Answer all the questions
1. A is a solution of a monobasic acid HX, suspected to be HCl or HNO3, containing 3.15g of the acid in 500 cm3 of solution.
B is 0.100 mol·dm−3 NaOH.
25.0 cm3 portions of B were titrated with A using methyl orange as indicator.
The following result was presented by one of the students:
Average vol. of A used:
Average = (25.1 + 24.8) / 2 = 49.9 / 2 = 24.95 cm3
The reaction equation is:
HX(aq) + NaOH(aq) → NaX(aq) + H2O(l)
[H = 1, N = 14, O = 16, Cl = 35.5, Na = 23]
1(a) List three errors in the results recorded by the student.
[6 marks]
1(b)(i) Calculate the concentration of A in mol·dm−3
[5 marks]
Solution:
Moles of NaOH = (0.100 × 25.0) / 1000 = 0.00250 mol
Since HX + NaOH → NaX + H2O, mole ratio is 1:1
Therefore, moles of HX = 0.00250 mol
Volume of HX used = 24.95 cm3 = 0.02495 dm3
Concentration = 0.00250 / 0.02495 = 0.1002 mol·dm−3
1(b)(ii) Calculate the molar mass of HX
[4 marks]
Solution:
Moles in 500 cm3 = 0.1002 × 0.500 = 0.0501 mol
Mass = 3.15 g
Molar mass = 3.15 / 0.0501 = 62.87 g/mol
1(b)(iii) Identify HX
[5 marks]
Solution:
Molar mass of HCl = 1 + 35.5 = 36.5 g/mol
Molar mass of HNO3 = 1 + 14 + (16 × 3) = 63 g/mol
Since 62.87 ≈ 63
HX is HNO3 (Nitric Acid)
2. P, Q, R and S are aqueous solutions of ZnCl2, K2CO3, Pb(NO3)2 and H2SO4 but not necessarily in that order.
The results obtained when these solutions were reacted with each other are as shown in the table below:
| P | Q | R | S | |
|---|---|---|---|---|
| P | X | ppt | ppt | ppt |
| Q | ppt | X | No ppt | effervescence |
| R | ppt | No ppt | X | ppt |
| S | ppt | effervescence | ppt | X |
From the results and your knowledge of the solubility of salts in water and the effect of acids on trioxocarbonates, identify, with reasons, each of the solutions.
2 Identify each solution with reasons
[16 marks]
Identification and Reasoning:
Final Identifications:
| Label | Identified Solution | Chemical Formula |
|---|---|---|
| P | Lead(II) nitrate | Pb(NO3)2 |
| Q | Potassium carbonate | K2CO3 |
| R | Zinc chloride | ZnCl2 |
| S | Sulfuric acid | H2SO4 |
3(a)(i) Describe one chemical test to distinguish between aqueous solutions of calcium trioxonitrate(V) and zinc trioxonitrate(V).
[4 marks]
Test: Add aqueous sodium hydroxide (NaOH) dropwise, then in excess.
Calcium trioxonitrate(V): A white precipitate of calcium hydroxide forms and does not dissolve in excess NaOH.
Equation: Ca2+ + 2OH- → Ca(OH)2 (s)
Zinc trioxonitrate(V): A white precipitate forms which dissolves in excess NaOH to form a colorless solution.
Equations:
Zn2+ + 2OH- → Zn(OH)2 (s)
Zn(OH)2 + 2OH- → [Zn(OH)4]2-
3(a)(ii) Describe one chemical test to distinguish between aqueous solutions of potassium chloride and potassium trioxocarbonate(IV).
[4 marks]
Test: Add dilute hydrochloric acid (HCl).
Potassium chloride: No observable reaction (neutral salt).
Potassium trioxocarbonate(IV): Effervescence due to carbon dioxide gas release.
Equation: CO32- + 2H+ → CO2 (g) + H2O (l)
3(b) Explain why the molar heat of neutralization of a strong base by a strong acid is constant.
[3 marks]
The molar heat of neutralization is constant (≈ −57 kJ/mol) because strong acids and bases completely ionize in water.
The net reaction is: H+ (aq) + OH- (aq) → H2O (l)
This reaction is the same in all strong acid–strong base reactions, so the heat change is always the same.
3(c) Name two gases that are soluble in water.
[3 marks]
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