Repost- Please help?This is the 3rd time I have posted this question to chegg. I

Question

Repost- Please help?This is the 3rd time I have posted this question to chegg. I cant find anyone to help me. I think he wants us to use the hinderson-hasselback equation ?

You are given 10 ml of an unknown acid of unknown concentration. You titrate it with 0.1 M KOH. When you first measure the pH you forget to write it down. It takes 40 ml to titrate to the endpoint. Fortunately you did remember to get a pH reading of 3.22 when you had added 20 ml of KOH. What acid do you have? what was the original concentration? what was the starting pH?

Explanation / Answer

V = 10 mL of unkown acid + M

a)

mmol of KOH = MV = (0.1)(40) = 4 mmol of base used

then

mmol of acid = 4

[HA] = mmol acid/ V = 4/10 = 0.4 M

now..

pH = pKa + log(A-/HA)

in the half point, A- = HA then

pH = 3.22

implies

pKa = pH = 3.22

a)

Nearest acids are either HF with pKa = 3.14 or HNO2 with pKa = 3.32

b)

initially, [HA] = 4/10 = 0.4 M

c)

starting pH:

HA <->H+ + A-

Ka = [H+][A-]/[HA]

(10^-3.22) = X*x/(0.4-x)

x = 0.015226

[H+] = x= 0.015226

pH = -log(0.015226) = 1.817

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