Question

-If you make up a solution of 100mL of 0.1M Hepes in the basic form, what...

-If you make up a solution of 100mL of 0.1M Hepes in the basic form, what will be the pH?

-Now you add 3mL of 1M HCl to the solution above, now what will by your pH?

-what can you conclude about the effect of dilution on the pH of a buffer?

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Homework Answers

Answer #1

Hepes pKa = 7.5

Hepes + H2O <-> HepesH+ + OH-

then

V = 0.1 L

M = 0.1 M

solve for

pH = -log[H+]

Kb = [HepesH+][OH-]/[HEPES]

10^-6.5 = x*x/(0.1-x)

x =1.1776*10^-4

pOH = -log(OH) = -log(1.1776*10^-4) = 3.9290

ph = 14-3.9290= 10.071

b)

if add 3 ml of 1 M Hcl then

conjugat acid = 0 + 3*1 = 3

base = 0.1*100 - 3*1 = 7

total V = 100+3 = 103

then

this is a buffer so

pOH = pKb + log(conjugate/base)

pOH = 6.5 + log(3/7) = 6.1320

ph = 14-6.1320 = 7.868

c)

change in pH

pH = 10.071-7.868 = 2.203

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