17: Concentrations of Solutions and Volumetric Analysis

Question 1

How is the concentration of a solution calculated given the number of moles of solute and volume of the solution?

Answer 1

Concentration (mol dm^-3) = number of moles of solute / volume of the solution (in dm³)

Question 2

Which quantity is conserved in dilutions?

Answer 2

MV (molarity x volume)

Question 3

What is the definition of standard solution?

Answer 3

A standard solution is a solution whose concentration is accurately known.

Question 4

What are the criteria of chemicals used to make primary standard solutions?

Answer 4

1. The chemical should be available in a high degree of purity.
2. The chemical should have a known chemical formula.
3. The chemical should be chemically stable.
4. The chemical should not absorb moisture from the air.
5. The chemical should be completely soluble in water.
6. The chemical should react rapidly and completely with other substances used in titrations.
7. The chemical should have a high molar mass to minimise error.

Question 5

Why is sodium hydroxide not suitable as primary standards?

Answer 5

It absorbs moisture and reacts with carbon dioxide in air.

Question 6

How is the concentration of sodium hydroxide solution determined?

Answer 6

Through a titration with an acid of primary standard (of accurate concentration).

Question 7

Why should the conical flask in a titration be rinsed with distilled water instead of the solution?

Answer 7

This is to ensure that the number of moles of solution inside the flask is conserved.

Question 8

Why should the volumetric flask in a titration be rinsed with distilled water instead of the solution?

Answer 8

This is to ensure that the number of moles of solution inside the flask is conserved.

Question 9

What is the meaning of concordant titre?

Answer 9

Concordant titres are titres within 0.10cm³ of each other for each pair of titre values.

Question 10

What is the equivalence point of a titration?

Answer 10

The equivalence point is the point at which sufficient acid has been added to just neutralise the alkali or vice versa.

Question 11

What is the end point of a titration?

Answer 11

The end point is the point at which the indicator just changes colour.

Question 12

What is the pH range for methyl orange to have a red colour?

Answer 12

pH < 3.2

Question 13

What is the pH range for methyl orange to have an orange colour?

Answer 13

3.2 < pH < 4.4

Question 14

What is the pH range for methyl orange to have a yellow colour?

Answer 14

pH > 4.4

Question 15

What is the pH range for phenolphthalein to be colourless?

Answer 15

pH < 8.2

Question 16

What is the pH range for phenolphthalein to be pale pink?

Answer 16

8.2 < pH < 10.0

Question 17

What is the pH range for phenolphthalein to be pink?

Answer 17

pH > 10.0

Question 18

Which indicator is suitable for the titration between a strong acid and a strong alkali and why?

Answer 18

Both indicators are suitable, since they both change colour within the pH range of the vertical part of the titration curve.

Question 19

Which indicator is suitable for the titration between a strong acid and a weak alkali and why?

Answer 19

The pH of the equivalence point is less than 7; an acidic salt is produced. Methyl orange is suitable, since it changes colour within the pH range of the vertical part of the titration curve (slightly below pH 7).

Question 20

Which indicator is suitable for the titration between a weak acid and a strong alkali and why?

Answer 20

The pH of the equivalence point is more than 7; an alkaline salt is produced. Phenolphthalein is suitable, since it changes colour within the pH range of the vertical part of the titration curve (slightly above pH 7).

Question 21

Which indicator is suitable for the titration between a weak acid and a weak alkali and why?

Answer 21

There is no sudden change in pH, thus no indicator is suitable.

Question 22

Why does the electrical conductivity decrease when an acid is neutralised by an alkali?

Answer 22

The concentrations of mobile ions from the acid decreases as it is neutralised by the alkali. Therefore, its electrical conductivity decreases.