Question 10.11: What is the pH of a 0.0032 M solution of NaOH? ANALYSIS Sinc...
What is the pH of a 0.0032 M solution of NaOH?
ANALYSIS Since NaOH is a strong base, the OH^- concentration is the same as the NaOH concentration. Starting with the OH^- concentration, finding pH requires either using the K_w equation to find [H_3O^+] or calculating pOH and then using the logarithmic form of the K_w equation.
BALLPARK ESTIMATE Because [OH^-] = 3.2 \times 10^{-3} M is close to midway between 1 \times 10^{-2} M and 1 \times 10^{-3} M the pOH must be close to the midway point between 2.0 and 3.0. Subtracting the pOH from 14 would therefore yield a pH between 11 and 12.
Taking the negative logarithm gives pH = – \log (3.1 \times 10^{-12}) = 11.51.
Alternatively, we can calculate pOH and subtract from 14.00 using the logarithmic form of the K_w equation. For [OH^-] = 0.0032 M ,
pH = 14.00 – 2.49 = 11.51
Since the given OH^- concentration included two significant figures, the final pH includes two significant figures beyond the decimal point.
BALLPARK CHECK The calculated pH is consistent with our estimate.