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Lecture Notes Week 05

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

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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 **************************************************************

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

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