Question

Calculate the pH of a solution that is made by mixing 0.80 L of 0.30 M formic acid (HCOOH, Ka=1.77X10-4) and 0.90 L of 0.20 M sodium formate (NaHCOO). [Hint: don’t forget that you are mixing, and so each is diluted upon mixing]

Answer 1

formic acid

before mixing                                                                                   after mixing

M1 = 0.3M                                                                                       M2 =

V1   = 0.8L                                                                                      V2 = 0.8+ 0.9 = 1.7L

       M1V1   = M2V2

       M2    = M1V1/V2   = 0.3*0.8/1.7    = 0.14M

sodium formate ( HCOONa)

before mixing                                                                                    after mixing

M1 = 0.2M                                                                                       M2 =

V1   = 0.9L                                                                                      V2   = 0.9 + 0.8 = 1.7L

                        M1V1   = M2V2

                           M2    = M1V1/V2    = 0.2*0.9/1.7   = 0.1058 M

        Ka   = 1.77*10^-4

       PKa   = -logKa

                = -log1.77*10^-4

                 = 3.7520

   PH    = Pka + log[HCOONa]/[HCOOH]

           = 3.752 + log0.1058/0.14

          = 3.752 -0.1216   = 3.6304 >>>>>answer