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Understand Titration

In this worksheet, students will learn why titrations are useful and how to do them.

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. Titration is the way we can carefully add acid so that we don't add too much.

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). The questions will help you think about these ideas some more.

 

 

 

What kind of reaction do we study with titrations?

displacement reaction

neutralisation

thermal decomposition

What do we use to measure exactly 25 cm3 of alkali?

pipette

burette

measuring cylinder

Which of these phrases describe a burette? Tick all the right answers.

contains alkali

contains acid

tap at the bottom

held horizontally

What chemicals do we place in the conical flask at the start of the titration experiment?

acid

alkali

indicator

Select a good indicator for titration, and the reason why it is preferable.

acid

alkali

indicator

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

Column A

Column B

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

Before the start of the titration experiment, there is 46.5 cm3 acid in the burette. Once the neutralisation has reached its endpoint, there is 22.4 cm3 acid left in the burette. How much acid was needed to neutralise the alkali?

Column A

Column B

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

Why do we swirl the conical flask?

So the reaction goes faster.

So the alkali and indicator mix.

So the acid and alkali mix.

The final part of an accurate titration is different to a rough titration. What is the difference, and why is that difference used?

So the reaction goes faster.

So the alkali and indicator mix.

So the acid and alkali mix.

Here are some results from a titration experiment:

Run initial volume (cm3) Final volume (cm3) Volume added (cm3)
1 49.6 27.8 21.8
2 27.8 6.1  
3 48.9 28.6  
4 28.6 6.8  

 

The volume added in run 2 was 21.7 cm3

The volume added in run 3 was 21.7 cm3

The volume added in run 4 was 21.9 cm3

The volume added in run 4 was 21.8 cm3

The runs which agree are 2, 3 and 4

The runs which agree are 1, 2 and 4

  • Question 1

What kind of reaction do we study with titrations?

CORRECT ANSWER
neutralisation
EDDIE SAYS
Titration is about how much acid we need to neutralise an alkali.
  • Question 2

What do we use to measure exactly 25 cm3 of alkali?

CORRECT ANSWER
pipette
EDDIE SAYS
A pipette has a single measuring line on it, so we can only use it to measure one specific volume. It does this very accurately.
  • Question 3

Which of these phrases describe a burette? Tick all the right answers.

CORRECT ANSWER
contains acid
tap at the bottom
EDDIE SAYS
We put acid in the burette, and hold it vertically above the flask of alkali. The tap at the bottom lets us control the flow of acid into the flask.
  • Question 4

What chemicals do we place in the conical flask at the start of the titration experiment?

CORRECT ANSWER
alkali
indicator
EDDIE SAYS
We start with the alkali and indicator in the flask, and gradually add acid until neutralisation reaches its end point.
  • Question 5

Select a good indicator for titration, and the reason why it is preferable.

CORRECT ANSWER
EDDIE SAYS
You need to stop adding acid at the exact instant that neutralisation is complete. Because Universal Indicator changes colour gradually, we can't tell when the indicator is the exact shade of green to show neutral pH.
  • Question 6

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 7

Before the start of the titration experiment, there is 46.5 cm3 acid in the burette. Once the neutralisation has reached its endpoint, there is 22.4 cm3 acid left in the burette. How much acid was needed to neutralise the alkali?

CORRECT ANSWER
EDDIE SAYS
This is a subtraction problem; 46.5 - 22.4 = 24.1
  • Question 8

Why do we swirl the conical flask?

CORRECT ANSWER
So the acid and alkali mix.
EDDIE SAYS
The hydrogen (acid) and hydroxide (alkali) ions need to meet to react and neutralise. Swirling the solution makes the acid move throughout the alkali, so the all the acid reacts with all the alkali.
  • Question 9

The final part of an accurate titration is different to a rough titration. What is the difference, and why is that difference used?

CORRECT ANSWER
EDDIE SAYS
In an accurate titration, we add the final acid drop-by-drop, so that we know how much acid is needed to the nearest drop. We still add most of the acid more quickly, or the titration would take far too long.
  • Question 10

Here are some results from a titration experiment:

Run initial volume (cm3) Final volume (cm3) Volume added (cm3)
1 49.6 27.8 21.8
2 27.8 6.1  
3 48.9 28.6  
4 28.6 6.8  

 

CORRECT ANSWER
The volume added in run 2 was 21.7 cm3
The volume added in run 4 was 21.8 cm3
The runs which agree are 1, 2 and 4
EDDIE SAYS
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.
---- OR ----

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