Titrations / Back Titrations

Question 1

What apparatus is used to measure volume in titrations?

Answer 1

Volumetric / graduated pipette

Burette.

Question 2

What are concordant results?

Answer 2

Results that are within +/- 0.1, and are used in calculations.

Any results beyond that are ignored.
Rough titrations are also ignored.

Question 3

The percentage by mass of calcium carbonate, CaCO3, in a sample of marble was determined by adding excess hydrochloric acid to ensure that all the calcium carbonate had reacted. The excess acid left was then titrated with aqueous sodium hydroxide. A student added 27.20 cm3 of 0.200 mol dm-3 HCl to 0.188 g of marble. The excess acid required 23.80 cm3 of 0.100 mol dm-3 NaOH for neutralization. Calculate the percentage of calcium carbonate in the marble.

Answer 3

1# Write the equation for the titration reaction:
HCl (aq) + NaOH (aq) → NaCl (aq) + H2O (l)

2# Calculate no. of moles of NaOH
n(NaOH) = 0.02380 dm3 x 0.100 mol dm-3 = 2.380 x 10-3 mol

3# Deduce the number of moles of the excess acid
Since the reacting ratio is 1:1 the number of moles of HCl is also 2.380 x 10-3 mol

4# Find amount of HCl in the original solution and then the amount reacted
n(HCl)original = 0.02720 dm3 x 0.200 mol dm-3 = 5.440 x 10-3 mol
n(HCl)reacted = 5.440 x 10-3 mol – 2.380 x 10-3 mol = 3.060 x 10-3 mol

5# Write the equation for the reaction with the calcium carbonate
2HCl (aq) + CaCO3 (s) → CaCl2 (aq) + CO2 (g) + H2O (l)

6# Deduce the number of moles of the calcium carbonate that reacted
Since the reacting ratio is 2:1 the number of moles of CaCO3 is (3.060 x 10-3 mol) ÷ 2
n(CaCO3) = 1.530 x 10-3 mol

7# Calculate the mass of calcium carbonate in the sample of marble
mass = moles x molar mass = 1.530 x 10-3 mol x 100.09 g mol-1 = 0.1531g

8# Calculate the percentage of calcium carbonate in the marble
% of CaCO3 in marble = (0.1531) / (0.188) x 100 =

81.5%

Question 4

If the acid used in the titration is sodium hydrogen sulfate

And the base used is sodium hydroxide

Then what will the pipette, burette and conical flask be rinsed with before it is used?

Answer 4

Pipette = sodium hydroxide

Burette = sodium hydrogen sulfate

Conical flask = deionisied / distilled water

Question 5

What is the formula for working out the percentage uncertainty?

Answer 5

% Uncertainty = (uncertainty) / (measured value) x 100

Question 6

What is the formula for working out the uncertainty?

Answer 6

Resolution / 2

Question 7

Name examples of indicators

Answer 7

Methyl orange

Phenolphthalein

Question 8

What is the indicator phenolphthalein only suitable for?

Answer 8

Weak acid - strong base titration

Question 9

What is the indicator methyl orange suitable for?

Answer 9

Strong acid - strong base
Strong acid - weak base

Basically strong acid all the way.
Phenolphthalein is only weak acid.

Question 10

Why is a white tile used in the titration?

Answer 10

To see a clear colour change of the solution from the indicator.

Question 11

What can be done to minimise any safety risks in this experiment?

Answer 11

Wear goggles - eye protection.

Lower the burette when pouring the solution - never fill it above your head.

Question 12

The student uses a funnel to fill the burette with sulfuric acid before starting the titration. After filling, the student forgets to remove the funnel from the top of the burette.

Suggest why this might affect the titre volume recorded. [1 mark]

Answer 12

Additional drops of solution could have entered the burette from the funnel…

making the value on the burette lower.

Question 13

State one advantage of using a conical flask rather than a beaker for the titration. [1 mark]

Answer 13

Less chance of losing/spilling any solution using a conical flask when swirling.

Question 14

The student used a wash bottle containing deionised / distilled water when approaching the end-point to rinse the inside of the conical flask.

Explain why this improved the accuracy of the titration [1 mark]

Answer 14

Some of the solution would be on the sides of the flask.

OR

Ensures all acid/alkali reactants are in the solution.

Question 15

Before adding the solution from the burette in the rough titration, there was an air bubble below the tap.

At the end of this titration the air bubble was not there.

Explain why this air bubble increases the final burette reading of the rough titration [1 mark]

Answer 15

Some solution replaces the air bubble.

OR

Solution fills below the tap.

OR

Air bubble takes up volume that would be filled by the solution.

Question 16

During the titration the student washed the inside of the conical flask with some distilled water.

Suggest why this washing does not give an incorrect result. [1 mark]

Answer 16

It does not change the number of moles of the solution.

OR

Does not react with the solution.

Question 17

Give the meaning of the term concordant titres.

Answer 17

Titres that are within 0.1cm³ of each other.