette
Indicating whether a chemical is acidic or alkalin...
Phenolphthalein
Addition and measurement of acid

If we have 22.5 cm3 of acid with concentration 0.1 mol per litre, what is the amount of acid? Enter your answer as a decimal, not in standard form.

Column A

Column B

Pipette
Measurement of 25 cm3 alkali
Burette
Indicating whether a chemical is acidic or alkalin...
Phenolphthalein
Addition and measurement of acid

We have 25.0 cm3 of sodium hydroxide, of unknown concentration. In the last question, you worked out that this was neutralised by 0.00225 mol of hydrochloric acid. What is the concentration of the sodium hydroxide?

The reaction is HCl (aq) + NaOH (aq) → NaCl (aq) + H2O (l).

Column A

Column B

Pipette
Measurement of 25 cm3 alkali
Burette
Indicating whether a chemical is acidic or alkalin...
Phenolphthalein
Addition and measurement of acid

We have 25.0 cm3 of potassium hydroxide, of unknown concentration. By titration, we know that we need 28.4 cm3 of hydrochloric acid with concentration 0.25 mol per litre to neutralise this. What is the concentration of the postassium hydroxide?

The reaction is HCl (aq) + KOH (aq) → KCl (aq) + H2O (l).

Column A

Column B

Pipette
Measurement of 25 cm3 alkali
Burette
Indicating whether a chemical is acidic or alkalin...
Phenolphthalein
Addition and measurement of acid

Imagine doing a titration to neutralise lithium hydroxide with hydrochloric acid.

We have 25.0 cm3 LiOH, with unknown concentration.

When we neutralise this with 0.1 moles per litre hydrochloric acid, the burette readings are 39.8 cm3 at the start, and 12.2 cm3 at the end point.

How many moles of HCl were added to the alkali? Enter your answer as a decimal, not in standard form.

Column A

Column B

Pipette
Measurement of 25 cm3 alkali
Burette
Indicating whether a chemical is acidic or alkalin...
Phenolphthalein
Addition and measurement of acid

Imagine doing a titration to neutralise lithium hydroxide with hydrochloric acid.

We have 25.0 cm3 LiOH, with unknown concentration.

When we neutralise this with 0.1 moles per litre hydrochloric acid, the burette readings are 39.8 cm3 at the start, and 12.2 cm3 at the end point.

In the last question, you showed this was 0.00276 moles.

The reaction equation is HCl (aq) + LiOH (aq) → LiCl (aq) + H2O (l)

What is the concentration of the lithium hydroxide? Give your answer to 3 significant figures.

Column A

Column B

Pipette
Measurement of 25 cm3 alkali
Burette
Indicating whether a chemical is acidic or alkalin...
Phenolphthalein
Addition and measurement of acid
  • Question 1

Which of these sentences about phenolphthalein are true?

CORRECT ANSWER
Phenolphthalein is pink in alkaline conditions
Phenolphthalein changes colour suddenly.
EDDIE SAYS
You need to stop adding acid at the exact instant that neutralisation is complete. The fact that phenolphthalein changes colour suddenly at a specific pH value means that it is more useful as an indicator in titration.
  • Question 2

Sort these steps for a rough titration into the right order.

CORRECT ANSWER

Column A

Column B

Step 1
Put on safety glasses.
Step 2
Pipette 25 cm3 alkali ...
Step 3
Fill a burette with acid. Note th...
Step 4
Gradually add acid to the flask, ...
Step 5
Record the volume of acid left in...
EDDIE SAYS
With the key experiments in GCSE science, you need to know the sequence of steps. Imagine yourself in a lab; think what has to be done first, what can only be done later?
  • Question 3

Match these aspects of the titration experiment with their reasons.

CORRECT ANSWER

Column A

Column B

We swirl the flask,
so that the acid and alkali mix w...
We add the last cm3 of...
so that the volume of acid is ver...
We repeat the experiment,
so we know the result is reliable...
EDDIE SAYS
Swirling the solution makes the acid move throughout the alkali, so the all the acid reacts with all the alkali. Results are accurate if they answer is reported in very small units, and a result is reliable when we are confident that the result is correct.
  • Question 4

Here are some results from a titration experiment:

Run initial volume (cm3) Final volume (cm3) Volume added (cm3)
1 49.6 27.8  
2 27.8 6.1  
3 48.9 28.6  
4 28.6 6.8  

 

CORRECT ANSWER
EDDIE SAYS
For run 1, 49.6 - 27.8 = 21.8 For run 2, 27.8 - 6.1 = 21.7 For run 3, 48.9 - 28.6 = 20.3 For run 4, 28.6 - 6.8 = 21.8 Runs 1, 2 and 4 agree to within 0.1 cm3, but run 3 is significantly different.
  • Question 5

Match these pieces of apparatus and chemicals with their purpose.

CORRECT ANSWER

Column A

Column B

Pipette
Measurement of 25 cm3 ...
Burette
Addition and measurement of acid
Phenolphthalein
Indicating whether a chemical is ...
EDDIE SAYS
The names "pipette" and "burette" sound quite similar, so take care not to mix them up.
  • Question 6

If we have 22.5 cm3 of acid with concentration 0.1 mol per litre, what is the amount of acid? Enter your answer as a decimal, not in standard form.

CORRECT ANSWER
EDDIE SAYS
This calculation uses amount = concentration x volume. Remember that you need the volume in litres, so we have to divide 22.5 cm3 by 1000. If you got 2.25, it's because you missed out that step.
  • Question 7

We have 25.0 cm3 of sodium hydroxide, of unknown concentration. In the last question, you worked out that this was neutralised by 0.00225 mol of hydrochloric acid. What is the concentration of the sodium hydroxide?

The reaction is HCl (aq) + NaOH (aq) → NaCl (aq) + H2O (l).

CORRECT ANSWER
EDDIE SAYS
There is 0.00225 mol hydrochloric acid, so there must be 0.00225 mol sodium hydroxide. That is diluted in 25.0 cm3 water, so the concentration is 0.00225 ÷ (25.0 / 1000) = 0.09 mol per litre.
  • Question 8

We have 25.0 cm3 of potassium hydroxide, of unknown concentration. By titration, we know that we need 28.4 cm3 of hydrochloric acid with concentration 0.25 mol per litre to neutralise this. What is the concentration of the postassium hydroxide?

The reaction is HCl (aq) + KOH (aq) → KCl (aq) + H2O (l).

CORRECT ANSWER
EDDIE SAYS
There is 0.0071 mol hydrochloric acid, so there must be 0.0071 mol potassium hydroxide. That is diluted in 25.0 cm3 water, so the concentration is 0.0071 ÷ (25.0 / 1000) = 0.284 mol per litre.
  • Question 9

Imagine doing a titration to neutralise lithium hydroxide with hydrochloric acid.

We have 25.0 cm3 LiOH, with unknown concentration.

When we neutralise this with 0.1 moles per litre hydrochloric acid, the burette readings are 39.8 cm3 at the start, and 12.2 cm3 at the end point.

How many moles of HCl were added to the alkali? Enter your answer as a decimal, not in standard form.

CORRECT ANSWER
EDDIE SAYS
The volume of acid added was 39.8 - 12.2 = 27.6 cm3. That means that the amount of HCl was 0.1 x (27.6 / 1000) = 0.00276 mol.
  • Question 10

Imagine doing a titration to neutralise lithium hydroxide with hydrochloric acid.

We have 25.0 cm3 LiOH, with unknown concentration.

When we neutralise this with 0.1 moles per litre hydrochloric acid, the burette readings are 39.8 cm3 at the start, and 12.2 cm3 at the end point.

In the last question, you showed this was 0.00276 moles.

The reaction equation is HCl (aq) + LiOH (aq) → LiCl (aq) + H2O (l)

What is the concentration of the lithium hydroxide? Give your answer to 3 significant figures.

CORRECT ANSWER
EDDIE SAYS
There are 0.00276 moles of HCl, so there must be 0.00276 moles of LiOH. This is in 25 cm3 water, so the concentration is 0.00276 ÷ (25.0 / 1000) = 0.1104, which is 0.110 to 3 significant figures. (The zero at the end is the third significant figure, so we leave it in).
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