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186 Chemistry: Matter and Change
Solving Problems: A Chemistry Handbook
SOLVING PROBLEMS:
A CHEMISTRY HANDBOOK
CHAPTER 18
The pH scale has values from 0 to 14. Acidic solutions have pH values between 0 and 7, with a value of 0 being the most acidic. The
pH of a basic solution is between 7 and 14, with 14 representing the
most basic solution. A neutral solution has a pH of 7.
Chemists have also defined a pOH scale to express the basicity
of a solution. The pOH of a solution is the negative logarithm of the
hydroxide ion concentration.
pOH ϭ Ϫlog [OH Ϫ ]
If either pH or pOH is known, the other may be determined by using
the following relationship.
pH ϩ pOH ϭ 14.00
The pH and pOH values for a solution may be determined if either
[H ϩ ] or [OH Ϫ ] is known. The following example problem shows
you how to calculate pH and pOH.
Example Problem 18-3
Calculating pH and pOH from [H ϩ ]
If a certain carbonated soft drink has a hydrogen ion concentration
of 7.3 ϫ 10 Ϫ4 M, what are the pH and pOH of the soft drink?
Because [H ϩ ] is given, it is easier to calculate pH first.
pH ϭ Ϫlog [H ϩ ]
pH ϭ Ϫlog [7.3 ϫ 10 Ϫ4 ]
pH ϭ Ϫ(log 7.3 ϩ log 10 Ϫ4 )
A log table or calculator shows that log 7.3 ϭ 0.86 and log 10 Ϫ4 ϭ
Ϫ4. Substitute these values in the equation for pH.
pH ϭ Ϫ[0.86 ϩ (Ϫ4)] ϭ Ϫ(0.86 Ϫ 4) ϭ 3.14
The pH of the soft drink is 3.14. Note that the number of decimal
places retained in the pH value equals the number of significant figures in the H ϩ ion concentration.
To find pOH, recall that pH ϩ pOH ϭ 14.00. Isolate pOH by
subtracting pH from both sides of the equation.
pOH ϭ 14.00 Ϫ pH
Substitute the value of pH and solve.
186 Chemistry: Matter and Change
Solving Problems: A Chemistry Handbook
SOLVING PROBLEMS:
A CHEMISTRY HANDBOOK
CHAPTER 18
The pH scale has values from 0 to 14. Acidic solutions have pH values between 0 and 7, with a value of 0 being the most acidic. The
pH of a basic solution is between 7 and 14, with 14 representing the
most basic solution. A neutral solution has a pH of 7.
Chemists have also defined a pOH scale to express the basicity
of a solution. The pOH of a solution is the negative logarithm of the
hydroxide ion concentration.
pOH ϭ Ϫlog [OH Ϫ ]
If either pH or pOH is known, the other may be determined by using
the following relationship.
pH ϩ pOH ϭ 14.00
The pH and pOH values for a solution may be determined if either
[H ϩ ] or [OH Ϫ ] is known. The following example problem shows
you how to calculate pH and pOH.
Example Problem 18-3
Calculating pH and pOH from [H ϩ ]
If a certain carbonated soft drink has a hydrogen ion concentration
of 7.3 ϫ 10 Ϫ4 M, what are the pH and pOH of the soft drink?
Because [H ϩ ] is given, it is easier to calculate pH first.
pH ϭ Ϫlog [H ϩ ]
pH ϭ Ϫlog [7.3 ϫ 10 Ϫ4 ]
pH ϭ Ϫ(log 7.3 ϩ log 10 Ϫ4 )
A log table or calculator shows that log 7.3 ϭ 0.86 and log 10 Ϫ4 ϭ
Ϫ4. Substitute these values in the equation for pH.
pH ϭ Ϫ[0.86 ϩ (Ϫ4)] ϭ Ϫ(0.86 Ϫ 4) ϭ 3.14
The pH of the soft drink is 3.14. Note that the number of decimal
places retained in the pH value equals the number of significant figures in the H ϩ ion concentration.
To find pOH, recall that pH ϩ pOH ϭ 14.00. Isolate pOH by
subtracting pH from both sides of the equation.
pOH ϭ 14.00 Ϫ pH
Substitute the value of pH and solve.