
Interpretation:
The validation corresponding to the fact that 0.1 M solution of acetic acid (pKa=4.76) turns litmus red, and that a 0.1 M solution of phenol (pKa=9.95) does not is to be stated.
Concept introduction:
The pH of a solution is represented as the negative logarithm of the concentration of proton.
pH=−log[H+]
This scale is defined from 1 to 14. It helps to determine whether the given solution is acidic, basic or neutral. The solutions with pH<7 are acidic in nature, with pH=7 are neutral in nature and with pH>7 are basic in nature.

Answer to Problem 20.35AP
The value of pH for 0.1 M solution of acetic acid is below 3 that is 2.89 and the value of pH for 0.1 M solution of phenol is above 3 that is 5.48. So, 0.1 M solution of acetic acid (pKa=4.76) turns litmus red, and that a 0.1 M solution of phenol (pKa=9.95) does not.
Explanation of Solution
The given pKa value 0.1 M solution of acetic acid is 4.76.
So, the value of Ka for 0.1 M solution of acetic acid is 10−4.76=0.0000174.
The ionization table for acetic acid for initial and final concentration is given below.
CH3CO2H→CH3CO−2+H+Initial concentration 0.1 0 0Final concentration 0.1−x x x
The expression to calculate the value of Ka is given below.
Ka=[H+][CH3CO−2][CH3CO2H]
Substitute the values of respective concentrations and Ka in the above expression.
0.0000174=[x][x][0.1−x]0=x2+0.0000174x−0.00000174x=0.0013
The value of pH solution is represented as follows.
pH=−log[H+] … (1)
Substitute the value of H+ as 0.0013 for 0.1 M solution of acetic acid in equation (1).
pH=−log[0.0013]=2.89
Therefore, the value of pH for 0.1 M solution of acetic acid is 2.89 that is below the pH value 3 and turns the litmus paper red.
The given pKa value 0.1 M solution of phenol is 9.95.
So, the value of Ka for 0.1 M solution of phenol is 10−9.95=1.1×10−10.
The ionization table for phenol for initial and final concentration is given below.
PhOH→H++PhO−Initial concentration 0.1 0 0Final concentration 0.1−x x x
The expression to calculate the value of Ka is given below.
Ka=[H+][PhO−][H+]
Substitute the values of respective concentrations and Ka in the above expression.
1.1×10−10=[x][x][0.1−x]0=x2+1.1×10−10x−1.1×10−11x=3.3×10−6
Substitute the value of H+ as 3.3×10−6 for 0.1 M solution of phenol in equation (1).
pH=−log[3.3×10−6]=5.48
Therefore, the value of pH for 0.1 M solution of phenol is 5.48 that is above the pH value 3 and does not turns the litmus paper red.
The 0.1 M solution of acetic acid (pKa=4.76) turns litmus red, and that a 0.1 M solution of phenol (pKa=9.95) does not because acetic acid has its pH value below 3 whereas, phenol has its pH value above 3.
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Chapter 20 Solutions
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