5.3.2 a qualitative analysis

Question 1

What is the formula of a carbonate?

Answer 1

CO3 (2-)

Question 2

What is the formula of a sulfate?

Answer 2

SO4(2-)

Question 3

What do you add to test for a carbonate? What is the result of a positive test?

Answer 3

Dilute HNO3 (aq), and bubble gas formed through limewater. Cloudy result means the presence of CO2 (and hence a carbonate)

Question 4

What do you add to test for a sulfate? What is the result of a positive test?

Answer 4

Add Ba(NO3)2 (aq). Barium 2+ ions react with sulfate ions to form BaSO4 which is a white precipitate.

Question 5

Why do you have to use BaNO3 instead of BaCl2?

Answer 5

Because the chloride ions would react with the test for chlorines, giving a false positive

Question 6

How do you test for halide ions? What is the result of a positive test for:

• Chloride
• Bromide
• Iodide

Answer 6

Add AgNO3.

White precipitate - AgCl forms
Cream precipitate - AgBr forms
Yellow precipitate- AgI forms

Question 7

How can you distinguish between chlorides, bromides and iodides if the colours all look very similar?
What is the result of a positive test for:
- Chloride
- Bromide
- Iodide

Answer 7

Add aq NH3.

Dissolves in dilute NH3
Dissolves in concentrated NH3
Doesnt dissolve in NH3

Question 8

How can you test for NH4+ ions?

Answer 8

Add NaOH, heat over bunsen burner, test gaseous product with pH strips - look for blue colour signifying alkaline presence.

Question 9

What order should you test for ions including:
SO42-
Cl-
I-
Br-
CO32-
NH4+

Answer 9

1. CO32- –> removes carbonate ions which could react with test for sulfates and produce a false positive.
2. SO42- –> needs to be carried out before halides to ensure that white precipitate is from reaction of sulfate not halides
3. Halides –> forms 3 coloured precipitates
4. NH4+ –> gas tested with pH strips - needs to be carried out last otherwise you lose all the sample

Question 10

Write the IONIC equation for the reaction of CO32- ions with HNO3 (aq)

Answer 10

CO3 (2-) + 2H+ –> CO2 (g) + H2O

Question 11

Write the IONIC equation for the reaction of chloride ions with silver nitrate

Answer 11

Cl- + Ag+ –> AgCl (s)

Question 12

Write the IONIC equation for the reaction of bromide ions with silver nitrate

Answer 12

Br - + Ag+ –> AgBr (s)

Question 13

Write the IONIC equation for the reaction of iodide ions with silver nitrate

Answer 13

I- + Ag+ –> AgI (s)

Question 14

Write the IONIC equation for the reaction of sulfate ions with barium nitrate.

Answer 14

SO4 (2-) + Ba 2+ –> BaSO4

Question 15

What is the formula of barium nitrate?

Answer 15

Ba(NO3)2

Question 16

Write the IONIC equation for the reaction of NH4+ with NaOH

Answer 16

NH4+ + OH- –> NH3 (g) + H2O