Question

0.25 M HCOOH solution is found to have a pH of 2.19. The ionisation constant of HCOOH is 

​a.  1.7 × 10^-4

​b.  4.3 × 10^-4

c.  5.3 × 10^-4

​d.  6.5 × 10^-3

^{Please explain in detail.............}