PH and KA calculations

[Awuah29]

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How do you guys set these problems up ?

14. What is the pH of a solution, which is 0.5 M in CH3COOH (HOAc) and 0.5 M in NaCH3COO (NaOAc)?
CH3COOH: Ka = 1.8 x 10-5

a) 2.9
b) 4.7
c) 5.9
d) 7.0
e) 8.9


What is the Ka of the weak acid phenylacetic acid, C6H5CH2COOH, if 2.1 % of the acid dissociates in a 0.12 M solution ?

a) 5.2 x 10-5
b) 3.7 x 10-5
c) 2.1 x 10-2
d) 1.8 x 10-1
e) 3.7 x 101

What is the pH of a 0.020 M citric acid , H3C6H8O7, solution ?
citric acid: Ka = 7.5 x 10-4

a) 1.6
b) 2.2
c) 3.1
d) 3.7
e) 4.4

 

[BodybldgDoc]

Use this formula,
pH=pka + log(conj base/weak acid)

NaCH3COO=base and CH3COOH=acid
so for #14, you would have pH= 4.74 + (0.5M/0.5M) = 4.74
(you get the pka by taking the log of 1.8E-5=4.74)

do the same thing for the rest of the problems.

 

[Awuah29]

Use this formula,
pH=pka + log(conj base/weak acid)

NaCH3COO=base and CH3COOH=acid
so for #14, you would have pH= 4.74 + (0.5M/0.5M) = 4.74
(you get the pka by taking the log of 1.8E-5=4.74)

do the same thing for the rest of the problems.

so basically, you can estimate that a log of 1.8E- 5 will be around. 4?

do we have to know logs?

 

[BodybldgDoc]

yeah you do have to be able to estimate the log of something. For example Ka=1.8E-5 then the pKa=5-log1.8 But Since 1.8 is a small number, its log will also be small and the answer will be closer to 5 than to 4. The kaplan blue book shows you how to make the estimation really well.