Question

You have available the buffer ADA: disodium salt a) using the buffer, explain, in some detail including numerical calculations, how to prepare 500ml of a buffer with a pH=6.50 and a total concentration of 50.0mM for the acid and conjugate base. b) What volume of either the NaOH or HCl solutions would you need to add to change the pHof 100mL of the buffer to pH 6.00

Answer 1

pH (buffer sol) = pKa + log base/acid

total conc of acid and base = 50.mM    let base conc = 20mM , acid conc = 30 mM

Here 6.50 =   pKa + log 20/30

         6.50 - log 20/30 = pKa

         6,50 - 0.176 = 6.324 = pKa

Now suppose pH = 6.00

so 6.00   = pKa + log base/acid

   6.00    = 6.324   + log base/acid

    6.00-6.324   = log (bse/acid)

       - 0.324      = log base/acid

base/acid = 10^-0.324    = 0.4742

If we again add 50 ml acid , base will be = 50* 0.4742 = 23.71 ml

In this way you can set your mind

if we prepared 50+23.71 = 73.71 ml of sol , you added 50 ml acid and 23.71 ml base

                                        100    ml          "         "    ( 50/73.71)*100 ml acid and ( 23.71/73.71)*100 ml base

                                       = 67.83 ml of acid and 32.17 ml of base