Find the pH of 0.020 M carbonic acid at 298 K, taking K_a1 = 4.3x10^-7 and neglecting the second ionisation and ionisation of water.

JEE Main levelSingle correct optionDifficulty 2 of 3ia025

Find the pH of 0.020 M carbonic acid at 298 K, taking $K_{a 1} = 4.3 \times 1 0^{- 7}$ and neglecting the second ionisation and ionisation of water.

[H X^{+}] = K_{a 1} c = 4.3 \times 1 0^{- 7} \times 0.020 = 9.274 e - 05

M, so pH

= 4.03

. The other choices are

p K_{a 1}

- lo g c

(full ionisation), and

- lo g (K_{a 1} c)

(square root omitted).