4.1

Question 1

.1 Identifying unknowns (1)• To identify the positive and negative inorganic ions in a mixture of salts, using appropriate apparatus and procedures.

Answer 1

You are given a mixture of two salts of the same Group 2 metal. You will first use qualitative tests to identify the negative ions in the two salts. These tests make use of reactions and observations that are specific for particular ions.

Next, you will identify the Group 2 metal ion in the two salts, making use of the fact that salts of different Group 2 metals have different solubilities. You will mix a solution of the unknown salts with two known salts, and observe whether any precipitation occurs. Using solubility data provided, you will be able to identify the positive ion.

Question 2

A mixture of barium chloride, BaCl2 and calcium carbonate, CaCO3.

Answer 2

Toxic if swallowed

Causes serious eye irritation

Question 3

Dilute nitric acid, HNO3(aq)

Answer 3

Causes skin irritation and serious eye irritation

Question 4

Silver nitrate solution, AgNO3(aq)

Answer 4

No hazard classification

Can stain the skin

Question 5

Dilute ammonia, NH3(aq)

Answer 5

Causes skin and eye irritation

Question 6

Aqueous sulfuric acid, H2SO4(aq)

Answer 6

Causes skin irritation

Causes serious eye irritation

Question 7

Potassium chromate solution, K2CrO4(aq)

Answer 7

May cause cancer
May cause genetic defects
May cause an allergic skin reaction

Question 8

Equipment

Answer 8

• wash bottle containing distilled (or de-ionised water)
• measuring cylinder (25 cm3)
• beaker (100 cm3)
• glass rod
• filter paper
• conical flask (100 cm3)• filter funnel
• test-tube holder
• four test tubes
• dropping pipettes
• spatula

Question 9

Health & Safety

Answer 9

• Wear eye protection (goggles) throughout.

* Wear protective gloves when handling K2CrO4.

Question 10

Procedure part 1

Answer 10

: Identifying the negative ions (anions) present in Mixture A

1. Using a measuring cylinder, transfer about 25 cm3 of distilled (or de-ionised) water into a beaker.
2. Tip Mixture A into the water in the beaker. Stir the mixture with a glass rod for about 1 minute.
3. Filter the mixture into a conical flask.
4. Add about 1 cm depth of the filtrate to a test tube.
5. Add five drops of HNO3(aq) followed by ten drops of AgNO3(aq). Record your observations.
6. Add NH3(aq) dropwise into the test tube. Keep adding NH3(aq) until no further change occurs. Record your observations.
7. Add a spatula measure of the solid residue from Step 3 to a test tube.
8. Add about 1 cm depth of HNO3(aq) then keep adding HNO3(aq) until no further change occurs. Record your observations.
9. Suggest the formulae for the two negative ions present in mixture A using your observations. If you do not know what your observations mean, try to find the relevant information in your textbook or online.
10. Write the equations for the reactions you have observed.

Question 11

Procedure part 2:

Answer 11

Identifying the Group 2 positive ion (cation) present in Mixture A

1. Add about 1 cm depth of the filtrate from Step 3 to a new test tube.
2. Add about the same volume of H2SO4(aq). Record your observations.
3. Add about 1 cm depth of the filtrate from Step 3 to a new test tube.
4. Add about the same volume of K2CrO4(aq). Record your observations.
5. Suggest the formula of the Group 2 positive ion present in mixture A using your observations and the solubility (in water) information from the table below.

Question 12

Addition of nitric acid to the residue:

Answer 12

BaCO3(aq) + 2HNO3 (aq)  Ba(NO3)2(aq) + CO2(g) + H2O(l)

Question 13

Addition of H2SO4 to filtrate:

Answer 13

Ba2+(aq) + SO42(aq)  BaSO4(s)

Question 14

Addition of K2CrO4 to filtrate:

Answer 14

Ba2+(aq) + CrO4(aq)  BaCrO4(s)

Question 15

action Add 5 drops HNO3 to filtrate, followed by 10 drops of AgNO3.

Answer 15

White precipitate forms.

Question 16

action Add NH3 dropwise until no further change happens.

Answer 16

White precipitate (mostly) dissolves.
Note that some of the precipitate may remain.

Question 17

action Add 1 cm depth HNO3 to residue.

Answer 17

Effervescence.

Question 18

actionKeep adding HNO3 until no further change.

Answer 18

A colourless solution forms.

Note that some precipitate may still be visible in the solution.

Question 19

action Add H2SO4 to filtrate.

Answer 19

White precipitate in a colourless solution.

Question 20

action Add K2CrO4 to filtrate.

Answer 20

Yellow precipitate in a yellow solution.