Question

A student must make a buffer solution with a pH of 1.00.

Determine which weak acid is the best option to make a buffer at the specified pH.

formic acid,K_a = 1.77 x 10⁻⁴, 2.00 M

sodium bisulfate monohydrate, K_a = 1.20 x 10⁻², 3.00 M

acetic acid, K_a = 1.75 x 10⁻⁵, 5.00 M

propionic acid, K_a =1.34 x 10⁻⁵, 3.00 M

Determine which conjugate base is the best option to make a buffer at the specified pH.

sodium acetate trihydrate, CH₃COONa⋅3H₂O

sodium propionate, CH₃CH₂COONa

sodium sulfate decahydrate, Na₂SO₄⋅10H₂O

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 of 1.00?

mass = Answer g

Based on this information, what volume of acid should the student measure to make the buffer solution?

volume = Answer mL

Answer 1

1) Buffer capacity range = pka - 1 to pka+1.

so that, To get a buffer with pH = 1. The pka of acid must be close to 1. Hence, among the given sodium bisulfate monohydrate is suitable for it.

pka of sodium bisulfate monohydrate = -logka = -log(1.20*10^−2) = 1.92

answer: sodium bisulfate monohydrate, Ka = 1.20 x 10−2, 3.00 M

2) Buffer = weak acid + its conjugate base.

conjugate base : sodium sulfate decahydrate, Na2SO4⋅10H2O

3)

pH of acidic buffer = pka + log(Na2SO4*10H2O/NaHSO4*H2O)

no of mole of NaHSO4*H2O present in buffer = 100*0.1 = 10 mmol

no of mole of Na2SO4*10H2O present in buffer = x mmol

    1 = 1.92 + log(x/10)

x = 1.2  

no of mole of Na2SO4*10H2O present in buffer = x = 1.2 mmol

mass of Na2SO4*10H2O present in buffer = n*M.wt = 1.2*10^-3*322.2

                   = 0.39 g

volume of NaHSO4*H2O must be taken = n/M = 10/3 = 3.33 ml