## Chemistry and Chemical Reactivity (9th Edition)

1. Calculate the pKa value for acetic acid. $pKa = -log(Ka)$ $pKa = -log( 1.8 \times 10^{- 5})$ $pKa = 4.74$ 2. Find the acid/base ratio: $pH = pK_a + log\frac{[Base]}{[Acid]}$ $5 = 4.74 + log\frac{[Base]}{[Acid]}$ ** Mathematically, we can invert a fraction inside a logarithm, as long as we multiply the log by "-1": $log\frac{[Base]}{[Acid]} = -log\frac{[Acid]}{[Base]}$ $5 - 4.74 = - log\frac{[Acid]}{[Base]}$ $0.26 = - log\frac{[Acid]}{[Base]}$ $\frac{[Acid]}{[Base]} = 10^{-0.26} = 0.55$ There is no "0.55" answer, but, we can do this: $\frac{[Acid]}{[Base]} = \frac{0.55}{1}$ ** Dividing the fraction by 0.55: $\frac{[Acid]}{[Base]} = \frac{1}{1.8}$ So, the correct answer is (c): 1/1.8