1. Problem 1: Quantitative estimation of $x$ g of $(NH_4)_2SO_4$ using titration data. 2. (a)(i) The laboratory apparatus used to measure the 25.0 cm$^3$ portion of solution C is a **pipette**. It should be used because it delivers an accurate and precise fixed volume of liquid, essential for reliable titration results. 3. (a)(ii) Phenolphthalein indicator is **suitable** because it changes color in the pH range of about 8.2 to 10, which corresponds to the endpoint of the titration between a strong base (NaOH) and a strong acid (H$_2$SO$_4$). 4. (a)(iii) Few drops of phenolphthalein are used to avoid excess indicator which can cause a false color change and affect the accuracy of the titration endpoint. 5. (a)(iv) The student needed to repeat the titration to ensure accuracy and reproducibility of results, as titrations can have slight variations. 6. (a)(v) Three experimental errors might include: - Incorrect reading of burette (parallax error). - Not mixing the solution thoroughly before titration. - Overshooting the endpoint by adding titrant too quickly. 7. (b)(i) Calculate concentration of excess NaOH solution: - Volume of H$_2$SO$_4$ used in titration average: $$\frac{20.55 + 20.85 + 20.25}{3} = 20.55\text{ cm}^3$$ - Convert to dm$^3$: $$20.55\text{ cm}^3 = 0.02055\text{ dm}^3$$ - Moles of H$_2$SO$_4$ used: $$0.02055 \times 0.195 = 0.00400575\text{ mol}$$ - Reaction: $$2\text{NaOH} + \text{H}_2\text{SO}_4 \to \text{Na}_2\text{SO}_4 + 2\text{H}_2\text{O}$$ - Moles of NaOH in 25.0 cm$^3$ portion: $$2 \times 0.00400575 = 0.0080115\text{ mol}$$ - Concentration of NaOH: $$\frac{0.0080115}{0.025} = 0.32046\text{ mol dm}^{-3}$$ 8. (b)(ii) Calculate $x$ in $(NH_4)_2SO_4$: - Initial moles of NaOH in 1.00 dm$^3$ of solution C: $$2.05\text{ mol dm}^{-3} \times 1.00\text{ dm}^3 = 2.05\text{ mol}$$ - Moles of NaOH remaining after reaction (excess): $$0.32046\text{ mol dm}^{-3} \times 1.00\text{ dm}^3 = 0.32046\text{ mol}$$ - Moles of NaOH reacted: $$2.05 - 0.32046 = 1.72954\text{ mol}$$ - Reaction between $(NH_4)_2SO_4$ and NaOH: $$(NH_4)_2SO_4 + 2NaOH \to Na_2SO_4 + 2NH_3 + 2H_2O$$ - Moles of $(NH_4)_2SO_4$ reacted: $$\frac{1.72954}{2} = 0.86477\text{ mol}$$ - Molar mass of $(NH_4)_2SO_4$: $$2(14 + 4 \times 1) + 32 + 4 \times 16 = 132\text{ g mol}^{-1}$$ - Mass $x$: $$0.86477 \times 132 = 114.07\text{ g}$$ 9. Problem 2: Complete the table for tests on G and H. | TEST | OBSERVATION | INFERENCE | |---|---|---| | a.i G(s) + distilled water + stir + filter | Partially soluble; black residue and colourless filtrate | G contains insoluble impurities; filtrate is aqueous solution | | a.ii Filtrate + litmus | Litmus turns red (acidic) | Filtrate is acidic | | a.iii Filtrate + HCl (warm) | SO$_2$ gas evolved | SO$_3^{2-}$ ions present | | a.iv Filtrate + NH$_3$ drops | White gelatinous precipitate | Presence of Al$^{3+}$ or similar | | a.iv Filtrate + NH$_3$ excess | Precipitate dissolves | Amphoteric hydroxide | | b.i Residue + dilute HCl | Effervescence; colourless odourless gas turns lime water milky; pale blue solution left | CO$_3^{2-}$ present; Cu$^{2+}$ ions present | | b.ii Solution from b(i) + NaOH drops | Blue gelatinous precipitate | Cu$^{2+}$ ions present | | b.ii Solution from b(i) + NaOH excess | Precipitate dissolves | Amphoteric hydroxide | | b.iii Solution from b(i) + NH$_3$ drops | Blue precipitate | Cu$^{2+}$ ions present | | b.iii Solution from b(i) + NH$_3$ excess | Precipitate dissolves | Cu$^{2+}$ confirmed | | c.i H(aq) + Million's reagent | Brick-red precipitate | Presence of Cu$^{2+}$ ions | | c.ii H(aq) + Conc. HNO$_3$ | No reaction or specific observation | Confirm absence of carbonate or other reactive ions | 10. Problem 3: Identify reagents and methods of preparation. | Reaction | Reagent | Identity | Method | |---|---|---|---| | i. H$_2$SO$_4$(aq) + D → ZnSO$_4$(aq) | D = Zn metal | Zinc metal | Reaction of metal with acid (metal-acid reaction) | | ii. AgNO$_3$(aq) + E → AgCl(s) | E = NaCl or KCl | Chloride salt | Precipitation reaction (double displacement) | | iii. HNO$_3$(aq) + F → KNO$_3$(aq) | F = KOH or K$_2$CO$_3$ | Potassium hydroxide or carbonate | Neutralization or acid-base reaction | | iv. Fe(s) + G → FeCl$_3$(s) | G = Cl$_2$ gas or HCl (with oxidation) | Chlorine gas or hydrochloric acid | Direct combination or oxidation reaction | 11. Reasons for practices: (i) Hydrogen peroxide is stored in amber-colored bottles to protect it from light which decomposes it. (ii) Ethyne gas is stored in propanone in steel cylinders because it dissolves in propanone, reducing explosion risk and stabilizing the gas. Slug: "ammonium sulfate titration" Subject: "chemistry" Desmos: {"latex":"","features":{"intercepts":true,"extrema":true}} q_count: 3