Acids, alkalis and titrations

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54

Question 54

A student prepares a solution of the soluble salt potassium sulfate, using the neutralization reaction between sulfuric acid and the alkali potassium hydroxide.

To make sure that she obtains a pure solution of the salt, she first carries out a titration to find the volumes of sulfuric acid and potassium hydroxide that react exactly together.

She uses this method in the titration:

use a pipette to add $cm325.0\text{ cm}^3$ of dilute sulfuric acid to a conical flask
add a few drops of phenolphthalein indicator
fill a burette with potassium hydroxide solution
add the potassium hydroxide solution from the burette until the indicator just changes colour

The diagram shows the burette readings in the experiment before and after adding the potassium hydroxide solution.

Two vertical diagrams showing a portion of a burette scale.

Use these readings to complete the following:

Burette reading in $cm3\text{cm}^3$ after adding alkali
Burette reading in $cm3\text{cm}^3$ before adding alkali
Volume in $cm3\text{cm}^3$ of alkali added

[3]

The student repeats the experiment using a different concentration of sulfuric acid. The table shows her results.

	Titration 1	Titration 2	Titration 3	Titration 4
Burette reading in $cm3\text{cm}^3$ after adding alkali	$24.65$	$24.70$	$24.50$	$25.15$
Burette reading in $cm3\text{cm}^3$ before adding alkali	$0.50$	$1.20$	$0.65$	$1.40$
Volume in $cm3\text{cm}^3$ of alkali added	$24.15$	$23.50$	$23.85$	$23.75$

The average (mean) volume of alkali should be calculated using only concordant results. Concordant results are those volumes that differ from each other by $cm30.20\text{ cm}^3$ or less.

(i) Identify which titration results should be used by selecting the appropriate columns. (ii) Use the appropriate titration results to calculate the average (mean) volume of alkali added.

[3]

In a titration using solutions of the same acid and alkali but of different concentrations, she recorded these results:

volume of sulfuric acid = $cm325.0\text{ cm}^3$
concentration of sulfuric acid = $mol/dm30.120\text{ mol/dm}^3$
average (mean) volume of potassium hydroxide solution = $cm324.00\text{ cm}^3$

The equation for the reaction is:

2\text{KOH} + \text{H}_2\text{SO}_4 \rightarrow \text{K}_2\text{SO}_4 + 2\text{H}_2\text{O}

(i) Calculate the amount, in moles, of $H2SO4\text{H}_2\text{SO}_4$ in $cm325.0\text{ cm}^3$ of $mol/dm30.120\text{ mol/dm}^3$ sulfuric acid. (ii) Calculate the amount, in moles, of $KOH\text{KOH}$ in the $cm324.00\text{ cm}^3$ of potassium hydroxide solution. (iii) Calculate the concentration, in $mol/dm3\text{mol/dm}^3$ , of $KOH\text{KOH}$ in the potassium hydroxide solution.

[4]

To prepare the solution of potassium sulfate, the student mixes together the volumes of acid and alkali obtained from the titration results. She then tests a sample of the potassium sulfate solution formed by adding a few drops of barium chloride solution (acidified with hydrochloric acid).

(i) Describe the observation she makes. (ii) State the name of the substance responsible for this observation.

[2]

Titration 1

Titration 2

Titration 3

Titration 4

Burette reading in

cm3\text{cm}^3

after adding alkali

24.65

24.70

24.50

25.15

Burette reading in

cm3\text{cm}^3

before adding alkali

0.50

1.20

0.65

1.40

Volume in

cm3\text{cm}^3

of alkali added

24.15

23.50

23.85

23.75

Acids, alkalis and titrations

Question 54

Acids, alkalis and titrations Questions

Acids, alkalis and titrations

Question 54

Acids, alkalis and titrations Questions