Chemical Forums

Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: Joules23 on March 08, 2008, 07:28:13 PM

Title: Calculating pH
Post by: Joules23 on March 08, 2008, 07:28:13 PM
Calculate the pH after 0.020 mol NaOH is added to 1.00 L of each of the solutions below.
(a) 0.121 M HONH2 (Kb = 1.1  10-8)
(b) 0.121 M HONH3Cl
(c) pure H2O
(d) a mixture containing 0.121 M HONH2 and 0.121 M HONH3Cl

My attempt for (a)

HONH2 +OH- -> HONH- + H2O
.121----.02----.121
.101----0------.141

pH=pKa + Log (B/A)
pH=6.04 + log (.141/.101)
pH=6.18

(c) -log(1E-14/.02)= 12.03

im not sure how to do part b, or d
Title: Re: Calculating pH
Post by: Borek on March 09, 2008, 05:30:35 AM
How does HONH2 react with water? Hint: in a way it is similar to ammonia, it is a Brønsted-Lowry base.

Are you sure you have no idea how to do c? If so, you should get back to the water ion product (http://www.chembuddy.com/?left=pH-calculation&right=water-ion-product).
Title: Re: Calculating pH
Post by: Joules23 on March 09, 2008, 12:22:58 PM
Ok i got part (c) thanks, but still stuck on part a..

HONH2 + H20 -> HONH3+ + OH-

so would adding NaOH decrease HONH2 and increase OH-

-------------HONH2 + H20 -> HONH3+ + OH-
Before NaOH--.121------------.121--------0
After NaOH---.101------------.121-------.02
Title: Re: Calculating pH
Post by: Borek on March 09, 2008, 01:30:45 PM
Quote from: Joules23 on March 09, 2008, 12:22:58 PM
Ok i got part (c) thanks, but still stuck on part a..

HONH2 + H20 -> HONH3+ + OH-

so would adding NaOH decrease HONH2 and increase OH-

Yes. Now, for obvious reasons NaOH is completely dissociated. HONH2 is a vey weak base. Will it be able to dissociate enough to change pOH just from NaOH? What is the pOH (or pH) of 0.121 HONH2 solution?
Title: Re: Calculating pH
Post by: Joules23 on March 09, 2008, 01:44:01 PM
Quote from: Borek on March 09, 2008, 01:30:45 PM
Will it be able to dissociate enough to change pOH just from NaOH?
No?

Quote from: Borek on March 09, 2008, 01:30:45 PM
What is the pOH (or pH) of 0.121 HONH2 solution?
pOH = 8.88
pH = 5.12
Title: Re: Calculating pH
Post by: Borek on March 09, 2008, 01:58:10 PM
Quote from: Joules23 on March 09, 2008, 01:44:01 PM
Quote from: Borek on March 09, 2008, 01:30:45 PM
What is the pOH (or pH) of 0.121 HONH2 solution?
pOH = 8.88
pH = 5.12

Show how you got these numbers. First of all - you have solution of base, yet your pH is acidic. But you have not simply switched pH with pOH - 8.88 is not a correct answer too :(
Title: Re: Calculating pH
Post by: Joules23 on March 09, 2008, 02:06:15 PM
ok i just redid it and i got pH = 9.56 .. i dont know what i did wrong last time, but i did it quickly so i probably enetred something incorrect int he calculator

1.1E-8=(x^2/.121)
x=3.648E-5=[OH-]
-log[OH-]=4.44=pOH
Title: Re: Calculating pH
Post by: Borek on March 09, 2008, 02:25:30 PM
9.56 it is :)

Now, what about b? What is HONH3Cl? How does it dissociate? Is there anything acidic/basic in the dissociation products?
Title: Re: Calculating pH
Post by: Joules23 on March 09, 2008, 02:32:54 PM
HONH3Cl   -->   HONH3+ + Cl-

no, so pH will be the same?
Title: Re: Calculating pH
Post by: Borek on March 09, 2008, 02:36:07 PM
No. HONH2 and HONH3+ are base and conjugated acid, just like NH3 and NH4+...
Title: Re: Calculating pH
Post by: Joules23 on March 09, 2008, 02:41:38 PM
HONH3+ + OH-  -->  HOHN2 + H2O
.121-------0-----------0
.101------.02----------0

?
Title: Re: Calculating pH
Post by: Borek on March 09, 2008, 02:46:36 PM
Or

HONH3+ <-> H+ + HONH2