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CHEM237L Chapter Notes - Chapter 2: Buffer Solution, Titration Curve, Burette

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12 Jul 2017

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Introduction: a buffer resists changes in ph that would other occur after adding a sb or sa which contain weak acids and their conjugate base salts (or vice versa). Capacity is higher when ha=a- and concentration is greater. Adding sa ha h+ + a- + oh- h2o. Ha h+ + a- ph = pka + log [cb]/[wa] = pka + log (cb/ca) When ca=cb, ph = pka + log 1 = pka ph equivalence #1 = pka1 + pka2 / 2. ), it is highly unlikely that the ph will be closer to pka 4 . Q1: explain what might happen when 2 and 2. 5 equivalents of strong base is added to the phosphoric acid. A1: if 1 equivalent to phosphoric acid gets h3po4 h2po4 with the first equivalence at a ph in between pka 1 and 2. The 2nd equivalent will get h2po4 hpo4 with the second equivalence at a ph between pka 2 and 3.

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Related Questions

Experiment 18: Buffers Purpose In this experiment you will prepare several buffers and investigate their properties. Discussion A solution of a weak acid and one of its salts, in approximately equal concentrations, resists changes in pH when acid or base is added to it. (Note: The pH of a solution is a measure of the concentration of free H3Ot ion; it does not indicate concentration of combined acid such as HA.) The same is true for a solution of a weak base and one of its salts. A solution with this property is called a buffer (see illustration next page). Bufter systems are extremely important in many chemical and biological systems. For example, your blood contains a buffer system utilizing carbonic acid and bicarbonate ion. The important equilibrium in an acidic buffer is HA (aq) H20 ) A (a)0 () acid conjugate base in which the concentrations of acid and conjugate base are approximately equal. If a small amount of free acid (H30) is added to this solution, it reacts with the conjugate base A (reaction 1 in reverse). The equilibrium constant for this reaction is very large, so the reaction proceeds to near completion, absorbing the added acid. Similarly, added base (OH-) reacts with the acid HA as shown below: 2. HA (aq) + OH-(aq) ATaq) + H2O (1) This reaction also proceeds to completion, effectively absorbing any added base. Since the H30 and OH- concentrations are kept constant, the solution thus resists pH changes when an acid or base is added to it. A similar argument can be made for a basic buffer The actual pH of a buffer can be determined from the value of the Ka and the concentrations of the acid and conjugate base obtained from an initial-change- equilibrium table. However, a shortcut for buffers is available, and is shown below. We begin by writing and rearranging the equilibrium constant expression for the acid dissociation. If the -log of both sides of equation (3) is taken, a relationship involving pH is then obtained. H30IA] HA] 3. HA HAI -loglH30'1- -log Ka A- 141