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sveta [45]
3 years ago
6

Phenol red is an indicator that changes color over a range from ph=6.4 to ph=8.0. what is ka of phenol red?

Chemistry
1 answer:
patriot [66]3 years ago
7 0
First, we will get the average pH of the two given values:
average pH = (6.4+8) / (2) = 7.2
At this average pH, the concentration of the acid from the phenol red is equal to the concentration of the base.
pH = 7.2
[H+] = 10^(-7.2) = 6.3 * 10^-8

Phenol red has the general formula HA, this gives us:
HA <.......> H+ + A- 
At pH = 7.2, [H+] = [A-]
<span>Ka = [H+][A-]/ [HA]
</span>Ka = [H+] = <span>6.3 x 10^-8</span>

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Acetic acid has a pKa of 4.74. Buffer A: 0.10 M HC2H3O2, 0.10 M NaC2H3O2 Buffer B: 0.30 M HC2H3O2, 0.30 M NaC2H3O2 Buffer C: 0.5
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Buffer B has the highest buffer capacity.

Buffer C has the lowest buffer capacity.

Explanation:

An effective weak acid-conjugate base buffer should have pH equal to pK_{a} of the weak acid. For buffers with the same pH, higher the concentrations of the components in a buffer, higher will the buffer capacity.

Acetic acid is a weak acid and CH_{3}COO^{-} is the conjugate base So, all the given buffers are weak acid-conjugate base buffers. The pH of these buffers are expressed as (Henderson-Hasselbalch):

pH=pK_{a}(CH_{3}COOH)+log\frac{[CH_{3}COO^{-}]}{[CH_{3}COOH]}

pK_{a}(CH_{3}COOH)=4.74

Buffer A:    pH=4.74+log(\frac{0.10}{0.10})=4.74

Buffer B:   pH=4.74+log(\frac{0.30}{0.30})=4.74

Buffer C:   pH=4.74+log(\frac{0.10}{0.50})=4.04

So, both buffer A and buffer B has same pH value which is also equal to pK_{a} . Buffer B has higher concentrations of the components as compared to buffer A, Hence, buffer B has the highest buffer capacity.

The pH of buffer C is far away from pK_{a} . Therefore, buffer C has the lowest buffer capacity.

     

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