Quantitative Analysis

Question 1

Outline the tests for these 4 gases:
- Hydrogen
- Oxygen
- Carbon Dioxide
- Chlorine

Answer 1

Hydrogen - a lit splint will burn with a squeaky pop

Oxygen - a glowing splint will re-light,

Carbon Dioxide - Turns limewater milky,

Chlorine - damp litmus paper is bleached white

Question 2

Outline the sodium hydroxide test for these metal ions:
- Aluminium
- Calcium
- Copper
- Iron (II)
- Iron (III)

Answer 2

• add dilute sodium hydroxide to the metal ions in solution. Some metals react to form solid, insoluble metal hydroxides precipitate such as:

Aluminium - White
Calcium - White
Copper - Blue
Iron (II) - Green
Iron (III) - Brown

• to distinguish between the Aluminium and Calcium, you then add more sodium hydroxide (in excess).
  -The aluminium hydroxides precipitate reacts to form a colourless solution
• The calcium hydroxide precipitate is unchanged.

Question 3

Outline the tests for the negative ‘Carbonate ions’

Answer 3

1) add a dilute acid to the test compound. If Carbonate ions are present, bubbles of Carbon Dioxide will be formed.
2) The gas produced is bubbled through limewater. If the gas is carbon dioxide, the limewater will turn cloudy.

Question 4

Outline the test for the negative ‘ Sulfate ions’

Answer 4

Barium ions react with sulfate ions to form an insoluble, white precipitate of barium sulfate.

1) Add a few drops of dilute HCl to the sample.
2) Add a few drops of dilute barium chloride solution.
- If sulfate ions are present, a white precipitate will form

Question 5

Outline and explain each step of the test for these halide ions:
- Chloride
- Bromide
- Iodine

Answer 5

1) Add dilute nitric acid. This removes other ions that could react with the silver nitrate, preventing other precipitates from forming. (Therefore preventing false positives).

2) Add dilute silver nitrate. The silver reacts with the Halide ions to form insoluble precipitates.

3) Observe the precipitate formed:

Chloride - White
Bromide - Cream
Iodine - Yellow

4) To double check your results, you can react these precipitates with ammonia:

AgCl- Precipitate dissolves in dilute ammonia solution

AgBr- Precipitate dissolves in concentrated ammonia solution

AgI- Precipitate doesn’t dissolve in ammonia

Question 6

Outline the test for ammonium ions

Answer 6

Ammonium ions react with hydroxide ions to form ammonia and water

1) Add dilute sodium hydroxide solution to the sample
2) Warm the mixture.
3) If ammonium ions are present, ammonia gas will be given off. Test for ammonia ( ammonia turns damp red litmus paper blue).

Question 7

Outline the basic test for sulfate ions

Answer 7

Barium chloride
Positive test=white precipitate

Question 8

Describe the basic test for carbonate ions

Answer 8

Add dilute acid - if positive, bubbles of CO2 form

Bubble through limewater - CO2 turns it cloudy

Question 9

Describe the basic test for certain halide ions

Answer 9

Silver nitrate
Chloride= white precipitate
Bromide= cream precipitate
Iodine=yellow precipitate

Question 10

Describe basic test for ammonium ions

Answer 10

Sodium hydroxide
If ammonium ions present, ammonia gas given off
Test for ammonia gas- damp red litmus paper turns blue

Question 11

Why must you add dilute nitric acid before the dilute silver nitrate when testing for halide ions

Answer 11

Removes other ions that could react with the silver nitrate - preventing other precipitates from forming