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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: schafer on October 05, 2014, 08:13:54 AM

Title: Calculating concentrating of OH vs. H
Post by: schafer on October 05, 2014, 08:13:54 AM
Hi,
I got stuck on a silly easy question, and have a test soon. Was hoping someone could assist me.

I need to calculate the pH of a solution containing 2g of NaNO2 in 500ml. I know the Kb.
I know I need to solve it with one of the formulas:
H+=(Ka X Ca)^0.5
or
HO-= (Kb X Cb)^0.5

I know I can calculate the Ka since I know the Kb (pKa+pKb=pKw=14)

My problem is:
I calculated the concentration of NaNO2 (2g/mw/2). I don't understand how do I tell if this is Ca or Cb...
any help?

thanks .

Title: Re: Calculating concentrating of OH vs. H
Post by: Hunter2 on October 05, 2014, 08:17:03 AM
You need Ka, because the acid is the important thing. KB is used if you have NH4Cl , then ammonia the base is the important thing.

NaOH strong base, HNO2 weak acid.
Title: Re: Calculating concentrating of OH vs. H
Post by: Borek on October 05, 2014, 08:46:44 AM
Quote from: schafer on October 05, 2014, 08:13:54 AM
I need to calculate the pH of a solution containing 2g of NaNO2 in 500ml.

Apparently your problem is with determining what is responsible for pH change in this solution.

Do you remember Bronsted-Lowry definition of an acid? Of a base?

Can you apply this definition to both ions present in the salt?

Quote from: Hunter2 on October 05, 2014, 08:17:03 AM
You need Ka, because the acid is the important thing. KB is used if you have NH4Cl , then ammonia the base is the important thing.

Kb is what is needed to solve this particular problem. Ka and Kb can be easily interconverted, but here it is definitely Kb that makes calculations easy. Question is - Kb of what?
Title: Re: Calculating concentrating of OH vs. H
Post by: schafer on October 05, 2014, 09:04:46 AM
I'll explain my problem better (having problems since I do not study in english, nor is it my first language):
As far as I understand, the salt is composed of two ions (Na+ and NO2-)...
My problem is that after I calculate the concentration of NaNO2 I don't know if this is the concentration of acids (then I'll use Ka to find pH) or bases (then I'll use Kb to find pOH).