Acid base Titration
What is the colour change of the methyl orange during the
A student is given a sample of a carbonate, M2CO3, where M is
titration?
a metal.
The colour changes from ......................... to ......................... [1]
He is asked to determine the relative atomic mass of M.
The student does three titrations. The diagrams below show
(a) A sample of the carbonate is added to a previously weighed
parts of the burette with the liquid levels at the beginning and
beaker which is then reweighed.
end of each titration.
mass of beaker + M2CO3 = 7.69 g
mass of beaker = 5.99 g
Calculate the mass of M2CO3.
........................................................ g [1]
(b) The student transfers the sample of M2CO3 to a beaker and
adds 50.0 cm3 of 1.00 mol / dm3 hydrochloric acid (an excess).
A gas is produced.
Name the gas and describe a test for this gas.
gas .....................................................................
test.......................................................................................[2]
(c) When the reaction has finished, the solution in the beaker is
(d) Use the diagrams to complete the following table.
transferred to a volumetric flask and made up to 250 cm3 with
distilled water. This is solution G.
Using a pipette, 25.0 cm3 of G is transferred to a conical flask
and a few drops of methyl orange indicator are added.
A burette is filled with 0.100 mol / dm 3 sodium hydroxide.
Aqueous sodium hydroxide is run into the conical flask until an
Summary
end-point is reached.
Tick (✓) the best titration results.
1
Using these results, the average volume of 0.100 mol / dm3
sodium hydroxide is
.................................................moles [1]
(h) Calculate the number of moles of hydrochloric acid
contained in the original 50.0 cm3 of 1.00 mol / dm3 hydrochloric
acid.
................................................... cm3. [4]
(e) Calculate the number of moles of sodium hydroxide in the
average volume of 0.100 mol / dm3 sodium hydroxide in (d).
.................................................moles [1]
(i) By subtracting your answer in (g) from your answer in (h),
calculate the number of moles of hydrochloric acid that reacts
with the sample of M2CO3.
.................................................moles [1]
.................................................moles [1]
(j) Using the equation, calculate the number of moles of M 2CO3
(f) Using the equation, calculate the number of moles of
that reacts with the number of moles of hydrochloric acid in
hydrochloric acid in 25.0 cm3 of G.
your answer in (i).
M2CO3 + 2HCl → 2MCl + CO2 + H2O
.................................................moles [1]
.................................................moles [1]
(g) Calculate the number of moles of hydrochloric acid in
250cm3 of G.
(k) Using your answers in (a) and (j) calculate the relative
formula mass of M2CO3 and hence the relative atomic mass of
M. [Ar: C, 12; O, 16]
2
relative formula mass of M2CO3 ..................................................
burette and added to the solution in the conical flask until an
relative atomic mass of M ...................................................... [2]
end-point is reached.
[Total: 16]
Phenolphthalein is colourless in acidic solution and pink in
alkaline solution.
8 A student is given a sample of an organic acid, G, and asked
What is the colour of the solution in the conical flask
to
(i) before the acid is added
• determine its relative molecular mass
...........................................................................................
• suggest its formula.
(ii) at the end-point?
(a) A sample of the acid is placed in a previously weighed
................................................................................................ [1]
container and reweighed.
(c) The student does three titrations. The diagrams below show
parts of the burette with the liquid levels at the beginning and
end of each titration.
Calculate the mass of G used in the experiment.
.............................................. g [1]
(b) The student transfers the sample to a beaker and adds 50.0
cm3 of 1.00 mol / dm3 sodium hydroxide, an excess. The
contents of the beaker are allowed to react and then transferred
to a volumetric flask. The solution is made up to 250 cm 3 with
distilled water.
This is solution H.
25.0 cm3 of H is transferred into a conical flask.
Use the diagrams to complete the following table.
A few drops of phenolphthalein indicator are added to the
conical flask. 0.100 mol / dm3 hydrochloric acid is put into a
3
(g) Using your answer from (f) calculate the number of moles of
sodium hydroxide in 250 cm3 of H.
...................................... moles [1]
(h) Calculate the number of moles of sodium hydroxide in 50.0
cm3 of 1.00 mol / dm3 sodium hydroxide.
Summary
Tick (✓) the best titration results.
Using these results, the average volume of 0.100 mol / dm3
hydrochloric acid is
...................................... moles [1]
(i) By subtracting your answer in (g) from your answer in (h),
.......................................... cm3 [4]
(d) Calculate the number of moles of hydrochloric acid in the
calculate the number of moles of sodium hydroxide that reacts
with the original sample of the organic acid, G.
average volume of 0.100 mol / dm3 hydrochloric acid from (c).
...................................... moles [1]
(e) Construct the equation for the reaction between hydrochloric
acid and sodium hydroxide.
...................................... moles [1]
................................................................................................ [1]
(j) One mole of G reacts with two moles of sodium hydroxide.
(f) Using your equation and the answer from (d), deduce the
Deduce the number of moles of G in the sample.
number of moles of sodium hydroxide present in 25.0
cm3 of
H.
...................................... moles [1]
4
...................................... moles [1]
[Total: 16]
(k) Using your answers from (a) and (j) calculate the relative
molecular mass of the acid G.
.................................................. [1]
(l) The acid G contains two carboxylic acid groups and has the
formula
HO2CCxHyCO2H
where x and y are whole numbers.
Deduce the values of x and y in the formula.
[Ar: H, 1; C, 12; O, 16]
x .................................................
y ................................................. [2]
5