I need help on the following: Design a buffer that has a pH of 6.51 using one of

Question

I need help on the following:

Design a buffer that has a pH of 6.51 using one of the weak acid conjugate base systems shown below. Weak Acid Conjugate Base Ka pKa 5.4 x 105 419 HC204 6.2 x 10-3 7.21 H PO4 HPO4 4.s x 10-11 10.32 HCO CO3 How many grams of the potassium salt of the weak acid must be combined with how many grams of the potassium salt of its conjugate base, to produce 1.00 L of a buffer that is 1.00 M in the weak base? grams potassium salt of weak acid grams potassium salt of conjugate base

Explanation / Answer

Question 1.

if pH = 6.51, then find the best weak acid/conjugate base required

so

pKa must be near the pH target, the best is H2PO4- and HPO4-2

fom

KH2PO4 and K2HPO4

apply buffer equation

pH= pKa2 + log(K2HPO4/KH2PO4 )

6.51 = 7.21 + log(K2HPO4/KH2PO4 )

10^(6.51-7.21) = K2HPO4/KH2PO4

0.1995262 = K2HPO4/KH2PO4

Assume: [K2HPO4] = 1 M so

KH2PO4 = [K2HPO4] / 0.1995262

KH2PO4 = 1/ 0.1995262 = 5.0118 M

for 1 Liter:

moles of KH2PO4 --> 5.0118 moles

moles of K2HPO4 --> 1 mol

mass of KH2PO4 --> 5.0118 *136.086 g = 682.03 g of KH2PO4

moles of K2HPO4 --> 1 mol *174.2 = 174.2 g

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