Chapter 16

# Chapter 16 - Buffer a solution of a weak acid or base and...

This preview shows pages 1–21. Sign up to view the full content.

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

View Full Document

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

View Full Document

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

View Full Document

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

View Full Document

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

View Full Document

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

View Full Document

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

View Full Document

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

View Full Document

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

View Full Document

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

View Full Document
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: Buffer : a solution of a weak acid or base and its conjugate base or acid. Buffers resist changes in pH. HA + OH- H 2 O + A- weak acid reacts with any added OH- A- + H 3 O + HA + H 2 O weak base reacts with any added H 3 O + for HA + H 2 O H 3 O + + A- K a = [H 3 O + ] = so pH = p K a + log [H 3 O + ][A- ] [HA] K a [HA] [A- ] [HA] [A- ] In a buffer system with HA and A- both present in large concentration, [HA] ≈ C a C a is the analytical concentration of the acid [A- ] ≈ C b C b is the analytical concentration of the base Henderson-Hasselbalch equation pH = p K a + log C b C a Calculate the pH of a solution of 0.20 M NH 3 and 0.30 M NH 4 Cl K b = 1.8 x 10-5 Calculate the pH of a buffer that is 0.25 M HCN and 0.15 M NaCN, K a = 4.9 x 10-10 Calculate the pH of a buffer that is 0.25 M HCN and 0.15 M NaCN, K a = 4.9 x 10-10 pH = -log(4.9 x 10-10 ) + log = 9.09 0.15 0.25 The Henderson-Hasselbalch equation pH = p K a + log can be rewritten as pH = p K a + log n b n a C b C a Determine the number of moles of CH 3 COONa that must be added to 250 mL of 0.16 M CH 3 COOH in order to prepare a pH 4.70 buffer. K a = 1.8 x 10-5 n a = MV = (0.16 M )(0.250 L) = 0.040 mol pH = p K a + log 4.70 = -log(1.8 x 10-5 ) + log-0.04 = log n b n a n b 0.040 mol n b 0.040 mol 0.91 = n b = 0.036 mol n b 0.040 mol How many moles of NaCN should be added to 100 mL of 0.25 M HCN to prepare a buffer with pH = 9.40? K a = 4.9 x 10-10 How many moles of NaCN should be added to 100 mL of 0.25 M HCN to prepare a buffer with pH = 9.40? K a = 4.9 x 10-10 Answer: 0.031 mol NaCN Calculate the initial and fnal pH when 10 mL oF 0.100 M HCl is added to (a) 100 mL oF water, and (b) 100 mL oF a buFFer which is 1.50 M CH 3 COOH and 1.25 M CH 3 COONa Initial pH of water is 7.00 [HCl] = [H 3 O + ] = C HCl = 0.0091 M pH = -log(0.0091) pH = 2.04 (0.110 L) (0.010L)(0.100 M ) n a = (0.100 L)(1.50 M ) = 0.150 mol n b = (0.100 L)(1.25 M ) = 0.125 mol p K a = -log(1.8 x 10-5 ) = 4.74 pH = 4.74 + log = 4.66 0.125 0.150 n(H 3 O + ) = (0.010 L)(0.100 M ) = 0.0010 mol CH 3 COO- + H 3 O + CH 3 COOH + H 2 O s 0.125 mol 0.001 mol 0.150 mol R-0.001 mol -0.001 mol +0.001 mol f 0.124 mol 0 mol 0.151 mol pH = 4.74 + log = 4.65 pH changes -0.01 units (0.124) (0.151) Calculate the fnal pH when 10 mL oF 0.100 M NaOH is added to 100 mL oF a buFFer which is 1.50 M CH 3 COOH and 1.25 M CH 3 COONa Ka = 1.8 x 10-5 Calculate the fnal pH when 10 mL oF 0.100 M NaOH is added to 100 mL oF a buFFer which is 1.50 M CH 3 COOH and 1.25 M CH 3 COONa Answer: pH = 4.67 Titration : an analysis based on measuring the amount of substance (the titrant ) required to react with a measured amount of analyte- the substance whose concentration is being determined...
View Full Document

## This note was uploaded on 02/21/2012 for the course CHEM 112 taught by Professor Chen during the Spring '09 term at South Carolina.

### Page1 / 81

Chapter 16 - Buffer a solution of a weak acid or base and...

This preview shows document pages 1 - 21. Sign up to view the full document.

View Full Document
Ask a homework question - tutors are online