ANSWERS Unit 14: Review Acids and Bases

Transcription

1 ANSWERS Unit 14: Review Acids and Bases 1) CH 3 COOH(aq) + H 2 0(l) H (aq) + CH 3 COO - (aq) In the equilibrium above, what are the two conjugate bases? A. CH 3 COOH and H 2 0 B. CH 3 COO - and H C. CH 3 COOH and H D. CH 3 C00 - and H 2 0 2) Which of the following is the weakest acid in aqueous solution? A. C 6 H 5 OH Ka = 1.3 x 10 - ' B. HCN Ka = 4.9 x 10-' C. H 2 Se Ka = 1.5 x 10-4 D. HF Ka = 6.9 x ) Which salt will produce the most alkaline solution when dissolved in water? A. KN0 3 B. MgCl 2 C. CH 3 CO 2 Na D. (NH 4 ) 2 S0 4 4) NH 3 (aq) + HN0 2 (aq) NH4 + (aq) + NO 2 - (aq) For this reaction, a Bronsted-Lowry acid is A. NH 3 (aq) because it contains the largest number of hydrogen atoms. B. NH 3 (aq) because it accepts a proton from HNO 2 (aq) C. HNO 2 (aq) because it has lone pairs of electrons on the oxygen atoms. D. HN0 2 (aq) because it donates a proton to NH 3 (aq). 5) The K a values of acids HX HY and HZ are given. What is the correct order when these acids are arranged in order of increasing strength (weakest first)? HX K a =1 x 10-4 HY K a =1x10-5 HZ K a = 3 x 10-5 A. HX < HY < HZ B. HX < HZ < HY C. HZ < HY < HX D. HY < HZ < HX 6) What is the ph of a buffer solution that contains 0.1 mol dm -3 HA and 0.1 mol dm -3 NaA? (HA K a =1x10-5 ) A. ph=3 B. ph = 4 C. ph = 5 D. ph = 6 7) 10.0 cm 3 of 0.200M H 3 PO 4 (aq) is converted into Na 2 HPO 4. What volume (in cm 3 ) of 0.200M NaOH(aq) is required? A B C D ) Which curve is produced by the titration of a 0.1M weak base with 0.1M strong acid? C

2 IB Chemistry HL: Unit 14: Acids & Bases ANSWERS 2/10 10) The ph value of a 1x10-3 M solution of sodium hydroxide is? A. 3. B. 8. C. 11. D ) Which salt would form a neutral solution when dissolved in water? A. FeCl 3 B. Na 2 CO 3 C. KBr D. NH 4 NO 3 12) Which of the following represents a conjugate acid-base pair in this reaction? CH 3 COOH(aq) H 2 O(l) CH 3 COO - (aq) + H 3 O + (aq) A. CH 3 COOH / H 2 O B. CH 3 COOH / CH 3 COO - C. CH 3 COOH / H 3 O + D. CH 3 COO - / H 3 O + 13) Which statement describes the Brønsted-Lowry behaviour of H 2 O molecules in aqueous solutions? A. They cannot act as either acids or bases B. They can act as acids but not bases C. They can act as acids or bases when reacting with each other D. They can act as acids when reacting with HCl molecules 14) 25.0 cm 3 of sulfuric acid solution reacts with 36.2 cm 3 of mol dm 3 sodium hydroxide solution. The concentration of the acid is C 15) When the ph of a solution changes from 2.0 to 4.0, the hydrogen ion concentration A. increases by a factor of 100. B. increases by a factor of 2. C. decreases by a factor of 2. D. decreases by a factor of ) Which will be the same for separate 1 mol dm 3 solutions of a strong acid and a weak acid? I. Electrical conductivity II. Concentration of H + ions A. I only B. II only C. Both I and II D. Neither I nor II 17) What is the K a of a 0.10M solution of a weak monoprotic acid if the [H + ]=2.0x10-3 mol dm 3 A. 2.0x10-2 moldm -3 B. 2.0 x10-4 moldm -3 C. 4.0 x10-5 mol dm -3 D. 4.0 x10-7 moldm -3 18) A buffer solution will be formed by combining equal volumes of 0.1 mol dm 3 solutions of A. hydrochloric acid and sodium hydroxide. B. hydrochloric acid and sodium ethanoate. C. ethanoic acid and sodium hydroxide. D. ethanoic acid and sodium ethanoate. 19) Which statement is not correct? A. Hydrochloric acid can have a ph value of zero B. ph paper contains more than one indicator C. The ph value of an acidic solution decreases when water is added to it D. Dilute hydrochloric acid conducts electricity

3 IB Chemistry HL: Unit 14: Acids & Bases ANSWERS 3/10 20) What is the relationship between and Ka and pk a? A 21) What is the concentration of OH ions (in M ) in an aqueous solution in which [H + ]=2.0 x 10-3 M? (K w =1.0 x10 14 M 2 ) A. 2.0x10 3 B. 4.0x10 6 C. 5.0x10 12 D. 2.0x ) A Brønsted-Lowry base is defined as a substance which A. accepts H + ions. B. produces OH - ions. C. conducts electricity. D. donates protons. 23) A buffer solution can be prepared by adding which of the following to 50 cm 3 of 0.10M CH 3COOH(aq)? I. 50cm of 0.10M CH 3COONa(aq) II. 25cm 3 of 0.10M NaOH(aq) III. 50cm 3 of 0.10M NaOH(aq) A. I only B. I and II only C. II and III only D. I, II and III 24) Which equation represents an acid-base reaction according to the Lewis theory but not according to the Brønsted-Lowry theory? B

4 IB Chemistry HL: Unit 14: Acids & Bases ANSWERS 4/10 Short Answers 1) (a) In the reaction 2H 2 0(l) H (aq) +OH - (aq) use the Bronsted-Lowry Theory to discuss the acidic and/or basic nature of water. [2] H 2 O + H 2 0 H 3 O + + OH - Water can donate a proton (BL acid) or receive a proton (BL base) (b) What is the conjugate base of the hydroxide ion, OH -? [1] O 2- (c) State one method which could be used to decide whether a solution of 0.10 mol dm -3 acid is strong or weak. Give the results expected in each case. [3] a) Measure conductivity of acid against 0.1M sol (equal volume) of a strong acid eg HCl [good conductor]and a weak acid eg ethanoic acid [bad conductor]. Which ever conductivity is closest to, it is that sort of acid. b) As above but compare reaction rates with strong/weak acid. Slower rate = weak, faster = strong. (d) In a titration experiment, 40.0 cm 3 of mol dm -3 NaOH was added to 60.0 cm 3 of mol dm -3 CH 3 (CH 2 ) 3 COOH (K a = 1.38 x 10-5 mol dm -3 ). Calculate the ph of this mixture. [4] How many moles of acid remains unreacted within the total volume? Conc x vol = mol of NaOH used = 0.15M x 40/1000 = 6x10-3 mol. Since ratio 1:1 6x10-3 mol of acid used up. Initial moles of acid = 0.2M x 60/1000 = 0.012mol Mol of acid remaining = x10-6 = 6x10-3 mol acid remaining Conc of acid remaining = mol/volume = 6x10-3 /[(40+60)/1000] =0.06M Ka = [H+][ CH 3 (CH 2 ) 3 COO-] / [CH 3 (CH 2 ) 3 COOH] = x 2 / 0.06M x since x is small compared to 0.06,, the bottom of equ can be simplified. X = 9.1 = [H+], ph=3

5 IB Chemistry HL: Unit 14: Acids & Bases ANSWERS 5/10 2) (a) Write an equation to show the ionisation of propanoic acid in water. [1] CH 3 CH 2 COOH + H 2 O CH 3 CH 2 COO- +H 3 O+ (b) Give the equilibrium expression for this reaction. [1] ka = [CH 3 CH 2 COO-] [H 3 O+] / [CH 3 CH 2 COOH] (c) Using information from Table 16 in the Data Booklet, determine the ph of a mol dm -3 solution of propanoic acid. State the approximation that you have made in arriving at your answer. ka = x 2 /0.2M x (ICE table), x is v small compared to 0.2M so assume 0.2M-x=0.2M to avoid quadratic pka = 4.87, ka = 1.3x10-5, so x = 1.6x10-3 = [H+], ph = 2.8 (d) What mass of sodium propanoate, Na + CH 3 CH 2 C00 -, is required in 500 cm 3 of a solution of mol dm- 3 propanoic acid to give a ph of 4.87? This is a buffer, so ph = pka + log [CH 3 CH 2 COO-] / [CH 3 CH 2 COOH] NB NOT IN DATA BOOK= LEARN IT! 4.87 = log [CH3CH2COO-] / 0.200M So, log [CH3CH2COO-] / 0.200M must = 0, [CH3CH2COO-] / 0.200M =1, thus CH3CH2COO- = 0.200M. Conc = mol/vol, 0.200M = mol/(500/1000), 0.1 mol of sodium propanoate, Na + CH 3 CH 2 C00 - needed. Mass = mol x Mmass = 0.1 x 96g/mol, =9.6g (e) Explain, with equations, why the ph of the above solution in (d) will remain relatively constant even if small amounts of acid or alkali are added. [2] CH 3 CH 2 COOH + H 2 O CH 3 CH 2 COO- +H 3 O+ Na + CH 3 CH 2 C00 - Na+ + CH 3 CH 2 C00 - And extra acid [H+] removed in 1st equ to form proanoic acid, excess alkali [OH-] removed by reaction with H3O+ in 1 st equation.

6 IB Chemistry HL: Unit 14: Acids & Bases ANSWERS 6/10 3) (a) State the definitions of an acid and a base according to the Brønsted.Lowry and Lewis theories. Give a different equation to illustrate an acid.base reaction for each theory, identifying clearly the acid and the base. State the type of bond formed in a Lewis acid-base reaction. [7] (b) State the difference between a strong acid and a weak acid and give one example of each. [2] (c) Explain qualitatively how an acid-base indicator works. [4] (d) Sodium hydroxide solution is added to aqueous hydrochloric acid. The graph of ph against volume of sodium hydroxide solution added is shown below: Sketch clearly labelled corresponding graphs for each of the following and suggest a suitable indicator in each case: (i) The addition of sodium hydroxide solution to aqueous ethanoic acid. [3]

7 IB Chemistry HL: Unit 14: Acids & Bases ANSWERS 7/10 (ii) The addition of ammonia solution to aqueous hydrochloric acid. [3] (e) 30cm 3 of 0.100M CH 3 COOH is placed in a beaker and mixed with 10cm 3 of moldm 3 NaOH (i) Explain, with the help of an equation, how the solution formed acts as a buffer solution when a small quantity of acid is added to it. [2] (ii) Calculate the ph of the buffer solution (Ka of CH 3 COOH = 1.74x10-5 moldm -3 ).[4] May 2001 SL p2

8 IB Chemistry HL: Unit 14: Acids & Bases ANSWERS 8/10 4)

9 IB Chemistry HL: Unit 14: Acids & Bases ANSWERS 9/10 [May 2003 HL p2]

10 IB Chemistry HL: Unit 14: Acids & Bases ANSWERS 10/10