Chemistry Club Vol 2/2 27/9/01 11:13 am Page 48
P U P I L S ’
W O R K S H E E T
26: Find the Combination of the Safe
The gold is locked away in a safe. The concentrations of four solutions hold the
secret that will undo the lock. Use your skill at titration to find the letters and
numbers of the combination. Work very accurately, you are ‘going for gold’.
What you will need
eye protection
must be worn
25 cm3 solution A
25 cm3 solution D
25 cm3 solution E (irritant)
25 cm3 solution L (irritant)
100 cm3 1 mol dm-3 hydrochloric acid
phenolphthalein indicator solution
(highly flammable)
eye protection
IRRITANT
solution E
solution L
2 x 25 cm3 measuring cylinders
250 cm3 conical flask
burette
plastic funnel for filling burette
clamp and stand
100 cm3 beaker
What you do
1
Measure out 25 cm3 of solution A using a measuring cylinder. Be careful,
this, and the other solutions may be corrosive.
HIGHLY
FLAMMABLE
phenolphthalein
indicator solution
2 Pour the measured solution A into the clean conical flask, then wash the
measuring cylinder well with water.
3
Add 3 drops of phenolphthalein to solution A. It should turn pink.
4 Use the plastic funnel to fill the burette with hydrochloric acid solution just
beyond the zero mark. (Your teacher will help you do this.) With a small beaker
under the tap of the burette, open the tap until the liquid is level with the zero
mark. Pour the acid in the beaker down the sink and wash the beaker well with
water.
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Chemistry Club Vol 2/2 27/9/01 11:13 am Page 49
P U P I L S ’
W O R K S H E E T
26: Find the Combination of the Safe (contd)
5
Place the conical flask containing solution A under the burette. Carefully and
slowly, let the acid out of the burette, about 1 cm3 at a time. Swirl the flask
gently after each addition.
6
Stop adding the acid when the liquid in the flask has JUST turned colourless.
7
Read off the level of the acid in the burette. Record this value in the table below.
This is the amount of acid needed to neutralise 25 cm3 of solution A.
Solution
Volume of acid needed to neutralise solution / cm3
eye protection
must be worn
A
D
IRRITANT
E
solution E
solution L
L
Total volume of acid used
8
9
Empty the conical flask by pouring the liquid down the sink. Wash the flask well
with water.
Repeat the experiment with each of the other solutions in turn (D, E and L).
Enter your results in the table.
HIGHLY
FLAMMABLE
phenolphthalein
indicator solution
10 In the first four boxes below, put the letters that correspond to the volume of
acid used in increasing order. Start with the letter for the smallest volume.
11 How much acid did you use altogether? Enter the value (to the nearest cm3) in
the fifth box. You should now have the combination of the safe.
Eye protection must be worn.
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