Announcements. Print worksheet #21 prior to your Thursday discussion section. LON-CAPA assignment #12 due NEXT Wednesday at 9am

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1 Announcements Print worksheet #21 prior to your Thursday discussion section LON-CAPA assignment #12 due NEXT Wednesday at 9am Dr. Nicely s office hours (205 Chem Annex): Mondays from 3-4pm, Fridays from 2-3pm or by appointment The online gradebook has been updated with all exam, clicker, quiz and LON-CAPA assignments Please check for accuracy!!

2 ph of Salt Solutions Solutions involving IA and IIA metal ions are NEUTRAL ex. Li +, Ca 2+ Solutions involving the conjugate base of a strong acid are NEUTRAL ex. Cl -, NO 3 - Solutions involving the conjugate acid of a weak base are ACIDIC ex. NH 4+, C 5 H 5 NH + Solutions involving the conjugate base of a weak acid are BASIC ex. NO 2-, OCl -

3 ph of Salt Solutions Classify the following as acidic, basic or neutral: LiNO 3 CH 3 NH 3 Cl Mg(C 2 H 3 O 2 ) NaClO 4 KNO 2

4 ph of Oxides Oxides containing group IA and IIA metals are BASIC Na 2 O(s) + H 2 O 2NaOH(aq) CaO(s) + H 2 O Ca(OH) 2 Metal oxides react with water to form metal hydroxides These are strong bases, so the overall solution will be basic

5 Buffers solutions that resist ph changes upon addition of H + or OH - Buffers always contain a weak acid/base conjugate pair: weak acid + conjugate base weak base + conjugate acid Which of the following are buffer systems? a) KH 2 PO 4 /H 3 PO 4 c) KCl/HCl b) NaClO 4 /HClO 4 d) KCHOO/CHOOH

6 ph of a Buffered Solution Calculate the ph of a solution containing 0.25 M HOCl (K a 3.5x10-8 ) and 0.25 M NaOCl Strategy: Identify the major species and then consider their acidic/basic properties HOCl: major species HOCl and H 2 O NaOCl: major species Na +, OCl - and H 2 O Na + does not affect the ph of the resulting solution HOCl and OCl - make a weak acid/base conjugate pair HOCl + H 2 O OCl - + H 3 O +

7 ph of a Buffered Solution Calculate the ph of a solution containing 0.25 M HOCl (K a 3.5x10-8 ) and 0.25 M NaOCl Initial Change Equilibrium K a HOCl + H 2 O OCl - + H 3 O x +x +x 0.25 x x x ( x)( 0.25) ( 0.25) [ H O ][ OCl ] + [ HOCl ] x ( x)( x) ( 0.25 x) ph -log(3.5x10-8 ) 7.46

8 ph of a Buffered Solution Does the initial concentration of the acid and its conjugate base matter? K a ( x)( 0.25) ( 0.25) [ H O ][ OCl ] + [ HOCl ] x ( x)( x) ( 0.25 x) Calculate the ph for a solution containing 0.50 M solutions of HOCl and NaOCl. What about 1.0 M solutions? The ph will always be 7.46 for this buffer unless we change the initial ratio of acid:base

9 Henderson-Hasselbalch Equation Calculate the ph of a buffer solution using the equation ph pk a where pk a - log (K a ) + [base] log [acid] If [A - ] [HA], then ph pk a Adjust ph by changing [A- ] [HA] The best buffering occurs when [A - ] [HA]

10 Henderson-Hasselbalch Equation For a buffer at ph 8.60, use an acid with a pk a 8.60 Acid K a pk a HA HB HC 2.7 x x x Use acid HC along with its conjugate base (C - ) to create a buffer with a ph centered at 8.59 How can we increase the ph to 8.60? Increase the ratio of base:acid

11 Calculate the ph of a buffer containing L of 1.0 M KCHOO and L of 1.0 M CHOOH (K a 1.8x10-4 ) ph ph pk a log 1.8 [base] + log [acid] ( 4 10 ) + log Calculate the ph of the buffer after the addition of L of 0.15 M HCl Strategy #1: Identify the major species and then consider their acidic/basic properties CHOOH, CHOO -, H 2 O, H 3 O +, Cl - Strong acids tend to react with available bases

12 Calculate the ph of the buffer after the addition of L of 0.15 M HCl H 3 O + + CHOO - H 2 O + CHOOH Strategy #2: When solutions are mixed, the change in volume changes all the concentrations Set up the ICE table using MOLES 0.15 mol H O O 1L soln mol H3O L mol H3 + mol CHOO - mol CHOOH L 1.0 mol 1L soln mol

13 Calculate the ph of the buffer after the addition of L of 0.15 M HCl moles Initial Change Equilibrium H 3 O + + CHOO - H 2 O + CHOOH ph ph pk a + log log 1.8 base acid ( 4 10 ) + log

14 Buffer Preparation 1. Mix weak acid + salt of conjugate base or Mix weak base + salt of conjugate acid (see previous example) 2. Partial neutralization of a weak acid with a strong base or a weak base with a strong acid There MUST be an excess of weak acid (base) Assume that ALL of the strong base (acid) reacts to form the conjugate base (acid)

15 Calculate the ph of a buffer prepared by mixing 40.0 ml of 1.0 M C 3 H 5 OOH (K a 1.3x10-5 ) 60.0 ml of 0.10 M NaOH moles Change 1.0 mol C3H5OOH mol C H5OOH L 1L soln mol OH Equilibrium L 0.10 mol OH 1L soln mol mol C 3 H 5 OOH + OH - H 2 O + C 3 H 5 OO - Initial

16 Calculate the ph of a buffer prepared by mixing 40.0 ml of 1.0 M C 3 H 5 OOH (K a 1.3x10-5 ) 60.0 ml of 0.10 M NaOH moles Change Equilibrium C 3 H 5 OOH + OH - H 2 O + C 3 H 5 OO - Initial ph ph pk a log log 1.3 base acid ( 5 10 ) + log

Announcements. Exam 1 is on Thursday, September 23 from 7-8:10pm; Conflict exam is from 5:35-6:45pm

Announcements. Exam 1 is on Thursday, September 23 from 7-8:10pm; Conflict exam is from 5:35-6:45pm Announcements Print worksheet #6 prior to your Tuesday discussion section LON-CAPA assignment 4 is now available Don t forget to bring your Clicker to class EVERY day The points from last week have been