Assignment 17 A 1- (i) (ii)

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1 Assignment 17 A 1- Does the ph increase, decrease, or remain the same on addition of each of the following? (i) NaNO 2 to a solution of HNO 2, (ii) HCl to a solution of NaC 2 H 3 O 2 a) (i) increase, (ii) remain the same b) (i) increase, (ii) increase c) (i) decrease, (ii) decrease d) (i) decrease, (ii) increase e) (i) increase, (ii) decrease (The NO 2 causes the formation of more HNO 2, which lowers the ph. The added H + from the HCl reacts with the OH from the hydrolysis of the C 2 H 3 O 2 ion, increasing the [H + ] and decreasing the ph.) 2- Calculate the ph of a buffer that is 0.20 M in formic acid and 0.15 M in sodium formate (K a = ). a) 0.82 b) 8.33 c) 3.62 d) e) 0.70 (The hydrogen-ion concentration equals M.) 3- A 50.0-mL sample of 0.50M acetic acid, HC 2 H 3 O 2, is titrated with a M NaOH solution. Calculate the ph after 25.0 ml of the base have been added (K a = ). a) 3.99 b) 3.92 c) 0.55 d) 2.62 e) 2.52 (This is the ph of the buffer formed by the acid-base reaction.) 4- If the molar solubility of CaF 2 at 35 C is mol/l, what is K sp at this temperature? a) b) c) d) (The [Ca 2+ ] = M and the [F ] = M. The K sp expression is [Ca 2+ ][F ] 2.) 5- Which of the following would you not expect to be more soluble in acid than in pure water? a) AgCl b) CuCN c) AlPO 4 d) BaCO 3 e) FeS (In order for a solid to be more soluble in acid than in pure water, the anion of the solid compound must be a weak base. Chloride is a neutral anion.) 6- If 0.1 M aqueous solutions of the following pairs of substances are mixed together, which pair could result in the formation of a precipitate? a) NaOH, HCl b) KCl, Al(NO 3 ) 3 c) Na 2 S, FeCl 3 d) NaOH, KNO 3 e) Ni(NO 3 ) 2, Mg(NO 3 ) 2

2 (Iron will form a precipitate with a basic sulfide solution.) 7- Which one of the following changes is incorrect? a) CaCl 2 added to an HCl solution does not change the ph. b) NaBr added to a solution of HBr will raise the ph. c) NaC 2 H 3 O 2 added to a solution of HC 2 H 3 O 2 raises the ph. d) HNO 3 added to CaC 2 H 3 O 2 lowers the ph. e) KNO 3 added to an HCl solution does not change the ph. (It does nothing to the ph, since HBr is a strong acid.) 8- How many moles of sodium hypobromite, NaBrO, should be added to 1.00 L of M hypobromous acid, HBrO (K a = ), to form a buffer solution of ph 8.80? Assume that no volume change occurs when the NaBrO is added. a) A buffer solution with this ph cannot be formed. b) 0.13 mol c) 0.32 mol d) 0.20 mol (The [BrO ] = K a [acid]/[h + ].) 9- Calculate the molar concentration of bromide ions in a saturated solution of mercury(ii) bromide (K sp = ). a) M b) M c) M d) M (The K sp equals 4S 3, where S equals the molar solubility. The concentration of bromide ion equals 2S.) 10- Calculate the solubility in mol/l of Cu(OH) 2 at ph = 7.0. K sp = a) b) c) d) e) (The hydroxide-ion concentration at ph 7.0 is M.) 11- K sp for AgI = , and K f for Ag(CN) 2 = Calculate the equilibrium constant for the following reaction: AgI(s) + 2CN (aq) Ag(CN) 2 (aq) + I (aq) a) b) c) 8.3 x d) e) (The reaction of interest is the algebraic sum of the dissolution of AgI and the formation reaction of Ag(CN) 2. Therefore, the equilibrium constant for the reaction of interest is the product of K sp K f.) 12- Calculate the minimum ph needed to precipitate Ni(OH) 2 so completely that the concentration of Ni 2+ is less than 1.0 µg/l (1.0 part per billion (ppb)) (K sp = ). a) b) 9.49 c) 7.97 d) 3.01

3 e) (The maximum allowable hydroxide-ion concentration is ) 13- In the course of various qualitative analysis procedures, the following mixture is encountered: Mg 2+ and K +. Suggest how this mixture might be separated. a) add dilute HCl b) It is not possible to separate the ions. c) add (NH 4 ) 2 HPO 4 to a basic solution d) add (NH 4 ) 2 S at ph 8 e) add 0.2 M HCl and H 2 S (This will precipitate only the Mg 2+.) 14- Consider the titration of 50. ml of M HN 3 (K a = ) with M NaOH. Calculate the ph of the solution after the addition of 29.7 ml of NaOH solution. a) 7.00 b) 4.51 c) 2.62,. d) 4.59 e) 2.88 (This is the ph of the buffer formed at the half-neutralization point in the titration.) 15- Calculate the ph of a solution that is both 0.50 M CH 3 COOH and 0.50 M CH 3 COONa (K a (CH 3 COOH) is ). a) 7.00 b) 2.52 c) 4.75 d) e) 9.26 (When the concentrations of the salt and acid are equal, the ph equals the pk a of the acid.) 16- What is the ph of a buffer prepared from 0.30 M formic acid and 0.15 M potassium formate (K a = )? a) 2.13 b) 4.04 c) 3.44 d) 3.74 e) 1.87 (The dissociation of formic acid is negligible.) 17- How many moles of sodium hypobromite, NaBrO, should be added to 1.00 L of M hypobromous acid, HBrO (K a = ), to form a buffer solution of ph 8.80? Assume that no volume change occurs when the NaBrO is added. a) 0.20 mol b) 0.13 mol c) 0.32 mol d) A buffer solution with this ph cannot be formed. (The [BrO ] = K a [acid]/[h + ]. 18- A buffer contains 0.30 M acetic acid and 0.20 M sodium acetate. What is the ph of the buffer as prepared and after mol/l of a strong acid or mol/l of a strong base are added (K a = )? a) 4.57, 4.46 with acid, 4.68 with base b) 4.57, 4.56 with acid, 4.56 with base c) 4.92, 5.03 with acid, 4.81 with base

4 d) 4.56, 4.68 with acid, 4.46 with base e) 4.74, 4.61 with acid, 4.88 with base (The added H + reacted with the acetate to lower the ph slightly, and the added OH reacted with the acetic acid to raise the ph slightly.) 19- Estimate the ph at the equivalence point of an HOAc solution if 25.5 ml of this solution required 37.5 ml of M NaOH to reach the equivalence point (K a = ). a) 4.31 b) 12.2 c) 7.00 d) 8.88 e) 2.07 (NaOAc is the salt of a weak acid, so its solution would be slightly basic.) 20- Calculate the ph of a solution formed by adding 50.0 ml of 6.0 M NH 3 to 75.0 ml of 1.0 M HCl. (K b (NH 3 ) = ). a) 4.27 b) 5.22 c) 9.26 d) 9.73 e) 8.79 (This is the ph of the buffer formed after the acid-base reaction.) 21- The solubility of Mg(OH) 2 is mol/l. Determine the K sp for Mg(OH) 2. a) b) c) d) e) (The hydroxide-ion concentration is double the magnesium-ion concentration. The K sp = [Mg 2+ ][OH ] 2.) 22- Calculate the number of mg of silver in 250 ml of a saturated solution of Ag 2 CO 3 (K sp = ). a) 14 mg b) 6.8 mg c) 27 mg d) 8.7 mg e) 3.4 mg (The K sp is 4x 3, where x is the molar solubility. The mass in milligrams of Ag equals the concentration (2x) converted to mg of Ag in 250 ml.) 23- Will Mn(OH) 2 precipitate from solution if the ph of a M solution of MnCl 2 is adjusted to 8.0 (K sp = )? a) no b) yes (Q = < K sp.) 24- In which case will a precipitate form? (i) a M MnCl 2 solution with ph = 8.00 (does Mn(OH) 2 precipitate?) K sp = (ii) 100. ml of M AgNO 3 is added to 20 ml of M Na 2 SO 4 (does Ag 2 SO 4 precipitate?) K sp = a) i, yes; ii, no b) i, no; ii, yes

5 c) i, no; ii, no d) i, yes; ii, yes (Q for (i) is compared to K sp = , while Q for (ii) is compared to K sp = ) 25- Consider a solution containing M lead (II) ions and M mercury(ii) ions. Calculate the maximum concentration of sulfide ions that can be in solution without precipitating any lead ions. K sp : lead sulfide is ; mercury(ii) sulfide is a) M b) M c) M d) M (This is the K sp for PbS divided by the lead concentration.)