Question

Determine the volumes of each solution you would use to prepare a 1.0L of a pH 5.00 buffer. Choose solutions from the following list. For the answer, list the solution and the volume of that solution.

0.10 M HCOOH Ka = 1.8 x 10^-4

0.10 M HCOONa

0.10 M CH₃COOH Ka = 1.8 x 10^-5

0.10 M CH₃COONa

0.10 M HCN Ka = 4.9 x 10^-10

0.10 M NaCN

Answer 1

pH= pKa+ log [ conjugate base/acid]

1.

pKa= -log (1.8/10000)= 3.74

5= 3.74+log ( HCOO-/HCOOH)

HCOO-/HCOOH= 18.2

Moles of HCOO-/ moles of HCOOH= 18.2

Moles of HCOO= 18.2* moles of HCOOH

Let x= volume of HCOOH, its moles = x*1= x ( Molarity* Volume(L)

Volume of HCOO- = 1-x, its moles = (1-x)

Hence x= 18.2-18.2x, 19.2x= 18.2, x= 18.2/19.2= 0.95L. Volume of HCOONa= 1-0.95=0.05L

2.

pKa= -log (1.8*10^-5) =4.74= 5+ log (CH3COO-/CH3COOH)

CH3COO- = 0.55*CH3COOH

Let x= volume of CH3COONa ( CH3COO- is coming form CH3COONa),

Moles of CH3COONa= x, moles of CH3COOH= (1-x)*1

Hence         x= 0.55-0.55x, 1,55x =0.55, x= 0.55/1.55=0.35L , Volume of CH3COOH= 1-0.35=0.65

3. pKa= 9.31= 5+log (CN-/HCN)

20417= x/(1-x). Where x= volume of NaCN

Hence x= 0.99 volume of CN- and Volume of HCN= 1-0.99=.01L