Tuesday
September 23, 2014

Homework Help: Plz -- Chem Help

Posted by A on Wednesday, March 18, 2009 at 10:54am.

Posted by Rushi on Sunday, March 15, 2009 at 3:35pm.

Question..

Nicotinic acid (niacin) is a monoprotic acid with the formula HC6H4NO2. A solution that is 0.012M in the nicotinic acid has a pH of 3.39 at 25C. what is the acid-ionization constant, Ka and pKa for this acid at 25C?


Attempt..

HC6H4NO2 <----> C6H4NO2- + H+

[H+] = 10^-3.30= 0.00501 M = [C6H4NO2-]
[HC6H4NO2] = 0.012 - 0.000501 =0.0115 M

Ka = ( 0.00501)^2 / 0.0115 = 0.00218
pKa = - log Ka =2.66

Responses

* Plz -- Chem Help - DrBob222, Sunday, March 15, 2009 at 5:47pm

Note you had pH 3.39 in the problem.
[H+] = 10^-3.30= 0.00501 M = [C6H4NO2-]
[HC6H4NO2] = 0.012 - 0.000501 =0.0115 M
This step is OK.

Ka = ( 0.00501)^2 / 0.0115 = 0.00218
pKa = - log Ka =2.66
Note here that you used 0.00501 and not 0.000501 in the numerator. So the digits are ok but the power is off. I think it should be 2.18 x 10^-5 for Ka with the corresponding change in pKa. Check my work.

* Plz -- Chem Help - Ed, Monday, March 16, 2009 at 11:22pm

What I got is a bit different... Can you please check my work?

[H+] = 10^-3.39 = 4.074*10^-4 = [C6H4NO2]
[HC6H4NO2] = 0.012-4.074*10^-4 = 0.0116 M

Ka = [4.074*10^-4]^2/0.0116 = 1.43*10^-5
pKa = -logKa = 4.84

Answer this Question

First Name:
School Subject:
Answer:

Related Questions

Plz -- Chem Help - Question.. Nicotinic acid (niacin) is a monoprotic acid with ...
CHEM HELP - Nicotinic acid (niacin) is a monoprotic acid with the formula ...
CHEM HELP! - Nicotinic acid (niacin) is a monoprotic acid with the formula ...
Urgent Chem Help - Question.. Nicotinic acid (niacin) is a monoprotic acid with ...
Chemistry 122 - A solution of nicotinic acid, HC6H4NO2, of concentration 0.012 M...
chemistry - write the formula of each acid and identify each as a diprotic, a ...
CHEM REPOST...PLEASE CHEK! - Posted by Kat on Monday, March 2, 2009 at 11:21am. ...
Chemistry Acid Question Ka - Enough of a monoprotic acid is dissolved in water ...
Chemistry - Explain why acetic acid has four hydrogens, but is a monoprotic acid...
chemistry - A. Strong Base 1.) What is the concentration of a solution of KOH ...

Search
Members