Phenylacetic acid is a weak monoprotic acid with an acid dissociation constant, Ka=4.90×10−5 mol dm−3K_{\text{a}} = 4.90 \times 10^{-5}\text{ mol dm}^{-3}Ka=4.90×10−5 mol dm−3 at 298 K.
What is the concentration, in mol dm−3\text{mol dm}^{-3}mol dm−3, of hydrogen ions in a 4.00 × 10-2 mol dm-3 solution of phenylacetic acid at 298 K?
1.96×10−61.96 \times 10^{-6}1.96×10−6
8.00×10−28.00 \times 10^{-2}8.00×10−2
1.40×10−31.40 \times 10^{-3}1.40×10−3
2.80×10−32.80 \times 10^{-3}2.80×10−3