Model Answers for Acids, Bases and Buffers!

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dm-3 solution added / cm³ B B C No D 14 (a) Which curve is produced by adding ammonia to 25 cm³ of hydrochloric acid? A 0 0 10 20 30 40 Volume 0.1 mol dm-³ solution added / cm³ AD (b) Which curve is produced by adding ethanoic acid to 25 cm³ of sodium hydroxide? C (continued from previous page) (c) An indicator with pKn 8.5 is suitable for the following titrations. A Titrations A and B only. 6a 6b 6c DB C Titration C only. D Titrations A, B, C and D. B Titrations A, B and D only. Select the correct pH for each of the following solutions. (a) 2 mol dm-³ nitric acid. A -2 B D +2 -0.3 +0.3 (b) 0.10 mol dm-³ barium hydroxide, Ba(OH)₂. K = 1.0 × 10-¹4 mol² dm-6. Kw = [H] [OH] ки [OH-] [ht] = DA 13.0 B 13.3 Вс 13.7 D 14.3 OA 0 B 0.30 V/c OD 7 (c) A mixture of 20 cm³ of 1.0 mol dm-³ hydrochloric acid and 10 cm³ of 1.0 mol dm-³ sodium hydroxide. 0.48 10-4 0.01 = Conc of 0.03 [H²]. Ans. ↓ 0.2 -log (Ans) ती clv = HCl + NaOH → NaCl + H₂O 20 10 n = CXU no 0.01 x 1 0.01 moles of leftover HCI. 8 Which of the following mixtures would form the best buffer solution with pH 9 for use in a school laboratory? D A Ethanoic acid and sodium ethanoate B Sodium chloride and sodium hydroxide OCHydrocyanic acid and sodium cyanide D Ammonium chloride and ammonia The reaction between concentrated sulfuric acid and pure ethanoic acid is CH₂COOH + H₂SO CH₂COOH + HSO The Brønsted-Lowry acids in this equilibrium are A CH₂COOH and H₂SO B CH₂COOH₂+ and HSO CH₂SO₂ and CH₂COOH₂ D CH₂COOH and HSO Total/12 ( O 3 6 Carbonic acid, H₂CO3, is a weak Bronsted-Lowry acid formed when carbon dioxide dissolves in water. Blood contains several buffer solutions and healthy blood is buffered to a pH of 7.40. The most important buffer solution in blood is a mixture of carbonic acid and hydrogencarbonate ions, HCO3. The equilibrium in the carbonic acid / hydrogencarbonate buffer system is shown below. H₂CO3(aq) H+(aq) + HCO₂¯(aq) K₂ = = 4.17x10-7mol dm-3 (a) Carbonic acid is a weak Bronsted-Lowry acid. What is meant by the following terms? (i) A Bronsted-Lowry acid. A proton (H) doner (ii) A weak acid. An acid that partially dissociates to from H+ ions in water [1] 2816/1 Jan04 [1] (iii) pH. A bogarithmic Scale representing the acidity. of a solution PH = -log [H] (iv) A buffer solution. A solution that resists changes in P.H. upon addition of small amounts of acid [11 or alkali. [1] For Examiner's Usa 7 (b) In this question, one mark is available for the quality of written communication. Explain how the carbonic acid / hydrogencarbonate buffer works. Use equations to help your answer. H* + HCO H₂ CO ₂ caps Upon addition of alkali: → [OH-] increases... Small concentration of Itions react with O.H ious → H² + OH → 12₂0 H₂CO₂ dissociates and the equilibrium shifts to the right restring most of the Hims. Upon addition of acid: →> [H.] increases... HCD.3-cagy reacts with the It ions. → Equilibrium shifts to the left, restoring ... 141 the Ht concencentration. Quality of Written Communication [1] (c) Calculate the ratio Ka [A] Ка [H] [HA] : = [HCO3(aq)] [H₂CO3(aq)] in healthy blood with a pH of 7.40. -7 10.48 4.17x10 3-98x10-8 = gca [H'] = 10-7.¹40 3. 98 x10-8 moldm=" : : [HCO ₂ cap)] = 10.48 [H₂CO3(aq)] 2816/1 Jan04 [4] [Total: 13] For Examine Use [Turn over 2 1 ка Answer all the questions. Methanoic acid, HCOOH, is a weak organic acid which occurs naturally in ants and stinging nettles. (a) Use an equation for the dissociation of methanoic acid to show what is meant by a weak acid. H.COOH = HLOO cag) Ка (b) A 1.50 x 10-² mol dm-3 solution of HCOOH has [H+] = 1.55 x 10-3 mol dm-³. (i) ..P.H. = . = 3 Calculate the pH of this solution and give one reason why the pH scale is a more convenient measurement for measuring acid concentrations than [H+). - 2.81 the -log (1.55.x 10²³) ..Using. ph. centures.. ..manageable... 2 [ht]² (ii) Write the expression for K₂ for methanoic acid. ка= [нсоон] (iii) Calculate the values of K₂ and PK for methanoic acid. [HA] ·(aq) = pka= -log(+) + = 1.60 x10¯¯ moldm-3 [H] [ HCOOT] ка numbers are more... .[2] : 2816/01 Janos .[1] (1.SS X10-31² 1.50×10-2 pka = 3.80 [1] [3] (iv) Estimate the percentage of HCOOH molecules that have dissociated in this aqueous solution of methanoic acid. [1] For Examiner's Use [Turn over (c) A student titrated the 1.50 x 10-2 mol dm-3 methanoic acid with aqueous sodium hydroxide. A 25.00 cm³ sample of the HCOOH(aq) was placed in a conical flask and the NaOH(aq) was added from a burette until the pH no longer changed. ↳1:1 ratio CXV (i) Write a balanced equation for the reaction between HCOOH(aq) and NaOH(aq). ....H.COOH. ..+... NaOH. HCĐÓ, Nh H₂O₁ "Cag) "Cag)" (ii) Part of the pH curve for this titration is shown below. PH 14 13 121 11 10 98765432T 1 0 0 5 10 1 n (1.50x10²x 0-025) = 3.75 x 10-4 moles. [H]. 10-14 Calculate the concentration, in mol dm-3, of the aqueous sodium hydroxide. 0.0125 4 15 20 25 30 35 40 45 50 volume NaOH(aq) added/cm³ :: 3.75 x 10-4 males of NaOH 0-03 = 0.0125 (iii) Calculate the pH of the aqueous sodium hydroxide. Kw = 1.00 x 10-14 mol dm-3 -13 8x10" pH = -log (8x1073) = 12.10 concentration = 0.0125, 2816/01 Jano6 pH = .........mol dm-³ [3] 12.10 . [2] For Examiner's Use 5 (iv) The pH ranges in which colour changes for three acid-base indicators are shown below. indicator metacresol purple 2,4,6-trinitrotoluene ethyl orange pH range 7.4 9.0 11.5-13.0 3.4 4.8 Explain which of the three indicators is suitable for this titration. Metacresol purple because the p.H.. range matches the vertical section. ...of.... I the clive... 2816/01 Jan06 .[2] [Total: 16] For Examiner's Use [Turn over 3 The K₂ values for three acids are shown in the table below. ethanoic acid acid phenol sulphurous acid 6 . (a) What information is provided by K₂ values? (i) In the spaces above CH₂COOH CH₂OH H₂SO3 H₂SO3(aq) + CH₂COOH(aq) = Acid I ...Base 2 Extent of dissociation of a weak ..acid. (b) When sulphurous acid and ethanoic acid are mixed together, an acid-base reaction takes place. K₂/moldm-3 1.70 x 10-5 1.28 x 10-10 1.50 x 10-² (0 label one conjugate acid-base pair as acid 1 and base 1. label the other conjugate acid-base pair as acid 2 and base 2. HSO₂ (aq) + CH3COOH₂(aq) Base 1 Acid 2 [2] (ii) Predict and explain the acid-base reaction that would take place if ethanoic acid were mixed with phenol. Include an equation in your answer. 2816:01 Jun06 CH₂COOH + C₂ H₂OH = CH₂ CD0%) + C₂ H₂OH₂ col (aq) (aq) (99) acid ... CH₂COOH is the. aad, so dissociates.... . Stronger. For Examiner's Use .[2] (c) The pH value of 0.0450 mol dm-3 hydrochloric acid is different from that of 0.0450 mol dm-3 ethanoic acid. Calculate the pH values of these two acids. Show all your working. -log (0.045) = 1.35 pH of HCl = pH of CH₂COOH = [H²] = √(1-7×10-5) × 0045 = x 7 8.746 x10-4 moldm-³ -3 -logl PH = = Ид +2ен соон мед 3.06 [5] (d) An excess of magnesium was added to 100 cm³ of 0.0450 mol dm-3 hydrochloric acid. The same mass of magnesium was added to 100 cm³ of 0.0450 moldm-³ ethanoic acid. 8-746x10-4) Both reactions produced 54 cm³ of hydrogen gas. measured at room temperature and pressure, but the reaction with ethanoic acid took much longer to produce this gas volume. Explain why the reactions produced the same volume of a gas but at different rates. Use equations in your answer. • Hydrochloric acid is a swong... → Ethanoic acid is a weak acid acid, so.. HC fully dissociates to form Ht ions, but CH₂COOH only pantally dissociates (so more Ht released with HC). → Same number of males of both HC and CH₂COOH reacting. So same volume of H₂ yielded. I mole of acid yields . halt..... ·2+ .[4] mole of H₂... +2CH₂COOH →→→> (CH ₂ COO") ₂2 Mg²" Jun06 + 2HCl → Mg@12+ H₂ + H₂ For Examiner's Use [Total: 14] [Turn over 4 . This question looks at two acids: methanoic acid, HCOOH, a weak organic acid; nitric acid, HNO3, a strong acid which can also act as a powerful oxidising agent. (a) Methanoic acid is a weak Brønsted-Lowry acid. Explain what is meant by a weak Bronsted-Lowry acid. A proton (H+) donor that to form H ions in water 6 pH = -log (1.987x10-2) = 2.70 [4²] =√1.58x10 4× 0-025 [H"] (b) Calculate the pH of a 0.025 mol dm3 solution of methanoic acid. Show your working. For HCOOH, K₂ = 1.58 x 10-4 mol dm-³. ..partially... : ⒸOCA 2007 12, ...dissociates.... 1.987 x10-3 -3 moldm 2.70 pH = [3] (c) Methanoic acid is a component of a buffer solution used in shampoos. The buffer solution can be made by mixing methanoic acid with another chemical. (i) State what is meant by a buffer solution. A buffer solution resists changes in pH upon. addition of small amounts of acid or alkali *154712106* [2] (iii) What factors determine the pH of a buffer solution? →Ka of weak acid → ratio of [A] to [HA] in the butter. solution. (ii) Suggest a chemical that could be added to methanoic acid to prepare a buffer solution. Explain your answer. 3 [1] Sodium metranoate A buffer solution can be famed from a weak acid and its conjugate. [12] base. [2] OCR OCR OCR OCR OCR OCR OCR OCR OCH OCR OCR OCR OCH JOCR OCR OCR OCR OCA OCR OCR OCR OCR OCR OCR OCR OCR OCR >CR CA CR JCR OCR OCR OCR OGR OCR OCR OCR OCR OCR OCA OCR OCR OCR OCR FOCR OCA OCF OCF OCF OCF OCF OCR OCR OCA OCR OCR. OCR OCR OCR OCR OCR OCR OCRI OCR OCR OCRI OCRI OCE ОСI OCRI OCR OCR OCR OCR OCR OCR OCR OCR, OCR OCR OCR OCR OCR OCR OCR! OCR OCR OCR OCR OCR OCR OCR OCA. OCR OCR OCF OC OCR OCR OCR OCR OCR OCR OCA! OCR OCR OCRI OCR OCR OCR OCR OCR OCR OCR OCR¹ OCR OCR OCR OCR OCP OC OCH OCR OCRI OCR OCA OCR (d) Nitric acid, HNO3, is sold by a chemical supplier as a 65% solution, by mass. As supplied, each cubic decimetre of this nitric acid has a mass of 1400g. Calculate the pH of this solution. 65x1400 100 910g pH = -logy (144) 2 OOCA 2007 (e) When dilute, nitric acid behaves as a typical acid. Write an equation for the reaction of nitric acid with limestone. ... 2. H.N.O.3.raat. Ca. Coges) + (9) Balan coff 7 ÷ 63 = 14.4 (2) (f) When nitric acid is added to methanoic acid, the acid-base equilibrium below is set up. Al -1.16 ANO +5 3 32 Use this equilibrium to explain what is meant by the term conjugate acid-base pairs. HNO3 = NO₂ HCOONHCOOH B1 HNO3 + HCOOH NO3 + HCOOH₂+ A2 pH = Species that can be interconvened by the transk of a ...Proton.... +4 -4 Ca(NO₂)₂ + CO₂ + H₂O₂ (ag) NO ₂ -1.16 (108 ( [3] entrated nitric acid is a powerful oxidising agent. Concentrated nitric acid oxidises phuto sulphuric acid. Nitrogen dioxide and another product are also formed. (i) Suggest a balanced equation for this reaction. HNO₂ + S → H₂S (ii) Deduce the change in oxidation number of nitrogen in this reaction. +1 -6 lost / gained ..probion.... ....... a *154712107* [3] H₂SO₂ + NO₂ + H₂O₁, [1] from +S. to +4 [1] [Total: 20] [Turn over Ethanedioic acid, H₂C₂O, is a dicarboxylic acid which occurs in many plants, for example in rhubarb leaves, and is used as a rust remover and strong descaler. The S structure of ethanedioic acid is shown below. H-O 12 HC ₂040 Ethanedioic acid is a much stronger acid than carboxylic acids such as ethanoic acid, having a pk of 1.38. The hydrogenethanedioate ion, HC₂O, is a weaker acid than ethanedioic acid, having a pK, of 4.28, although slightly stronger than ethanoic acid. ii (a) (i) Write an equation for the reaction of the hydrogenethanedioate ion with water to form an acidic solution. Include state symbols in your equation. + (ag) -O-H SONA CO K₁ = [H²] [C₂0₂²"] ка с негой з 2 24 + Ht (99) (ii) Write the expression for the acid dissociation constant, K, of the weak acid, HC₂0. 2.. (iii) A solution containing hydrogenethanedioate ions behaves as a typical weak acid. Use your answer to (a)(ii) and the pK, of the hydrogenethanedioate ion to calculate the pH of a 0.050 mol dm-³ solution of sodium hydrogenethanedioate, NaHC₂O₁. 2 ka = [H²] ² [HA] Ka= 10 -pka ka = 10 -4.28 [H] = √5.25x10 ³ ×0.050 [H'rg]am [H²] op едm egm (b) (i) State two approximations used in the calculation of pH in (a)(iii). [HA cags Jeam = [HA cag)] start. = = 5.25x10 PH = -log(t) 1.620 x10 = 2.79 · S *(ii) Explain why the calculation of the pH of a solution of sodium hydrogenethanedioate gives a more accurate value than a similar calculation for ethanedioic acid. -3 13 Turn over I I (c) 25 cm³ of a 0.050 mol dm-³ solution of sodium hydrogenethanedioate was titrated with a sodium hydroxide solution of the same concentration. (i) On the axis below, sketch the curve for this titration. 14 pH 14 12 10 8 6 4 2 0 40 Volume of NaOH / cm³ 20 60 *When 25 cm³ of a 0.050 mol dm-³ solution of ethanedioic acid is titrated with sodium hydroxide solution of the same concentration using phenolphthalein as the indicator, the end point is 50 cm³. When methyl yellow indicator is used, the colour changes at around 25 cm³. Using the information given at the start of the question and quoting data from page 19 of your data booklet, suggest why these volumes are different. (2) 5. (Total for Question 18 = 15 marks) 6 (c) Zinc carbonate, ZnCO3, reacts with dilute hydrochloric acid. A student reacts a sample of ZnCO3 with an excess of dilute hydrochloric acid in a test-tube. (i) Describe what the student would see during this reaction. →... •effervescence... 11 a........ .. ZnCD. dissolving ⒸOCR 2016 (ii) Write the equation for the reaction between ZnCO3 and dilute hydrochloric acid. H₂.0.¹¹ EnCO₂ + → ZnCl₂ + + Cọ, + HO 3(5) ................ (ag) cags 2 cg), (d) Magnesium will undergo redox reactions with aqueous salts of less reactive metals. (i) A student reacts magnesium with aqueous copper(II) sulfate. .8 Moso (aq) Mg(s) ++Zuso (aq) →→ Cu(s) + MgSO Explain, in terms of numbers of electron transferred, the redox processes taking place in this reaction. [1] Magnemium is oxidised and loses 2 elections. ...from... an oxidation state of Oin Myrs to +2. in Mg Souce Copper is reduced and gains 2 electrons from an oxidation state of +2 in.. CASO ay to Oike Cues). (ii) The student also noticed that the magnesium started fizzing. The student thought the fizzing was due to the magnesium reacting with water in the mixture. Write the equation for the reaction of magnesium with water. Include state symbols. Mg + 2 H₂O + Mg(OH)2 + 2 Ma H₂.0 [2] Turn over 12 (e) A student reacts 35.0 cm³ of 3.00 x 10-2 moldm-³ H₂SO₂(aq) with an excess of Al. An equation for this reaction is shown. 2Al(s) + 3H₂SO4(aq) →→→ A½(SO4)³(aq) Calculate the mass, in g, of Al2(SO4)3 formed in solution. Give your answer to three significant figures. Show your working. A = CXV n (1₂ 504)= 3 x 10-² M Mr I n X 0.035 = 1.05 × 10-³ mal x ⒸOCR 2016 3:1 ratio .. 1.05x10-3 3 + 3H₂(g) " 3.5 x10 - 4 mol of Al₂(SO₂), L3-sxlo-4x 342.3. L3-SX10-4 0.119805. mass= 0.120 g [4] (f) Compounds of calcium have many uses. (i) Identify a compound of calcium that could be used to convert a soil pH from 5.8 to 7.5. C₁ (OH) [1] "2" (ii) Calcium phosphide, Ca3P₂, is an ionic compound used in rat poison. Calcium phosphide can be prepared by reacting calcium metal with phosphorus, P4- Write the equation for the reaction of calcium with phosphorus to form calcium phosphide. 6. Ca + Py 2. Саз Ра . [1] Tz