Unit 2
DTG
Task 1
Name:
..............................................................
Titration – Finding the concentration of an alkali solution
Introduction
A titration involves a reaction between two solutions, one acid and one alkali. The concentration of
one solution can be calculated if that of the other is known.
Scenario
You are a laboratory technician in an analytical chemistry laboratory. In one of the laboratory’s
cupboards you find a bottle of a sodium hydroxide solution. The bottle’s label doesn’t have the exact
concentration of the solution. It states only “sodium hydroxide solution (alkali) thought to have a
concentration of between 0.05 and 0.15 mol dm−3”. Your task is to find the solution’s actual
concentration.
The following method describes what you should do in general terms. You are expected, however, to
carry out the method in ways that are safe and which are likely to ensure that your results are as
accurate and precise as possible.
Hazards
Sodium hydroxide solution, 0.05 and 0.15 mol dm−3
Irritant
Equipment
50 cm3 beaker (2)
sodium hydroxide solution, between 0.05 and
250 cm3 conical flask
0.15 mol dm−3 (150 cm3)
burette
hydrochloric acid acid, between 0.05 and 0.15
25 cm pipette and pipette filler
mol dm−3 (150 cm3)
white tile
phenolphthalein indicator solution
3
distilled water
Method
1
Always wear safety goggles
2
Rinse the burette with distilled water and hydrochloric acid
3
Fill the burette with hydrochloric acid (acid solutions usually go into the burette)
4
Check that the burette has no air bubbles
5
Draw up a suitable table of results that will allow you to record the initial burette reading, the
final burette reading and the difference of the two readings (titre) for this experiment and repeat
experiments.
P1
6
Record your initial burette reading.
7
Transfer about 100 cm3 of sodium hydroxide solution to a clean, dry 250 cm3 beaker. Use a
pipette and pipette filler to transfer 25.0 cm3 of this solution to a clean 250 cm3 conical flask.
8
Place a white tile beneath the conical flask
9
Add three drops of phenolphthalein indicator to show the colour of the alkaline solution
10
Add the acid solution from the burette, swirling the flask, until the indicator changes colour.
11
Record your final burette reading in your table of results
12
Calculate the actual volume of alkali used or ‘titre’ in this rough titration
13
This ‘rough’ titration allows you to find an approximate value for the volume of alkali needed to
M1
react with the hydrochloric acid solution. Repeat the titration, but this time add the acid drop by drop
when you get near to the end point. If in doubt about the endpoint, then take a reading and add two
more drops of the acid.
14
Rinse the conical flask well with distilled water before using it to repeat the titration. Repeat
the titration a number of times until you feel that you have achieved reliable results.
15
At the end of the practical, wash all the solutions in beakers/conical flasks down the drain.
Wash and rinse glassware and place on drying rack. Empty the burette of acid and wash it with
distilled water. Place the burette upside down in the clamp stand and leave the tap open to drain.
16
titres.
Select suitable titres to calculate the mean average titre. Explain why you have chosen these
M2
17
Calculate the mean average titre for the experiments. Show your calculation of the mean
average titre. Give your answer to two decimal places.
P2
Working out the alkali concentration
18
Convert the mean average titre volume from cm3 to dm3.
19
Calculate the number of moles of hydrochloric acid titrated using the following equation
M4
number of moles (mol) = concentration (mol/dm3) x volume (dm3)
Hint: Use your answer to 17.
20
P4
The word equation for the reaction between sodium hydroxide and hydrochloric acid is:
Sodium hydroxide + hydrochloric acid  sodium chloride + water
Try to write the chemical equation for this reaction.
21
The reacting ratio between sodium hydroxide and hydrochloric acid is 1:1. This means that the
number of moles of hydrochloric acid should be equal to the number of moles of sodium hydroxide.
Using this information you can work out the concentration of the sodium hydroxide.
a) Rearrange the following equation to make concentration the subject.
number of moles (mol) = concentration (mol/dm3) x volume (dm3)
b) Convert the volume of sodium hydroxide used (25 cm3) to dm3
M4
c) Calculate the concentration of the sodium hydroxide.
Hint: Use the number of moles calculated in 18, the volume of sodium hydroxide calculated in 20. b)
and substitute them into the equation that you have rearranged in 20. a)
D3
Evaluating your results and procedures
22
Comment on the limitations of the procedure, or the aspects of the procedures that are
important in ensuring the accuracy of your results.
D1