Question

4. Out of the following, which is the best acid/base to use to prepare a buffer with a pH of 11.00? a. hydrogen peroxide b. phenol c. propylamine What ratio of masses of the weak acid/base and its conjugate should you use to make the buffer of the required pH? Use the sodium salt of the conjugate base if you chose a weak acid or the chloride salt of the conjugate acid if you chose a weak base.

Answer 1

Solution :-

The pka of the propylamine is 10.71 which is very close to the given pH value.

Propylamine is the weak base therefore we need to use the chloride salt of the propyl amine as the conjugate acid of it

Propyl amine = C3H9N , molar mass = 59.11 g /mol

Chloride salt = C3H9NHCl ,molar mass = 95.57 g/mol

Pka = 10.71

Using the Henderson equation lets calculate the ratio of the concentration of the weak base to conj.acid

pH= pka + log ([base]/[acid])

11.0 = 10.71 + log ([base]/[acid])

11.0-10.71 = log ([base]/[acid])

0.29 = log ([base]/[acid])

10^(0.29)= ([base]/[acid])

1.95 =([base]/[acid])

Therefore ratio of the concentrations is 1.95 M base : 1 M acid

Lets assume we have to make 1 L solution

Then moles of acid and base are same as their molarities.

Therefore we need

1.95 mol base C3H9N

1 mol conj.acid C3H9NHCl

Now lets convert the moles to mass of each

Mass = moles x molar mass

Mass of base propyl amine = 1.95 mol * 59.11 g per mol

                                                 = 115 g C3H9N

Mass of conj acid salt C3H9NHCl = 1 mol * 95.57 g per mol

                                                          = 95.6 g C3H9NHCl

Therefore ratio of masses is   115 g base : 95.6g acid salt