analysis of inorganic compounds

Question 1

test for the presence of carbonate and hydrogencarbonate

Answer 1

1. bubble gas through limewater - will turn limewater cloudy
2. add any dilute acid and observe fizzing/bubbling

e.g. 2HCl + NaCO3 = NaCl + H2O + CO2
2H+ + CO3 2- = H2O + CO2

    HCl + NaHCO3 = NaCl + H2O + CO2
    H+ + HCO3 - = H2O + CO2

Question 2

testing for presence of a sulfate

Answer 2

1. acidified BaCl2 solution is used as a reagent to test for sulfate ions
   Ba+ (aq) + SO4 2- (aq) = BaSO4 (s)
2. if barium chloride is added to a solution that contains sulfate ions, a white precipitate is formed

Question 3

testing for halide ions with silver nitrate

Answer 3

1. add nitric acid
2. add silver nitrate solution

Question 4

what is the role of the nitric acid?

Answer 4

react with any carbonates present to prevent formation of precipitate Ag2CO3. this would mask the desired observations
2HNO3 + Na2CO3 = 2NaNO3 + H2O + CO2

Question 5

why can hydrochloric acid not be used when testing using silver nitrate?

Answer 5

it contains chloride ions which would form a precipitate

Question 6

how can silver halide precipitates be trated with ammonia solution to help differentiate between them if the colours look similar

Answer 6

silver chloride dissolves in dilute ammonia to form a complex ion
AgCl2 (s) + 2NH3 (aq) = [Ag(NH3)2]+ (aq) + Cl- (aq) forms colourless solution

silver bromide dissolves in concentrated ammonia to form a complex ion
AgBr2 (s) + 2NH3 (aq) = [Ag(NH3)2]+ (aq) + Br- (aq) forms colourless solution

silver iodide doesn’t react with ammonia - it’s too insoluble

Question 7

test for positive ions (cations)

Answer 7

test for ammonim ion (NH4+) by reaction with warm NaOH (aq) forming NH3
NH4 + OH- = NH3 + H2O

Question 8

how can ammonia gas be indentified?

Answer 8

pungent smell or by turning damp red litmus paper blue