A student must make a buffer solution with a pH of 2.5. Determine which of the 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 Making a buffer with a weak acid that has a pKa nearest to the desired pH is best. Consider the equation below O phosphoric acid, Ka-7.52x10-3, 1.00 M O acetic acid, Ka-1.75x10-5, 5.00 M sodium disulfate monohydrate, Ka-1.20x10-2, 3.00 M O formic acid, Ka-1.77104, 2.00 M Conjugate bases sodium dihydrogen phosphate monohydrate, O NaH2PO4. H20 O sodium formate, HCOONa o sodium sulfate decahydrate, Na2SO4. 10H20 sodium acetate trihydrate, CH3COONa 3H2O Scroll down to view more

Explanation / Answer

if pH = 2.5

then

we need a pKa nearest to 2.5

i.e.

Ka = 10^-pH = 10^-2.5 = 0.003162 = 3.16*10^-3

from the list

phosphoric acid migt be the best option, since Ka is nearest to the value reuqired

b)

conjugate base/acid must be:

H3PO4 + H2PO4- which could come from NaH2PO4 salt, which is shown as a hydrate

so choose

NaH2PO4*H2O

Q2

apply

pH = pKa + log(A-/HA)

pKA = -log(7.52*10^-3) = 2.12

2.5 = 2.12 + log(A-/HA)

(A-/HA) = 10^(2.5-2.12 ) = 2.398

[HA] = 0.1 then

[A-] = 2.398*0.1 = 0.2398 M

V = 0.1 L so

mol of A- = 0.02398 mol

mass of Na2HPO4*H2O = 141.9588 + 18 = 159.9588

mass = mol*MW = 159.9588*0.02398 = 3.835 g of Na2HPO4*H2O

mol of acid = requires initial concentration

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