Chemical Forums

Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: Budge on April 02, 2019, 08:51:36 PM

Title: Calculating pH of a weak base given pKb
Post by: Budge on April 02, 2019, 08:51:36 PM

Could anybody lend me a hand with this calculation:

Pyridine is a weak organic base with pKb = 8.75.  Determine the pH of a 0.33 M aqueous solution of pyridine at 25.0 °C

When I have attempted this, I end up with a pH greater than 14. Any help would be much appreciated!

Title: Re: Calculating pH of a weak base given pKb
Post by: Borek on April 03, 2019, 02:48:40 AM

Hard to say what you did wrong not seeing your work,. Show how you got the wrong result and we will start from there.

Title: Re: Calculating pH of a weak base given pKb
Post by: Budge on April 04, 2019, 12:47:22 AM

My working out is attached. I tried to use an ICE table to find the concentration of hydroxide ions so I could use that to calculate pOH and hence pH.

Title: Re: Calculating pH of a weak base given pKb
Post by: Borek on April 04, 2019, 02:56:01 AM

What is the difference between Kb and pKb?

Title: Re: Calculating pH of a weak base given pKb
Post by: Budge on April 04, 2019, 07:57:00 AM

Oh true, I can see I've substituted pKb into the equation where I should've put in Kb, thanks!

I would just need to take the negative log of pKb to find Kb right?

Title: Re: Calculating pH of a weak base given pKb
Post by: Borek on April 04, 2019, 09:54:32 AM

Quote from: Budge on April 04, 2019, 07:57:00 AM

I would just need to take the negative log of pKb to find Kb right?

Nope. You need to brush up your math.

What is the definition of the logarithm?