Question 8.6: (a) The [H3O+] of a certain liquid detergent is 1.4 × 10^-9 ...
(a) The [H_{3}O_{+}] of a certain liquid detergent is 1.4 × 10^{-9} M. What is its
pH? Is this solution acidic, basic, or neutral?
(b) The pH of black coffee is 5.3. What is its [H_{3}O^{+}]? Is it acidic, basic, or neutral?
Strategy
To determine the pH when given the concentration of a hydrogen ion, just take the negative of the log. If it is less than 7, the solution is acidic. If it is greater than 7, it is basic.
If given the pH, you can immediately determine if it is acidic, basic, or neutral according to how the number relates to 7. To convert the pH to the [H_{3}O^{+}], take the inverse log of -pH.
(a) On your calculator, take the log of 1.4 × 10^{-9}. The answer is -8.85. Multiply this value by -1 to give the pH of 8.85. This solution is basic.
(b) Enter 5.3 into your calculator and then press the +/- key to change the sign to minus and give -5.3. Then take the antilog of this number. The [H_{3}O^{+}] of black coffee is 5 × 10^{-6}. This solution is acidic.