Calculating Ka from Measured pH

A student prepared a 0.10 M solution of formic acid (HCOOH) and found its pH at 25 °C to be 2.38. Calculate Ka for formic acid at this temperature.

A 0.50 M solution of an acid HA has pH = 2.24. What is the value of Ka for the acid?

(a) 1.7 x 10-12, (b) 3.3 x 10-5, (c) 6.6 x 10-5, (d) 5.8 x 10-3, (e) 1.2 x 10-2