Name period Unit 9: acid/base equilibrium 1. What is the difference between the Arrhenius and the BronstedLowry definition of an acid? Arrhenious acids give H + in water BronstedLowry acids are proton donors 2. Why are strong acids also strong electrolytes (strong conductors or electricity)? Strong acids completely ionize so there are a lot of ions in solution so it will conduct 3. Name each of the following and label them as a weak/strong acid or weak/strong base a. H 3 PO 4 b. Ca(OH) 2 c. Al(OH) 3 d. HNO 3 e. NH 3 f. LiOH a.phosphoric acid, weak b.calcium hydroxide, strong base c. aluminum hydroxide, weak d. nitric acid, strong e. ammonia, weak base f. lithium hydroxide, strong 4. Which of the following solutions will be the poorest conductor of electricity? a. HCl b. CH 3 COOH c. NaOH d. NaCl 5. Which of the following has the highest concentration of [H + ]? 0.1 M HCl or 0.1 M HF 6. What is the [OH ] in each? a. 0.5 mol/l NaOH b. 0.03 mol/l Ca(OH) 2 0.5M 0.06M 7. NH 3 (g) and HCl(g) react to form the ionic solid NH 4 Cl(s). Which substance is the bronstedlowry acid in this reaction? Which is the BronstedLowry base? Acid HCl; basenh 3 8. What is the difference between the Arrhenius and the BronstedLowry definition of a base? Arrhenius bases give OH in water BronstedLowry bases are proton acceptors 9. Give the conjugate base of the following a. H 2 SO 3 b. HC 2 H 3 O 2 c. H 2 AsO 4 + d. NH 4 e. H 2 O HSO 3 C 2 H 3 O 2 2 HAsO 4 NH 3 OH 10. Give the conjugate acid of the following a. C 7 H 5 O 2 b. BrO 2 c. NH 3 d. H 2 AsO 4 2 e. CO 3 HC 7 H 5 O 2 HBrO 2 + NH 4 H 3 AsO 4 HCO 3 11. Identify the BronstedLowry acid and the BronstedLowry base in each reaction and the conjugate acid/base pair a. NH 4 + + CN HCN + NH 3 acid base conjugate acid conjugate base b. (CH 3 ) 3 N + H 2 O (CH 3 ) 3 NH+ + OH base acid conjugate acid conjugate base c. HCHO 2 + PO 4 3 CHO 2 + HPO 4 2 acid base conjugate base conjugate acid 12. a. HC 2 O 4 is amphoteric. Write a balanced equation showing how it acts as an acid toward water and another equation showing how it acts as a base toward water. HC 2 O 4 + H 2 O H 2 C 2 O 4 + OH HC 2 O 4 + H 2 O C 2 O 4 2 + H 3 O + b. What is the conjugate acid of HC 2 O 4? What is the conjugate base? H 2 C 2 O 4 ; C 2 O 4 2
13. Calculate the ph of a solution in which the [H 3 O + ] equals and tell if they are acidic or basic: a) 1.0 M 0.00 acidic b) 0.1M 1.0 acidic c) 0.01 M 2.0 acidic d) 1.0 X 10 7 M 7.0 neutral e) 3.0 X 10 7 M 6.50 acidic f) 4.0 X 10 10 M 9.40 basic 14. Calculate the [OH ] of a solution if the ph= a) 11.00 1.0 x 10 3 M c) 4.68 4.79 x10 10 M b) 7.89 7.76 x 10 7 M d) 3.54 3.47 x 10 11 M 15. What is the hydrogen ion concentration of a solution with a ph of 5? Is it an acid or a base? 1 x 10 5 M; acid 16. Which of the following solutions would have the lowest ph? 0.10 M HCl, 0.009 M HNO 3, 0.340 M HBr 17. Determine the ph of the following solutions and label if they are acidic or basic a. [H + ] = 1 x 10 6 b. [OH ] = 1 x 10 3 c. [H + ] = 2.3 x 10 3 d. [OH ] = 3.4 x 10 6 6 ; acidic 11; basic 2.64; acidic 8.5; basic 18. Determine the [H + ] for the following solution and label if they are acidic or basic a. [OH ] = 1 x 10 8 b. ph = 3.4 c. ph = 2 d. poh = 5 1 x 10 6, acidic 4.0 x 10 4, acidic 1 x 10 2, acidic 1 x 10 9, basic 19. What is the ph of a 0.0034 M HNO 3 solution? 2.50 20. Benzoic acid, C 6 H 5 COOH, is an organic acid whose sodium salt, C 6 H 5 COONa, has long been used as a safe food additive to protect beverages and many foods against harmful yeasts and bacteria. The acid is monoprotic. Write the equation for the K a of the acid. Ka = [H + ][C 6 H 5 COO ]/[C 6 H 5 COOH] 21. What is the percent ionization of a 0.15 M solution of HF? What is the ph of this ionization? (K a =6.8 x 10 4 ) 6.7% ; 2.00 22. Lactic acid, HC 8 H 7 O 2, has one acidic hydrogen. A 0.10M solution has ph of 2.44. Calculate the K a. 1.4 x 10 4 23. A particular sample of vinegar has a ph of 2.90. Assuming the vinegar contain only acetic acid (K a = 1.8 x 10 5 ), calculate the concentration of acetic acid. 0.089 M 24. Find the ph of the following solutions: (K a for propionic acid = 1.3 x 10 5 and K b for pyridine = 1.7 x 10 9 ) a. 0.095 M propionic acid, HC 3 H 5 O 2 b. 0.120 M pyridine, C 5 H 5 N 2.95 9.15 25. Saccharine is a weak monoprotic acid with a pka = 2.32. What is the ph of a 0.10 M solution? 1.71 26. Write a chemical equation and K b expression for the ionization of the following bases b. (CH 3 ) 2 NH 2 b. CO 3 (CH 3 ) 2 NH +H 2 O (CH 3 ) 2 NH + 2 + OH 2 CO 3 + H 2 O HCO 3 + OH K b = [(CH 3 ) 2 NH + 2 ][ OH ]/[(CH 3 ) 2 NH ] K b = [HCO 3 ][ OH ]/ [CO 2 3 ] 27. Calculate the molar concentration of hydroxide ions in 0.075 M solution of C 2 H 5 NH 2 (K b =6.4x10 4 ). Calculate the ph of this solution. 6.9 x 10 3 ; 11.84 28. Write the hydrolysis reaction for the salt, NaCN. CN + H 2 O HCN + OH 29. Calculate the hydroxide ion concentration an ph of a 0.10 M NaCN solution (K a for HCN =4.9 x 10 10 ) 11.15 30. Predict whether aqueous solutions of the following compounds are acidic, basic or neutral c. NH 4 Br b. FeCl 3 c. Na 2 CO 3 d. KClO 4 Acidic acidic basic neutral
31. HCN is a weak acid (K a = 6.2 x 10 10 ). NH 3 is a weak base (K b = 1.8 x 10 5). A 1.0 M solution of NH 4 CN would be (A) strongly acidic (B) weakly acidic (C) neutral (D) weakly basic 32. Which of the following mixtures forms a buffer when dissolved in 1.0 L of water? (A) 0.2 mol NaOH + 0.2 mol HBr (C) 0.4 mol HNO 2 + 0.2 mol NaNO 2 (B) 0.2 mol NaCl + 0.3 mol HCl (D) 0.5 mol NH 3 + 0.5 mol NH 4 Cl 33. A buffer solution is prepared in which the concentration of NH 3 is 0.30 M and the concentration of NH 4 + is 0.20 M. What is the ph of this solution? The equilibrium constant, K b for NH 3 equals 1.8 x 10 5. 9.43 34. Calculate the ph a buffer made by mixing 85 ml of 0.13M lactic acid with 95 ml of 0.15 M sodium lactate (K a for lactic acid= 1.4 x 10 4 ) 3.96 35. a) A formic acid, sodium formate solution is made up by dissolving 0.2 mole of formic acid and 0.3 mole of sodium formate in 500 ml of water? What ph will the resulting solution be at? (K a for formic acid= 1.8 x10 4 ) 3.92 b. Explain why buffers can resist changes to ph when acids or bases are added. Buffers has an acid and a base in the mixture so when an acid is added to the buffer it is neutralized by the base already in the buffer and when a base is added to the buffer it is neutralized by the acid already in the buffer 36. Buffer 1: a solution containing 0.10 M NH 4 Cl and 1 M NH 3. Buffer 2: a solution containing 1 M NH 4 Cl and 0.10 M NH 3. Which buffer would be better able to hold a steady ph on the addition of strong acid, buffer 1 or buffer 2? Explain. Buffer 1 because it has a higher concentration of the base to react with the strong acid being added 37. What color is phenolphthalein in an acid? In a base? Clear; pink 38. For which titration would the use of phenolphthalein introduce a significant error? D 39. A 0.100 M solution of acetic acid (K a = 1.8 x 10 5 ) is titrated with a 0.1000 M solution of NaOH. What is the ph when 50% of the acid has been neutralized? (A) 2.38 (B) 4.74 (C) 5.70 (D) 7.00 40. For the titration of 50.0 ml of 0.200 M HNO 3 with 0.100 M NaOH calculate the ph of the solution at the following selected points of the titration: a) NO NaOH has been added: 0.70 b) 10.0 ml of NaOH has been added: 0.82 c) 50.0 ml (total as opposed to additional) of NaOH has been added: 1.30
d) 100.0 ml (total) of NaOH has been added: 7.00 e) 200.0 ml (total) of NaOH has been added: 12.6 41. gas (HCN), a powerful respiratory inhibitor, is highly toxic. It is a very weak acid (Ka = 6.2 10 10 ) when dissolved in water. If a 50.0 ml sample of 0.100 M HCN is titrated with 0.100 M NaOH, calculate the ph of the solution a) after 8.00 ml of 0.100 M NaOH has been added. 8.47 b) at the halfway point of the titration. 9.21 c) at the equivalence point of the titration. 10.95 Review: 42. Consider these slightly soluble salts: i) PbS K sp = 8.4 x 10 28 ii. PbSO 4 K sp = 1.8 x 10 8 iii. Pb(IO 3 ) 2 K sp = 2.6 x 10 13 a) Which is the most soluble? PbSO 4 b) Calculate the solubility in moles/l for PbSO 4. 1.34 x 10 4 M c) How many grams of PbSO 4 dissolve in 1 L of solution? 0.041 g d) What is the concentration in moles/l of PbS in a saturated solution of the salt? 2.9 x10 14 M 43. Write net ionic equations for the following reactions a. phosphoric acid + potassium hydroxide H 3 PO 4 + 3K + + 3OH 3K + +PO 4 3 + 3H 2 O b. i. Solid magnesium hydroxide is added to a solution of hydrobromic acid. Mg(OH) 2 + 2H + 2 H 2 O + Mg 2+ ii. What volume, in ml, of 2.00M hydrobromic acid is required to react completely with 0.10 mol of solid magnesium hydroxide? 100 ml c. hydrobromic acid + magnesium carbonate 2H + + MgCO 3 Mg 2+ + CO 2 + H 2 O 44. Hydrogen gas burns in air according to the equation. 2 H 2 (g) + O 2 (g) 2 H 2 O(l) (a) Calculate the standard enthalpy change, ΔH 298 ⁰, for the reaction represented by the equation above. (The molar enthalpy of formation, ΔH f ⁰ for H 2 O(l) is 285.8 kj mol 1 at 298 K.) 571.6 kj mol 1 (b) Calculate the amount of heat, in kj, that is released when 10.0 g of H 2 (g) is burned in air. 1.42 10 3 kj (c) Given that the molar enthalpy of vaporization, ΔH vap ⁰, for H 2 O(l) is 44.0 kj mol 1 at 298 K, what is the standard enthalpy change, ΔH 298 ⁰, for the reaction 2 H 2 (g) + O 2 (g) 2 H 2 O(g)? 483.6 kj 45. rate = k[no] 2 [Cl2] k= 9.12 M 2 hr 1 46. What is the value of Kc for the reaction 2A(g) + 3B(g) 2C(g) + D(g) if at equilibrium [A] = 0.60 M, [B] = 0.30 M, [C] = 0.10 M and [D] = 0.50 M? Kc = 0.051 47. Consider the reaction 2NO 2 (g) N 2 O 4 (g). A reaction is initiated at 100 ⁰C with 2.35 moles NO 2 in a 3.0 L reactor (no N 2 O 4 present). If the equilibrium concentration of N 2 O 4 is 0.25 M, What is the value of Kc at 100 C? Kc = 3.2
Practice FRQ: 48. A 1.22 g sample of a pure monoprotic acid, HA, was dissolved in distilled water. The HA solution was then titrated with 0.250 M NaOH. The ph was measured throughout the titration, and the equivalence point was reached when 40.0 ml of the NaOH solution had been added. The data from the titration are recorded in the table below. Volume of 0.250 M NaOH added (ml) ph of titrated solution 0.00? 10.0 3.72 20.0 4.20 30.0? 40.0 8.62 50.0 12.40 a) Explain how the data in the table above provide evidence that HA is a weak acid rather than a strong acid. The ph at the equivalence point is above 7, which indicates that HA is a weak acid. b) Write the balanced netionic equation for the reaction that occurs when the solution of NaOH is added to the solution of HA. HA+ OH A + H2O c) Calculate the number of moles of HA that were titrated. At the equivalence point, the number of moles of base added equals the number of moles of acid initially present. 0.0400 L x 0.250 mol NaOH x 1 mol HA = 0.0100 mol HA 1 L 1 mol NaOH d) Calculate the molar mass of HA. MM= mass of acid= 1.22 g= moles of acid 0.0100mol 122g/mol The equation for the dissociation reaction of HA in water is shown below. HA(aq) + H 2 O(l) H 3 O + (aq) + A (aq) K a = 6.3 x 10 5 e) Assume that the initial concentration of the HA solution (before any NaOH solution was added) is 0.200 M. Determine the ph of the initial HA solution. Ka = [H + ][A ]/[HA] 6.3 x 10 5 = x 2 /(0.200x) ; assume that x << 0.200 M X = [H + ]= 3.5x10 3 M ph = log (3.5 x 10 3 ) = 2.45 f) Calculate the value of [H 3 O + ] in the solution after 30.0 ml of NaOH is added and the total volume of the solution is 80.0 ml HA + OH A + H 2 O Mol before rxn 0.0100 0.0075 0 Mol after the rxn 0.00250 0 0.00750 [HA] = 0.00250 mol/0.0800l = 3.13 x 10 2 M [A] = 0.00750 mol/0.0800 L = 9.38 x 10 2 M Ka = [H + ][A ]/[HA] 6.3 x 10 5 = x(9.38x10 2 + x)/(3.13 x 10 2 x) assume x is small 6.3 x 10 5 = x(9.38x10 2 )/(3.13 x 10 2 ) X= [H + ] = 2.1x10 5 M