A student must make a buffer solution with a pH of 5.0. Determine which of the a

Question

A student must make a buffer solution with a pH of 5.0. Determine which of the acids and conjugate bases listed below are the best options to make a buffer at the specified pH.

(Select One)

Weak Acids:

formic acid, Ka=1.77x10–4

acetic acid, Ka=1.75x10–5

ammonium citrate, Ka=4.06x10–7

propionic acid, Ka=1.34x10–5

(Select One)

Conjugate Bases:

sodium citrate dihydrate C6H5O7Na3•2H2O

sodium propionate CH3CH2COONa

sodium acetate trihydrate CH3COONa•3H2O

sodium formate HCOONa

The final volume of buffer solution must be 100.00 mL and the final concentration of the weak acid must be 0.100 M. Based on this information, what mass of solid conjugate base should the student weigh out to make the buffer solution with a pH=5.0?

_____g

Explanation / Answer

pH = 5.0

for making best buffer of pH = 5.0 .the weak acid pKa should be nearer to pH (5.0) value.

so here among all the weak acids given, propionic acid pKa (4.87 ) is nearer to given pH (5.0) value. so it is suitable

for making best buffer at pH = 5.0

propionic acid, Ka=1.34x10^–5

pKa of propionic acid = 4.87

the corresponding salt formed from the propanoinc acid is sodium propionate CH3CH2COONa

pH = pKa + log [salt / acid]

5.0 = 4.87 + log [salt / acid]

[salt / acid] = 1.349

acid moles = 100 x 0.1 / 1000

                  = 0.01

salt / acid = 1.349

salt / 0.01 = 1.349

salt (conjugate base) moles = 1.349 x 0.01

                    = 0.0135

conjugate base (CH3CH2COONa) molar mass = 96.06 g/mol

mass = moles x molar mass

          = 0.0135 x 96.06

          = 1.296 g

mass of conjugate base = 1.296 g

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