Chemical Tests
6 marks · NormalA student is given two separate white solids, labelled X and Y. X is known to be either sodium carbonate or ammonium chloride. Y is a mixture of calcium carbonate and ammonium chloride.
Describe a test to identify which solid, X or Y, contains the carbonate ion. Calculate the minimum volume of 1.0 mol/dm³ hydrochloric acid required to completely react with 0.05 mol of carbonate present in one of the solids. Show your working.
State what you would observe when dilute hydrochloric acid is added to a carbonate.
Write the ionic equation for the reaction of a carbonate ion with an acid.
Calculate the minimum volume (in cm³) of 1.0 mol/dm³ hydrochloric acid required to react completely with 0.05 mol of carbonate ion. (Assume carbonate is CO₃²⁻.)
Solid Y is warmed with sodium hydroxide solution. Ammonia gas is produced.
Describe the test and give a calculation to show how many moles of ammonium ions would be present if 672 cm³ of ammonia gas is collected at room temperature and pressure. (1 mol of gas occupies 24,000 cm³ at r.t.p.)
Describe how to test for ammonia gas.
Calculate the number of moles of ammonia gas in 672 cm³ at r.t.p.
Write the ionic equation for the reaction of ammonium ions with hydroxide ions.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
Show answer
(a(i)) Effervescence/fizzing due to gas (carbon dioxide) produced.
(a(ii)) CO3^{2-} (s or aq) + 2H^{+} (aq) → CO2 (g) + H2O (l)
(a(iii)) 100 cm³ cm^3
(b(i)) Use damp red litmus (or universal indicator) paper; ammonia turns it blue.
(b(ii)) 0.028 mol
(b(iii)) NH4^+ (aq) + OH^- (aq) → NH3 (g) + H2O (l)
Also accept: Using limewater to test for carbon dioxide in carbonate test; Equation with state symbols correct or omitted
Common mistakes: Confusing carbonate and sulfate reactions; Not multiplying by stoichiometric coefficients; Forgetting to convert volume units
Show mark scheme and worked solution
Marking points
B1 (a(i)) · 1 mark · States effervescence/fizzing (or gas given off) when acid is added to carbonate.
B2 (a(ii)) · 1 mark · Correct ionic equation for acid-carbonate reaction.
M1 (a(iii)) · 1 mark · Correctly calculates required volume using stoichiometry; allows ecf for moles used. (follow-through)
B3 (b(i)) · 1 mark · States use of damp red litmus/universal paper turns blue for ammonia.
M2 (b(ii)) · 1 mark · Converts cm³ to moles of gas using molar volume formula. (follow-through)
B4 (b(iii)) · 1 mark · Correct ionic equation for ammonium ion and hydroxide reaction.
Worked solution
a(i) When dilute hydrochloric acid is added to a carbonate, it effervesces (fizzes) as carbon dioxide gas is produced.
a(ii) The ionic equation is:
a(iii) From the equation, 1 mol CO₃²⁻ reacts with 2 mol H⁺. 0.05 mol CO₃²⁻ needs 0.10 mol HCl. 1.0 mol/dm³ = 1.0 mol in 1000 cm³, so:
b(i) Test for ammonia gas: Hold damp red litmus paper at the mouth of the test tube; it turns blue if ammonia is present.
b(ii) Moles of ammonia gas =
b(iii) Ionic equation for the reaction: