Chemical Forums

Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: TheMatroid on April 08, 2020, 01:49:53 PM

Title: Help with a buffer question
Post by: TheMatroid on April 08, 2020, 01:49:53 PM

Consider a buffer consisting of 0.75 M NH3 and 0.50 M NH4Cl. If 0.10 M H2SO4 is added to the solution with no change in volume, what is the resulting pH?

I'm not certain how to go about this one. The pkb oh NH3 isn't on the tables provided. Ammonia's pka is given (36) , but I'm not sure on how to use that. I know pOH = pKb + log (b+/boh). I assume I need this formula to find the initial pH, but I'm not sure. Thanks.

Title: Re: Help with a buffer question
Post by: chenbeier on April 08, 2020, 01:57:49 PM

If the pKa is given then pKb is also given. Do you know relationship between pKa and pKb? Tipp
, similar pH and pOH.

You need Henderson Hasselbalch equation and the values of ammonia and ammonium after adding sulfuric acid.

Title: Re: Help with a buffer question
Post by: Babcock_Hall on April 08, 2020, 02:12:06 PM

The pK_a of 36 for ammonia refers to ammonia as the conjugate acid and the amide ion as the conjugate base.  This is not the equilibrium of interest. 

Can you write the equilibrium acid-dissociation expression for which you need the pK_a?

Title: Re: Help with a buffer question
Post by: Borek on April 08, 2020, 02:27:03 PM

https://www.chembuddy.com/?left=buffers&right=pH-change