12C A-B1 homework due 30/9/16
1.
/28
%
NAME:
Sodium hydrogensulfate, NaHSO4, reacts with sodium hydroxide, NaOH, as shown below.
NaHSO4(aq) + NaOH(aq) → Na2SO4(aq) + H2O(l)
0.0100 mol of sodium hydrogensulfate is neutralized with dilute sodium hydroxide,
concentration 0.200 mol dm–3.
Calculate the volume of sodium hydroxide required.
A
20.0 cm3
B
50.0 cm3
C
100 cm3
D
500 cm3
(Total 1 mark)
2.
25.0 cm3 of a solution contains 0.020 mol of ethanoic acid.
Calculate its concentration in mol dm–3.
(Total 1 mark)
The Westlands School
1
12C A-B1 homework due 30/9/16
3.
/28
%
NAME:
A student carried out an experiment to determine the concentration of ethanoic acid in a solution
of vinegar.
•
The student used a measuring cylinder to measure out 25.0 cm3 of the vinegar solution.
•
This solution was then transferred to a 250 cm3 volumetric flask and the liquid level was
carefully made up to the mark with distilled water.
•
A pipette was used to transfer 25.0 cm3 portions of the acidic solution to conical flasks.
•
The solution was then titrated with sodium hydroxide solution, concentration 0.100 mol
dm–3, using phenolphthalein as the indicator.
CH3COOH(aq) + NaOH(aq) → CH3COONa(aq) + H2O(l)
Results
Titration number
1
2
3
4
Burette reading (fi nal) / cm3
28.55
28.00
40.35
28.05
Burette reading (initial) / cm3
0.00
0.05
12.30
0.05
Volume of NaOH used / cm3
28.55
27.95
28.05
28.00
(a)
In this titration, what is the colour change of the phenolphthalein indicator?
From .................................................. to ..................................................
(2)
(b)
Explain why the mean titre should be based only on titrations 2, 3 and 4.
........................................................................................................................
........................................................................................................................
........................................................................................................................
(1)
(c)
Calculate the mean titre in cm3.
(1)
The Westlands School
2
12C A-B1 homework due 30/9/16
(d)
(i)
/28
%
NAME:
Using your answer to (c), calculate the number of moles of sodium hydroxide in
the mean titre.
(1)
(ii)
Hence state the number of moles of ethanoic acid, CH3COOH, in 25.0 cm3 of the
diluted solution used in the titration.
(1)
(iii)
Calculate the concentration of the diluted acid solution in mol dm–3.
(1)
(iv)
Hence calculate the concentration of the ethanoic acid in the original vinegar
solution in mol dm–3.
(1)
(v)
Use your answer from (d)(iv) to state the concentration of the ethanoic acid in the
original vinegar solution in units of g dm–3.
[The molar mass of the ethanoic acid is 60 g mol–1.]
(1)
(e)
Suggest, with a reason, how the student’s method of preparing the diluted solution could
be improved.
Improvement
.....................................................................................................................................
.....................................................................................................................................
Reason
.....................................................................................................................................
.....................................................................................................................................
(2)
The Westlands School
3
12C A-B1 homework due 30/9/16
(f)
/28
%
NAME:
The burette used in the titration had an uncertainty for each reading of  0.05 cm3.
(i)
Identify, by letter, which ONE of the following should be regarded as the true
value of the titre in titration number 2?
X
Between 27.90 and 28.00 cm3
Y
Between 27.925 and 27.975 cm3
Z
Between 27.85 and 28.05 cm3
...........................................................................................................................
(1)
(ii)
Suggest ONE reason why a student may obtain volumes outside the uncertainty of
the burette when performing a titration.
...........................................................................................................................
...........................................................................................................................
(1)
(Total 13 marks)
4.
(a)
In an experiment to standardise an aqueous solution of sodium hydroxide 0.25 g of solid
sulphamic acid, NH2SO3H, was dissolved in distilled water in a conical flask. When the
aqueous sodium hydroxide was run into the flask from a burette 23.45 cm3 was required
to exactly react with the sulphamic acid solution. The equation for the reaction is:
NH2SO3H(aq) + NaOH(aq)  NH2SO3Na(aq) + H2O(l)
(i)
Calculate the amount (number of moles) of sulphamic acid in 0.25 g.
Mr(NH2SO3H) = 97.0.
(1)
The Westlands School
4
12C A-B1 homework due 30/9/16
(ii)
/28
%
NAME:
State the amount (number of moles) of sodium hydroxide in 23.45 cm3 of solution
and hence calculate the concentration of the solution in mol dm–3.
(3)
(b)
The balance used to weigh the sulphamic acid is accurate to ±0.01 g. Calculate the
percentage error in the mass of the sulphamic acid weighed.
(1)
(c)
An alternative method to that described in (a) involves making an aqueous solution of
sulphamic acid of accurately known concentration.
Describe a procedure bv which you would prepare 250 cm3 of aqueous sulphamic acid of
accurately known concentration. Assume that you are provided with a weighing bottle
containing between 2.40 g and 2.50 g of sulphamic acid and that this is a suitable mass to
use.
In your answer give full practical details including the name of each piece of apparatus
used, how each would be prepared for the procedure and how you would calculate the
concentration (in mol dm–3) of the sulphamic acid solution. State, with a reason, one
appropriate safety precaution that should be taken.
....................................................................................................................................
....................................................................................................................................
....................................................................................................................................
(Allow one lined page)
(8)
(Total 13 marks)
The Westlands School
5