Question

What is the pH of a solution containing 0.30 M HCOOH and 0.52 M HCOOK?

How do i get the pKa?? I know its 3.77 but how??

Help!

Edited; Nvm. figured it out, thanks

Answer 1

According to Henderson's Equation , $pH = pK_{a}+ log\frac{\left [ salt \right ]}{\left [ acid \right ]}$

                                                         $pH = pK_{a}+ log\frac{\left [HCOOK ]}{\left [ HCOOH ]}$

Given [HCOOK] = 0.52 M

          [ HCOOH] = 0.30 M

The value of acid dissociation constant for formic acid from the standard list is, K_a =1.7x10 ^-4

So pK_a = - log K_a

             = - log 1.7x10 ^-4

             = 3.77

Plug these values in Henderson's equation we have $pH = 3.77+ log\frac{\left 0.52}{\left 0.30}$

                                                                                

                                                                                

Therefore the pH of the resulting solution is 4.01