Acids and Bases 3

Acids and Bases 3 - BUFFER SOLUTIONS 03/03/08 Acids &...

Info iconThis preview shows pages 1–7. Sign up to view the full content.

View Full Document Right Arrow Icon
03/03/08 1 BUFFER SOLUTIONS
Background image of page 1

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

View Full DocumentRight Arrow Icon
03/03/08 2 BUFFER SOLUTIONS Both acid-base indicators and pH buffers involve mixtures of weak acids (or bases) and their salts. Acid-base Indicators are present in low concentrations. - they are sensitive to the pH of a solution, but do not change it. Buffer solutions are present in high concentrations and control the pH. Buffer solutions provide an excellent example of Le Chatelier’s principle, since they are designed to stabilize pH – the equilibrium concentrations shift in such a way that they oppose the effects of added acid or base.
Background image of page 2
03/03/08 3 3 3 2 3 2 ( ) ( ) ( ) + a H O [ aq ][CH CO aq ] K = [CH CO H aq ] - 3 2 3 3 2 ( ) ( ) ( ) a [CH CO H aq ] K [H O aq ] = [CH CO aq ] + - Rearranging, we get : Consider an acetic acid/sodium acetate buffer. H 2 O + CH 3 CO 2 H (aq) CH 3 CO 2 - (aq) + H 3 O + (aq) CH 3 CO 2 Na (aq) Na + (aq) + CH 3 CO 2 - (aq) Converting to base-10 logs pH =− log 10 [ H 3 O aq ]= pK a log 10 [ CH 3 CO 2 H aq ] [ CH 3 CO 2 aq ]
Background image of page 3

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

View Full DocumentRight Arrow Icon
03/03/08 4 or we may write: This is often referred to as the Henderson-Hasselbalch equation. Because the pH depends on the ratio of the concentrations of the acid and base, dilution should have no effect on the pH of a buffer , which is one of its most important properties. Thus, in order to set the pH of a solution, one needs to make up the solution with the appropriate ratio of acid and conjugate base concentrations. pH = pK a log 10 [ CH 3 CO 2 aq ] [ CH 3 CO 2 H aq ]
Background image of page 4
03/03/08 5 Table of Acetate Buffers [CH 3 CO 2 - ] [CH 3 CO 2 H] pH [CH 3 CO 2 - ] /[CH 3 CO 2 H] - 0.1 2.8 - 0.01 0.1 3.7 0.1 0.05 0.1 4.4 0.5 0.1 0.1 4.7 1.0 0.5 0.1 5.4 5.0 1.0 0.1 5.7 10.0 0.1 - 8.8 - Note: K a (CH 3 CO 2 H) = 1.8 x 10 -5 pK a = 4.74 Note that the most effective range of the buffer solution is ± 1 unit of pK around the pK a value. Outside this range, one of the concentrations may be too small.
Background image of page 5

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

View Full DocumentRight Arrow Icon
6 Again, these are measured values of the pH, which will not necessarily agree with predictions from H-H. They will be fairly close, though.
Background image of page 6
Image of page 7
This is the end of the preview. Sign up to access the rest of the document.

This note was uploaded on 04/29/2008 for the course HIST 101 taught by Professor Wormer during the Spring '08 term at UPenn.

Page1 / 19

Acids and Bases 3 - BUFFER SOLUTIONS 03/03/08 Acids &...

This preview shows document pages 1 - 7. Sign up to view the full document.

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