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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: hvard78 on April 17, 2011, 06:18:34 PM

Title: pH problem
Post by: hvard78 on April 17, 2011, 06:18:34 PM

The pKa value for HCN is 9.21. What molar concentration of NaCN is required to make a solution with a pH of 11.75?

I know:
pKa + pKb= 14
so pKb= 4.79
also, Kb= Kw/Ka
so Kb= 1.62*10^-5

Any help is appreciated.

Title: Re: pH problem
Post by: scientifics on April 17, 2011, 10:17:02 PM

If you look at your two components, you will see there is a common-ion. This makes it a common-ion effect problem.  What is the balanced equation? From there, you should be able to set up an ICE table.

Title: Re: pH problem
Post by: AWK on April 18, 2011, 01:44:15 AM

http://www.chembuddy.com/?left=pH-calculation-questions&right=pH-weak-base-hydrolysis-q1

Title: Re: pH problem
Post by: Borek on April 18, 2011, 04:09:03 AM

Quote from: scientifics on April 17, 2011, 10:17:02 PM

If you look at your two components, you will see there is a common-ion. This makes it a common-ion effect problem.  What is the balanced equation? From there, you should be able to set up an ICE table.

There are no two components.

Title: Re: pH problem
Post by: scientifics on April 18, 2011, 08:53:52 PM

The two components are your two compounds (HCN and NaCN). Sorry for any confusion.