practical 4-cations + anions

Question 1

How to test for the presence of a sulfate (gp7)

Answer 1

reagent: BaCl2 acidified w HCl

if acidified BaCl2 added to sulfate solution/=white precipitate of BSO4 formed.

Ba^2+(aq) + SO4^2-(aq) –> BaSO4(s)

Question 2

why is BaCl2 acidified w HCl.why is H2SO4 not used

Answer 2

carbonate impurities found in salts form a white barium carbonate precipitate giving a false positive.

sulfate ions in H2SO4 would give false positive

Question 3

Give equation for the removal of impurities

what is observed

Answer 3

HCl + Na2CO3 –>2NaCl + H2O + CO2

observation: fizzing due to CO2
- if carbonate was present.

Question 4

How is the presence halide ions detected

Answer 4

test solution acidified w nitric acid and then silver nitrate solution is added dropwise.

Ag+(aq) + Cl- (aq)—> AgCl(s) white precipitate

Ag+(aq) + Br- (aq)—> AgBr(s) cream precipitate

Ag+(aq) + I (aq)—> AgI(s) pale yellow precipitate

Question 5

why is test solution acidified with nitric acid

Answer 5

reacts w carbonate ions to prevent formation of Ag2CO3= false positive

2HNO3 + Na2CO3—-> 2NaNo3 + H2O + CO2