Exam 2 Review Guide Chapter 15 Typícal Math Problems 1) What is the pH of a 0.75

Question

Exam 2 Review Guide Chapter 15 Typícal Math Problems 1) What is the pH of a 0.750M solution of HCI? 2) a) What is the pH of a 0.750 M solution of a weak monoprotic acid with a pK, 6.15? b) What is the %-ionization of this weak acid? c) What is the corresponding pKs for the conjugate base of this weak acid? 3) a) What is the pk, for a weak base if the pH of a 0.125M solution of this weak base was 4) When a weak acid is added to water to make a 0.250M solution, the acid was found to found to be 8.161? be 1.5% ionized. What is the Ka and pKa of this weak acid? At 60°C, K,-9.6x10-14 (Kw increases as the temperature increases). a) For pure water at 60°C, what is the [Hjand pH. b) If a sample of an aqueous solution was found to have a pH of 7.00 at 60°c, is this solution acidic, basic, or neutral given your answer to a)? t Prohlems

Explanation / Answer

1) HCl is a strong acid , and for strong acids [acid] =[H+] as it ionises almost completely.

pH of a strong acid is given by

pH = -log [H+]

= -log (0.750)

= 0.1249

2)a) Weak acids do not dissociate completely and thir pH is given by

pH = 1/2 (pKa -logC]

= 1/2 [6.15 -log 0.75]

= 3.1374

2b) the degree of ionization of weak acid is calculated as

degree of ionization = (Ka/C)1/2

Ka of acid = antilog (-6.15)

= 7.08x10-7

Thus degree of ionization = (7.08x10-7 x 0.75)1/2

= 7.28x10-3

and % ionization = degree of ionisation x100

=0.728 %

2c) For an acid and conjugate base pair

pKa + pkb = pKw

Thus pkb = pkw -pka

= 14 -6.15

= 7.85

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