A student must make a buffer solution with a pH of 1.5. Determine which of the a
ID: 498319 • Letter: A
Question
A student must make a buffer solution with a pH of 1.5. Determine which of the acids and conjugate base listed below are the best options to make a buffer at the specified pH. Weak acids: propionic acid, K_a = 1.34 times 10^-5 3.00 M formic acid, K_a = 1.77 times 10^-4, 2.00 M acetic acid, K_a = 1.75 times 10^-5, 5.00 M sodium bisulfate monohydrate, K_a = 1.20 times 10^-2, 3.00 M Conjugate bases: sodium propionate, CH_3CH_2COONa sodium acetate trihydrate, CH_3COON+ 3H_2O sodium formate, HCOONa sodium sulfate decahydrate, Na_2SO_4+ 10H_2O 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 = 1.5? Based on this information, what volume of acid should the student measure to make the 0.100 M buffer solution?Explanation / Answer
For a buffer capacity at pH , try a pKa near pH, that is pKA near = 1.5
pKa = -log(Ka) =
pKa = -log(Ka) = -log(1.2*10^-2) = 1.92
pKa = -log(Ka) = -log(1.77*10^-4) = 3.75
this is too far, the best acid is the one with pKa = 1.92
choose sodium disulfate monohydrate
its conjugate base NA2SO4*10H2O
2)
VT = 100 ml
M = 0.1 HA
M = ? A-
mass of A-
pH = 1.5
substitute in ph equation for buffers
pH = pKa + log(-/HA)
1.5= 1.92 + log(A-/0.1)
Solve for A-
10^(1.5-1.92) = (A-/0.1)
A- = 0.038 M
V = 100 ml so
mol of A- = M*V = 0.038*0.1 = 0.0038 mol of A-
MW of sodium sulfate decahydrate = 322.2 g/mol
We have 0.0038mol so
322.2*0.0038 = 1.224 grams of sodium sulfate decahydrate is needed.
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