11 — QA

Question 1

Similarities and differences betw NaOH and NH3

Answer 1

Similarity: both r alkalis, both dissociates in water to give OH- ions

Differences:
NaOH is a strong alkali which dissociates fully in water to give a high conc of OH- ions
NH3 is a weak alkali which dissociates partially in water to give a low conc of OH- ions

Question 2

NH4+

Answer 2

NaOH few drops:
No precipitate formed. On warming, effervescence observed and gas evolved turns moist red litmus paper blue. Ammonia gas is produced.

Question 3

Cu2+

Answer 3

Few drops of NaOH:
Light blue precipitate of Cu(OH)2 formed.

Excess NaOH:
Precipitate is insoluble in excess NAOH

Few drops of NH3:
Light blue precipitate of Cu(OH)2 formed

Excess NH3:
Precipitate is soluble in excess NH3, resulting in formation oof a dark blue solution

Question 4

Fe2+

Answer 4

Few drops of NaOH:
Green precipitate of Fe(OH)2 formed

Excess NaOH:
Insoluble in excess NaOH

Few drops of NH3:
Green precipitate of Fe(OH)2 formed

Excess NH3:
Insoluble in excess NH3

Question 5

Fe3+

Answer 5

Few drops of NaOH:
Red-brown precipitate of Fe(OH)3 formed

Excess NaOH: insoluble in excess

NH3:
Red-brown precipitate of Fe(OH)3 formed

Excess NH3:
Insoluble in excess

Question 6

Ca2+

Answer 6

NaOH:
White precipitate

Excess NaOH:
Insoluble in excess

NH3:
No precipitate formed

Question 7

Zn2+

Answer 7

NaOH:
White precipitate of Zn(OH)2 formed

Excess:
Soluble in excess NaOH(aq), resulting in the formation of a colourless solution

NH3:
White precipitate of Zn(OH)2 formed

Excess NH3:
Precipitate is soluble in excess NH3, resulting in the formation of a colourless solution

Question 8

Al3+

Answer 8

NaOH:
White precipitate of Al(OH)3 formed

Excess NaOH:
Precipitate soluble in excess NaOH resulting in the formation of a colourless solution

NH3:
White precipitate of Al(OH)3 formed

Excess NH3:
Insoluble in excess

Question 9

Why r zinc and aluminium hydroxides soluble in excess NaOH

Answer 9

They have amphoteric characteristics and r able to react w both strong alkali n strong acid to form a stable compound

Question 10

Why is there no precipitate observed when aq ammonia is added to samples containing calcium ions?

Answer 10

Unlike NaOH, aq ammonia is a weak alkali which dissociates to give a low conc of OH- ions. Hence only a small amt of Ca(OH)2 is formed. Since calcium hydroxide is sparingly soluble, no obvious precipitate will be observed.

Question 11

CO3^2-

Answer 11

Add dilute acid. Bubble the gas given off into limewater.

Obsv:
Effervescence observed. Gas given off forms white precipitate (CaCO3) in limewater. CO2 gas is produced

Question 12

NO3-

Answer 12

Add aq NaOH then add apiece of aluminium. Warm the mixture carefully. Test the gas given off w a piece of damp red litmus paper.

Obsv:
Effervescence observed. Gas given off turns damp red litmus paper blue. Ammonia gas is produced.

Question 13

Cl-

Answer 13

Add dilute nitric acid then add aq silver nitrate

Obsv:
White precipitate of AgCl formed

Question 14

I-

Answer 14

Add dilute nitric acid then add aq silver nitrate

Obsv:
Yellow precipitate of AgI formed

Question 15

SO4^2-

Answer 15

Add dilute nitric acid then add aq barium nitrate

Obsv:
White precipitate of BaSO4 formed

Question 16

Why the addition of dilute nitric acid before adding aq silver nitrate or barium nitrate to Cl-,I-/SO4^2-?

Answer 16

To acidify the salt samples. Dilute nitric acid is added to remove interfering ions such as carbonates and hydroxides as they r mostly insoluble. Presence of interfering ions could lead to false positive results.

Dilute nitric acid is used as its salts r alw soluble.

Question 17

H2 gas

Answer 17

Colour and odour:
Colourless and odourless gas

Test:
Place a burning splint at the mouth of test tube

Obsv:
Burning splint is extinguished w a ‘pop’ sound

Question 18

O2

Answer 18

Colour and odour:
Colourless and odourless gas

Test:
Lace growing splint at the mouth of test-tube

Obsv:
The growing splint rekindled

Question 19

CO2

Answer 19

Colour and odour:
Colourless and odourless gas

Test:
Bubble the gas given off into limewater

Obsv:
White precipitate is formed in the limewater

Question 20

CL2

Answer 20

Colour and odour:
Yellow green gas w pungent smell

Test:
Place a piece of damp blue litmus paper at the mouth of test-tube

Observations:
Damp blue litmus paper turns red and is then bleached

Question 21

SO2

Answer 21

Colour and odour:
Colourless gas w pungent smell

Test:
Place a piece of filter paper soaked w acidified potassium manganate (VII) at the mouth of test-tube

Observations:
Purple acidified potassium manganate (VII) solution turns colourless

Question 22

NH3

Answer 22

Colour and odour:
Colourless gas w pungent smell

Test:
Place a piece of damp red litmus paper at the mouth of test tube

Observations:
Damp red litmus paper turns blue

Question 23

Explain why aqueous ammonia can only be used to identify one of the ions in the solution containing Zn2+, Pb2+ and Al3+ ions. [2]

Answer 23

Zn ions form a white precipitate which dissolves in excess ammonia to give a colourless solution when AQUEOUS ammonia is added to the solution.
When aqueous ammonia is added, lead and aluminium ions gives the same observations where white precipitate is formed which is insoluble in excess aqueous ammonia.
Hence aqueous ammonia can only be used to identify zinc ions.

Question 24

Sea water often contains large percentage of NaCl. Suggest a method to measure the amount of chloride ions present in a sample of sea water

Answer 24

Add excess silver nitrate, dry and weigh the precipitate formed

Question 25

Based on the graph above, the student concluded that the anion is sulfate ion, but
not carbonate ion.
Do you agree with the student?
Explain your answer with reference to the graph.
Graph depicts that on adding aq BaCl2, precipitate is formed but no precipitate is formed when HNO3 is added.

Steps:
1 Add aqueous barium chloride to a test tube containing solution T.
2 Measure the height of precipitate formed after 5 minutes.
3 Add excess dilute nitric acid to the above mixture.
4 Measure the height of the precipitate formed after 5 minutes.

Answer 25

Don’t agree with student.
Upon adding of barium chloride, ppt formed could be due to sulfate or carbonate ions. (✓)
Height of ppt decreases/ppt dissolves upon adding nitric acid (✓) and this means that the ppt reacted with nitric acid. (✓)
Hence the ppt could be BaCO3 which reacted with acid since BaSO4 (✓) cannot react with acid.
4 (✓) – [3]; 2 – 3 (✓) – [2], 0 – 1 (✓) – [1]