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

Metal hydroxides

Answer 2

Most metal hydroxides r insoluble — produces precipitate
Soluble metal hydroxides: Group 1, calcium, barium

Question 3

Steps to test for cations

Answer 3

1. Prepare a small amount of the unknown sample
2. Add a few drops of NaOH or NH3
3. Note any observations (formation of precipitate and its colour, gas evolved)
4. Add excess NaOH or NH3 and observe if precipitate dissolves.

Question 4

NH4+

Answer 4

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

Question 5

Cu2+

Answer 5

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 6

Fe2+

Answer 6

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 7

Fe3+

Answer 7

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 8

Ca2+

Answer 8

NaOH:
White precipitate

Excess NaOH:
Insoluble in excess

NH3:
No precipitate formed

Question 9

Zn2+

Answer 9

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 10

Al3+

Answer 10

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 11

Why r zinc and aluminium hydroxides soluble in excess NaOH

Answer 11

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

Question 12

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

Answer 12

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 13

CO3^2-

Answer 13

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 14

NO3-

Answer 14

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 15

Cl-

Answer 15

Add dilute nitric acid then add aq silver nitrate

Obsv:
White precipitate of AgCl formed

Question 16

I-

Answer 16

Add dilute nitric acid then add aq silver nitrate

Obsv:
Yellow precipitate of AgI formed

Question 17

SO4^2-

Answer 17

Add dilute nitric acid then add aq barium nitrate

Obsv:
White precipitate of BaSO4 formed

Question 18

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

Answer 18

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 19

H2 gas

Answer 19

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 20

O2

Answer 20

Colour and odour:
Colourless and odourless gas

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

Obsv:
The growing splint rekindled

Question 21

CO2

Answer 21

Colour and odour:
Colourless and odourless gas

Test:
Bubble the gas given off into limewater

Obsv:
White precipitate is formed in the limewater

Question 22

CL2

Answer 22

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 23

SO2

Answer 23

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 24

NH3

Answer 24

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