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Chapter 9: Q9DE (page 207)

The pH of 0.010 M o-cresol is 6.16. Find pKa for this weak acid.

Short Answer

Expert verified

ThepKa value of the given weak acid is 10.32 .

Step by step solution

01

Define the acid dissociation constant

The acid dissociation constant (Ka) is used to distinguish between strong and weak acids. As the Ka increases, the acid dissociates more. As a result, strong acids must dissociate more in water. In contrast, a weak acid is less likely to dissociate and release a hydrogen ion, resulting in a less acidic solution.

The acid dissociation constantKa is given by

Ka=[H+][A-][HA]

A weak acid does not undergo complete dissociation.

The negative logarithm of an equilibrium constant is called pKa.

02

Calculate the amount of solution required

It is given that

H+=10-pH=10-6.16=6.9×10-7M

A-=6.9×10-7HA=0.010-H+=0.010KavalueKa=H+A-HA

Ka=6.9×10-720.010Ka=4.8×10-11

The value of pKa :

pKa=-logKa=-log4.8×10-11=10.32

Thus, thepKa value of the given weak acid is 10.32.

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