Acid-Base Worksheet Answers are given at the end of all worksheets. Solve the problems before looking at the answers.

Transcription

1 Acid-Base Worksheet Answers are given at the end of all worksheets. Solve the problems before looking at the answers. 1. What is ph of M HCl? 2. What is ph of M NaOH? 3. The concentration of calcium hydroxide in a saturated solution is M. What is the ph of the solution? Is it acidic or basic? 4. The concentration of OH ions in a sample of seawater is 5.0 x 10 6 M. a. Calculate the concentration of H 3 O + ions and classify the solution as neutral, acidic, or basic. b. Calculate the poh of the above solution, and indicate whether a small or large poh value is indicative of a basic solution.

2 5. Write the formula for each of the following: + a. the conjugate base of NH 4 b. the conjugate base of H 3 PO 4 c. the conjugate acid of CO 3 e. the conjugate acid of H 2 O f. the conjugate base of HSO 4 g. the conjugate base of H 2 PO 4 h. the conjugate base of HC 6 H 3 O 5 i. the conjugate acid of C 2 H 6 NH 6. The K a of HNO 2 is 7.1 x Identify its conjugate base and determine the K b of that base. 7. The following is a list of weak acids and their K a values: HOCl hypochlorous acid 3.5 x 10 8 H 2 S hydrogen sulfide 1.1 x 10 7 HCN hydrocyanic acid 4.0 x HNO 2 nitrous acid 4.5 x 10 4 a. Which acid given above is the strongest? Explain your choice. b. Write the K a expression for the strongest acid. c. Which acid has the strongest conjugate base? Explain your choice.

3 8. Complete the following: I. In each of the following reactions, identify the conjugate acid-base pairs. II. Using K a or K b values from the textbook, if you mix equal concentrations of reactants and products, determine which of the following reactions would proceed to the right or to the left as written. Briefly explain your choice in each case. a. HF (aq) + NO 3 (aq) HNO 3 (aq) + F (aq) b. NH 4 + (aq) + CO 3 (aq) HCO 3 (aq) + NH 3 (aq) c. HCO 3 (aq) + H 2 S (aq) H 2 CO 3 (aq) + HS (aq) 9. Decide which of each of the following would give an acidic, basic or neutral solution when added to water. Explain your reasoning. a. NaNO 3 b. K 3 PO 4 c. NaHCO 3 d. NH 4 F e. FeCl 3

4 10. What is ph of M acetic acid (HC 2 H 3 O 2 ), K a = 1.8 x 10 5? 11. A M solution of a monoprotic acid, HA, is 0.150% dissociated in water at 25ºC. a. What is the K a of HA? b. What is the resulting ph of the solution?

5 12. A M solution of a weak acid (HA) has a ph of What is the K a of the acid? 13. Dimethylamine, (CH 3 ) 2 NH, a key intermediate in detergent manufacture, has a K b of 5.9 x What is the ph of 1.5 M (CH 3 ) 2 NH?

6 14. Calculate the ph of a solution that is 1.0x10 8 M in HCl. Does this answer make sense? 15. Answer the following questions regarding H 3 PO 4 : a. Write the balanced equation for the reaction of this acid with water, where one proton is transferred. b. Is the resulting ion from the above reaction acidic, basic or amphiprotic? Explain your answer. c. What are the equilibrium concentrations of H 3 O +, H 2 PO 4, HPO 4 and PO 4 3 in M H 3 PO 4?

7 16. Calculate the ph of M solutions of each of the following salts. a. Mg(C 2 H 3 O 2 ) 2 b. (CH 3 ) 2 NH 2 Cl 17. Without using K a s (just the formulas), determine which is the weaker acid in each of the following pairs. Explain why. a. H 2 S or H 2 Te b. H 3 AsO 4 or H 3 PO 4 c. HIO 2 or HIO 3

8 Answers Basic 4. a. [H 3 O + ] = 2.0 x 10 9 M; this solution is basic b. poh = 5.30; A small poh is indicative of a basic solution 5. a. NH 3 b. H 2 PO 4 c. HCO 3 e. H 3 O + f. SO 4 g. HPO 4 3 h. C 6 H 3 O 5 6. The conjugate base is NO 2 i. + C 2 H 6 NH 2 K b = 1.4 x a. Nitrous acid, HNO 2, is strongest because it has the highest K a b. K a = [H 3O + ][NO 2 ] [HNO 2 ] c. Hydrocyanic acid, HCN, has the strongest conjugate base because it is the weakest acid (lowest K a ). 8. a. HF (aq) + NO 3 (aq) HNO 3 (aq) + F (aq) acid base acid base 6.8 x 10 4 strong weaker acid weaker base stronger acid stronger base conjugate acid/base pairs are HF/F and HNO 3 /NO 3 (acid listed first) The reaction would proceed to the left because HNO 3 is a strong acid and HF is a weak acid. Reactions favor the weaker conjugate acid or base. b. NH + 4 (aq) + CO 3 (aq) HCO 3 (aq) + NH 3 (aq) acid base acid base K a = 5.7 x K a = 4.7 x stronger acid weaker acid conjugate acid/base pairs are NH 4 + /NH 3 and HCO 3 /CO 3 The reaction would proceed towards the right because NH 3 is the weaker base and HCO 3 is the weaker acid. c. HCO 3 (aq) + H 2 S (aq) H 2 CO 3 (aq) + HS (aq) base acid acid base 9 x x 10 7 weaker base weaker acid stronger acid stronger base conjugate acid/base pairs are H 2 CO 3 /HCO 3 and H 2 S/HS The reaction will favor the left because H 2 S is a weaker acid that H 2 CO 3 (these K a values were found in table 18.5) 9. a. neutral Na + is a group IA cation (neutral); NO 3 is the anion of a strong acid (neutral) b. basic K + is a group IA cation (neutral); PO 4 3 is the anion of a weak acid (base) c. basic The K a of HCO 3 (which is the K a2 of H 2 CO 3 ) is lower than the K b of HCO 3 (which is K w /K a1 of H 2 CO 3 )

9 d. slightly acidic NH + 4 is the conjugate acid of a weak base (acidic); F is the conjugate base of a weak acid (basic). The compound is overall slightly acidic because NH + 4 has a higher K a (calculated using the K b of NH 3 ) than the K b of F (calculated using the K a of HF) e. acidic Fe 3+ is a cation with a high charge density (acidic); Cl is the anion of a strong acid (neutral) 10. ph = 3.01 Please take time to notice the difference in ph between M HCl, NaOH (problems 1&2), and HC 2 H 3 O a. K a = 2.25 x 10 7 b. ph= K a = 2.1 x ph = If you just use ph = log[hcl], you get ph = No, that does not make sense because HCl is an acid, so its ph should be below 7. This is an anomaly of the calculation. You have to remember that the HCl is dissolved in water and that in water [H 3 O + ] = 10 7 M. Therefore, in 1.0 x 10 8 M HCl, [H 3 O + ] = 10 7 M = 1.0 x 10 8 M = 1.1 x 10 7 M, and ph = a. H 3 PO 4 + H 2 O H 2 PO 4 + H 3 O + b. It is amphiprotic because it can either donate another H + to become HPO 4 or it can gain an H + to become H 3 PO 4 again. Amphiprotic is the same thing as amphoteric. c. [H 3 O + ] = [H 2 PO 4 ] = 2.32 x 10 2 M [HPO 4 ] = K a2 = 6.3 x 10 8 [PO 3 4 ] = 1.1 x M 16. a. ph = 9.11 (Note: this answer is correct. If you got ph = 8.96, you did not take into account that there are two acetate ions per magnesium acetate). b. ph = a. H 2 S S is smaller than Te. The smaller bond length causes the bond to be stronger, so the H is held more tightly. On the exam, you must have this full explanation, not just the part about size difference. b. H 3 AsO 4 is the weaker acid because As is less electronegative than P. Since P is more electronegative, it will draw the electrons that it shares with oxygen towards itself. This will make the oxygen more electronegative so it will draw the electrons more towards itself and away from the hydrogen. The rest is like the answer above. Do not confuse the rules for oxoacids with those for binary acids. In oxoacids, the H bonded to the O, not to the other atom. Therefore, the electronegativity of the other atom is important, not its size. c. HIO 2 is the weaker acid because it has fewer oxygens. The more oxygens on an oxyacid, the stronger the acid because oxygen is very electronegative. The oxygens will pull shared electrons away from the central atom, which will then pull electrons away from the oxygen bonded to the hydrogen, which will then pull the electrons away from the hydrogen. The rest is as above.