Part 4 Acid and Base

Question 1

What is meant by the term “acid” ?

Answer 1

Acid is a Hydrogen containing substance that dissolves in water and ionises to form H+ ions as the only cation.

Question 2

What is meant by the term “alkali” ?

Answer 2

Alkali is a substance that dissolves in water and dissociates to give OH- ions as the only anion. // Alkali is a water soluble base that reacts with acid to give salt and water only

Question 3

What is meant by the term “base” ?

Answer 3

Base is a compound that react with acid to form salt and water only.

Question 4

What is meant by the term “Strong acid” ?

Answer 4

Strong acid is a hydrogen-containing substance that dissolves in water and completely ionises to give H+ ions as the only cation.

Question 5

What is meant by the term “weak acid” ?

Answer 5

Weak acid is a hydrogen containing compound that dissoves in water and partially ionises to give H+ ions as the only cation.

Question 6

What is meant by the term “standard solution” ?

Answer 6

A solution with accurately known concentration

Question 7

What is meant by the term “basicity” ?

Answer 7

Basicity of an acid is the maximum number of hydrogen ions produced by one molecule of the acid.

Question 8

State which 3 metal ions will redissolve to become colourless again when excess NaOH is added.

Answer 8

Al3+ , Zn2+, Pb2+
(All 升 le)
(All –> Al)
(升–> Zinc)
(le –> Lead)

Question 9

State which 3 metal ions will redissolve to become colourless again when excess NH3 is added.

Answer 9

Zn2+, Ag+, Cu2+

Question 10

State which metal ion will redissolve to become deep blue when excess NH3 is added.

Answer 10

Cu2+

Question 11

State, if any, the observable change when NaOH is added to Fe2+.

Answer 11

Dirty green precipitate forms

Question 12

State, if any, the observable change when NaOH is added to Fe3+.

Answer 12

Brown precipitate forms

Question 13

State, if any, the observable change when NaOH is added to Cu2+.

Answer 13

Pale blue precipitate forms

Question 14

State, if any, the observable change when NaOH is added to Ag+.

Answer 14

Dark brown precipitate forms

Question 15

State, if any, the observable change when NH3 is added to Ca2+

Answer 15

No observable change

Question 16

State, if any, the observable change when NH3 is added to Fe2+.

Answer 16

Dirty green precipitate forms

Question 17

State, if any, the observable change when NH3 is added to Fe3+.

Answer 17

Brown precipitate forms

Question 18

State, if any, the observable change when NH3 is added to Cu2+.

Answer 18

Pale blue precipitate forms

Question 19

State, if any, the observable change when NH3 is added to Ag+.

Answer 19

Dark brown precipitate forms

Question 20

Suggest a method to compare the strength of acids.

Answer 20

Measure the pH value of the acids ( with the same molarity ). The one with lower pH value is the stronger acid. //
Connect with the same numbers and same types of light bulb, and same voltage of power supply. The bulb with a brighter light is the stronger acid.
(accept other reasonable answer)

Question 21

Give one application of neutralisation

Answer 21

Soil pH control. //
Fertilizer production

Question 22

Suggest a chemical that can be used to adjust the soil pH.

Answer 22

CaO,CaCO3, Ca(OH)2, (NH4)2SO4

Question 23

Suggest why NaOH cannot be used as primary standard.

Answer 23

NaOH can absorb water. // It can react with carbon dioxide in the air. Mass of the pellets cannot be accurately measured.

Question 24

Suggest why NH3(aq) cannot be used as primary standard.

Answer 24

It is volatile. It would vapourise during preparation, leading to inaccurate measurement of the concentration.

Question 25

Suggest why conc. H2SO4 cannot be used as primary standard.

Answer 25

It can absorb water vapour from air, leading to inaccurate measurement of the concentration.

Question 26

Suggest why conc. HCl cannot be used as primary standard.

Answer 26

It is volatile. It would vapourise during preparation, leading to inaccurate measurement of concetration.

Question 27

Suggest why conc. HNO3 cannot be used as primary standard.

Answer 27

It is volatile and would decompose by light.

Question 28

Suggest why HNO3 has to be stored in a brown glass bottle.

Answer 28

HNO3 would decompose to give NO2. As the decomposition can be speeded up by light, HNO3 is usually stored in a brown bottle to slow down the decomposition.

Question 29

Suggest one criteria to be a good primary standard.

Answer 29

High solubility // Chemically stable // Involatile and non-toxic // Does not absorb moisture from the air // (accept other reasonable answer)

Question 30

Briefly describe the procedure in preparing standard solution.

Answer 30

Weigh accurately the amount of chemical needed. Dissolve the solid in distilled water. Transfer the solution to a volumetric flask. Make up to the grauation mark with distilled water.

Question 31

State the liquid(s) that should be used to rinse the volumetric flask.

Answer 31

Distilled water

Question 32

State the liquid(s) that should be used to rinse the conical flask.

Answer 32

Distilled water

Question 33

State the liquid(s) that should be used to rinse the pipette.

Answer 33

Distilled water followed by dilute sample.

Question 34

State the liquid(s) that should be used to rinse the burette

Answer 34

Distilled water followed by dilute sample.

Question 35

Suggest a suitable indicator for titration between a strong acid and a weak alkali.

Answer 35

Methyl orange.

Question 36

Suggest a suitable indicator for titration between a weak acid and a strong alkali

Answer 36

Phenolphthalein