A student must make a buffer solution with a pH of 2.5. Determine which of the a
ID: 550090 • Letter: A
Question
A student must make a buffer solution with a pH of 2.5. Determine which of the acids and conjugate bases listed below are the best options to make a buffer at the specified pH. Weak acids: Incorrect phosphoric acid, Ka-7.52x10-3, 1.00 M formic acid, Ka=1.77x10-4, 2.00 M sodium disulfate monohydrate, O Scroll down for more. Ka-1.20x102, 3.00 M O acetic acid, Ka=1.75x10-5, 5.00 M Conjugate bases: sodium dihydrogen phosphate monohydrate NaH2PO H20 sodium formate, HCOONa sodium acetate trihydrate, CH3COONa 3H20 O sodium sulfate decahydrate, Na2SO4 10H20 Scroll down to view more.Explanation / Answer
if pH = pka, the buffer capacity is more.
so that, for pH = 2.5
the suitable weak acid = phosphoric acid, Ka = 7.52*10^-3,1.00 M
conjugate bases = sodium dihydrogen phosphate monohydrate,NaH2PO4*H2O
no of mol of buffer = M*V = 0.1*100/1000 = 0.01 mol
no of mol of conjugate base = x mol
pH = pka + log(conjugate base/acid)
pka = -logka
= -log(7.52*10^-3)
= 2.124
pH = 2.5
2.5 = 2.124 + log(x/(0.01-x))
x = 0.007
no of mol of conjugate base = x = 0.007 mol
mass of conjugate base must add = 0.007*138 = 0.966 g
no of mol of weak acid = 0.01-x = 0.01-0.007 = 0.003 mol
volume of phosphoric acid = n/v = 0.003/1 = 0.003 L
= 3 ml
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