Lecture Notes Week 05

Lecture Notes Week 05 - WEEK 5 What is a Buffer Solution? A...

Info iconThis preview shows pages 1–3. Sign up to view the full content.

View Full Document Right Arrow Icon
1 – WEEK 5 – What is a Buffer Solution? A solution which resists change in ______ when either acid or base is added. It MUST have _______________________________________ present in " substantial amounts " ( or _________________________________________). Definition of “substantial amounts”: Conjugate pair ratio must in the range of 1/10 to 10/1. i.e., if the conjugate pair ratio (acid/base) in your solution is between 0.1 and 10, then you have what is known as a BUFFER solution ! Mathematical treatment: HA(aq) + H 2 O(l) H 3 O + (aq) + A (aq) [I] C HA 0 C A– [C] x + x + x [E] C HA x X C A– + x Note : For a buffer solution, we have an amount for the molecule on the reactant side and an amount for the conjugate on the product side. K a = [H 3 O + ][ A ]/[HA] = x(C A– + x )/( C HA x ) x = [H 3 O + ] = K a (C HA x )/(C A– + x ) Taking the log of the above relationship: log [H 3 O + ] = log K a + log (C HA /C A– ) Multiply the entire equation by –1: –log [H 3 O + ] = –log K a – log (C HA /C A– ) Substitute: pH = pK a + log (C A– / C HA ) this relationship is known as the Henderson–Hasselbalch Equation Also note: pH = pK a + log (n A– /V)/(n HA /V) = pK a + log (n A– /n HA ) In the case of a buffer solution, because the volume remains constant, we can just enter the amount of moles for the conjugate base and the conjugate acid into the Henderson– Hasslbach equation. We don’t need to convert to concentration units. This is one of the advantages of the Henderson– Hasselbach equation over using the K a expression. Also the equation gives us pH directly, no matter what type of buffer is used. Over what range is a buffer effective in controlling pH? Buffer Limits: log(conj. base/acid) = log(0.1) = 1 log(conj. base/acid) = log(10) = +1 ************************************************************** Buffer pH Range: pH = pK a ± 1 **************************************************************
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
2 Mode of Buffer Action: How does a Buffer Buff? We can add either acid or base to a buffer sol’n. What happens when you add some strong acid? – added acid reacts with ________________________________________ – net ionic equation: Does the equilibrium lie to the LHS or to the RHS? What is K for the process?
Background image of page 2
Image of page 3
This is the end of the preview. Sign up to access the rest of the document.

This note was uploaded on 04/17/2011 for the course CHEM 1040 taught by Professor Prokiacpak during the Spring '08 term at University of Guelph.

Page1 / 10

Lecture Notes Week 05 - WEEK 5 What is a Buffer Solution? A...

This preview shows document pages 1 - 3. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online