7.2. Brønsted–Lowry theory of acids and bases
A subsection of Chemistry, 9701, through 7. Equilibria
Listing 10 of 133 questions
Explain what is meant by the term buffer solution. Write an expression for the acid dissociation constant, Ka, for ammonium ions, NH4 +. Ka =  Write two equations to describe how a solution containing ammonium ions, NH4 +, and ammonia, NH3, can act as a buffer. The numerical value of Ka for NH4 +is 5.6×10–10 at 298 K. A buffer solution was prepared by adding 0.80 dm3 of 0.25 mol dm–3 ammonia, an excess, to 0.20 dm3 of 0.20 mol dm–3 hydrochloricacid. Calculate the pH of the buffer solution formed at 298 K.  pH = 
9701_s20_qp_41
THEORY
2020
Paper 4, Variant 1
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Explain what is meant by the term buffer solution. Write an expression for the acid dissociation constant, Ka, for ammonium ions, NH4 +. Ka =  Write two equations to describe how a solution containing ammonium ions, NH4 +, and ammonia, NH3, can act as a buffer. The numerical value of Ka for NH4 +is 5.6×10–10 at 298 K. A buffer solution was prepared by adding 0.80 dm3 of 0.25 mol dm–3 ammonia, an excess, to 0.20 dm3 of 0.20 mol dm–3 hydrochloricacid. Calculate the pH of the buffer solution formed at 298 K.  pH = 
9701_s20_qp_43
THEORY
2020
Paper 4, Variant 3
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Ibuprofen is one of the most commonly used non-steroidal anti-inflammatory drugs, used to treat chronic arthritic pain caused by inflammation of the joints. ibuprofen Draw a circle around any chiral centrein the above structure. Write down the molecular formula of ibuprofen. Calculate the Mr of ibuprofen and use it to calculate how many grams are needed to make 100 cm3 of a 0.15 mol dm–3 solution. Vigorous oxidation of ibuprofen produces a dibasic acid A. A solution containing 0.10 g of A required 12.0 cm3 of 0.10 mol dm–3 NaOH for neutralisation. Suggest a structure for A, showing your working. The Ka value for ibuprofen is 6.3 ×10–6mol dm–3. Write an expression for Ka. Use the Ka value to calculate the pH of a 0.15 mol dm–3 solution of ibuprofen. Use CH CH2 CH3 CH3 C O CH OH CH3 [T To avoid problems with digestive irritation over a long period of use, research is being carried out into ways of administering ibuprofen using skin patches. For this use the compound is dissolved in a hydrophilic gel which acts as a buffer. What do you understand by the term buffer? The buffer used in the pharmaceutical preparation is a solution containing Na2HPO4 and NaH2PO4. These salts contain the HPO4 2– and H2PO4 – ions respectively. Write equations to show how this buffer reacts with H+ ions, OH– ions. A buffer solution containing equal concentrations of the two sodium phosphate salts has a pH of 7.20. Calculate the pH of a pharmaceutical preparation containing 0.002 mol dm–3 of Na2HPO4 and 0.005 mol dm–3 of NaH2PO4.
9701_w06_qp_4
THEORY
2006
Paper 4, Variant 0
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When an aqueous solution of compound G, NH2CH2CH2CH2NH2, is titrated with HCl, two successive acid-base reactions take place. Write equations for these two acid-base reactions. A 0.10 mol dm–3 solution of G has a pH of 11.3. When 30 cm3 of 0.10 mol dm–3 HCl is added to 10 cm3 of a 0.10 mol dm–3 solution of G, the final pH is 1.6. Using the following axes, sketch the pH changes that occur during this addition of HCl. pH volume of HCladded / cm3
9701_w10_qp_43
THEORY
2010
Paper 4, Variant 3
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The enthalpy change of solution, , of the Group2 sulfates becomes more endothermic down the group. State and explain the trend in the solubility of the Group 2 sulfates down the group. Write the expression for Kw , the ionic product of water. Kw =  The numerical value of Kw increases with increasing temperature. Place a tick () in the appropriate column in each row to show the effect of increasing the temperature of water on the pH and on the ratio [H+] : [OH–]. effect of increasing temperature of water decrease stay the same increase pH ratio [H+] : [OH–]  An aqueous solution of sodium hydroxide has a pH of 13.25 at 298 K. Calculate the concentration of this sodium hydroxide solution.  concentration = mol dm–3 Buffer solutions are used to regulate the pH of a solution to keep its pH value within a narrow range. Write two equations to describe how hydrogencarbonate ions, HCO3 –, and carbonic acid, H2CO3, control the pH of blood. The Ka for ethanoic acid is 1.75×10–5 mol dm–3 at 298 K. When ethanoic acid is dissolved in water, an equilibrium mixture containing two acid‑base pairs is formed. Write an equation for this equilibrium. In the boxes label each species acidic or basic to show its behaviour in this equilibrium. CH3CO2H + +  A buffer solution was prepared by adding 30.0 cm3 of 0.25 mol dm–3 ethanoic acid, an excess, to 20.0 cm3 of 0.15 mol dm–3 sodiumhydroxide. Calculate the pH of the buffer solution formed at 298 K. Give your answer to one decimal place.  pH = Titration curves for two different acid-base reactions, M and N, are shown. pH volume of acid added / cm3 pH volume of acid added / cm3 reaction M reaction N Use the titration curve for reaction M to deduce the volume of acid added at the end‑point for this titration.  volume of acid added at the end-point = cm3 The table shows some acid-base indicators. name of indicator pH range of colour change malachite green 0.2–1.8 bromocresol green 3.8–5.4 bromothymol blue 6.0–7.6 thymolphthalein 9.3–10.6 Name a suitable indicator for each of the acid-base titrations M and N. Explain your answers. reaction M reaction N explanation  
9701_w18_qp_41
THEORY
2018
Paper 4, Variant 1
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The enthalpy change of solution, , of the Group2 sulfates becomes more endothermic down the group. State and explain the trend in the solubility of the Group 2 sulfates down the group. Write the expression for Kw , the ionic product of water. Kw =  The numerical value of Kw increases with increasing temperature. Place a tick () in the appropriate column in each row to show the effect of increasing the temperature of water on the pH and on the ratio [H+] : [OH–]. effect of increasing temperature of water decrease stay the same increase pH ratio [H+] : [OH–]  An aqueous solution of sodium hydroxide has a pH of 13.25 at 298 K. Calculate the concentration of this sodium hydroxide solution.  concentration = mol dm–3 Buffer solutions are used to regulate the pH of a solution to keep its pH value within a narrow range. Write two equations to describe how hydrogencarbonate ions, HCO3 –, and carbonic acid, H2CO3, control the pH of blood. The Ka for ethanoic acid is 1.75×10–5 mol dm–3 at 298 K. When ethanoic acid is dissolved in water, an equilibrium mixture containing two acid‑base pairs is formed. Write an equation for this equilibrium. In the boxes label each species acidic or basic to show its behaviour in this equilibrium. CH3CO2H + +  A buffer solution was prepared by adding 30.0 cm3 of 0.25 mol dm–3 ethanoic acid, an excess, to 20.0 cm3 of 0.15 mol dm–3 sodiumhydroxide. Calculate the pH of the buffer solution formed at 298 K. Give your answer to one decimal place.  pH = Titration curves for two different acid-base reactions, M and N, are shown. pH volume of acid added / cm3 pH volume of acid added / cm3 reaction M reaction N Use the titration curve for reaction M to deduce the volume of acid added at the end‑point for this titration.  volume of acid added at the end-point = cm3 The table shows some acid-base indicators. name of indicator pH range of colour change malachite green 0.2–1.8 bromocresol green 3.8–5.4 bromothymol blue 6.0–7.6 thymolphthalein 9.3–10.6 Name a suitable indicator for each of the acid-base titrations M and N. Explain your answers. reaction M reaction N explanation  
9701_w18_qp_43
THEORY
2018
Paper 4, Variant 3
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Silver sulfide, Ag2S, is very insoluble in water. Write an expression for the solubility product, Ksp, of Ag2S. Ksp =  The solubility of Ag2Sin water at 298 K is 1.16 × 10–17 mol dm–3. Calculate the numerical value of the solubility product, Ksp, of Ag2Sat 298 K.  Ksp = Calculate the minimum volume of water needed to dissolve 1.00 g of Ag2Sunder standard conditions.  volume = dm3 Bromic(acid, HOBr, is a weak acid. Its Ka is 2.0 × 10–9 mol dm–3. Calculate the pH of 0.20 mol dm–3 HOBr.  pH = 5.0 cm3 of 0.20 mol dm–3 potassium hydroxide, KOH, are added to 20.0 cm3 of 0.20 mol dm–3 HOBr. Calculate the pH of the buffer solution produced.  pH = 
9701_w19_qp_41
THEORY
2019
Paper 4, Variant 1
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