Chem 109 

Spring 2009 
Lecture Notes:: 27 April 


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Acids and Bases, cont.
The concentration of hydronium ion in water is extremely influential on all kinds of chemistry. The range of hydronium ion concentration in water is also vast, with extremes of about 10M to about 10^{15}M, and commonly ranging from 1M  10^{14}M. Imagine plotting [H_{3}O^{+}] vs. volume of acid added to a base solution in a titration from 10^{14}M  1M. If you had one cm on the graph paper = 10^{14}M, then you would need a piece of paper 10^{9} km long (greater than the distance from the Sun to Jupiter) to plot this titration! Obviously a more convenient measure is needed. This is easily accomplished by looking instead at the logarithm of [H^{+}] and defining a new term,
Because of the equilibrium dissociation of water to H^{+} + OH^{}, the concentration of hydrogen ion in water is related to the concentration of hydroxide ion:
But the concentration of water remains essentially the same in dilute solution, so by convention we define the dissociation constant or ion product for water:
Let's look at some general characteristics of pH in aqueous solution.
Examples:
Note that the significant figures are correct, 1 is the power of ten, only the figures to the right are significant.
Note that the "p" has the more general meaning of "log[]". Thus pOH is log [OH^{}], pCa = log [Ca^{2+}], etc.
Weak acid dissociations involve equilibria. The equilibrium constants have a specific symbol = K_{a}.
Example: What is the pH of a 0.10 M solution of acetic acid. K_{a} = 1.8 x 10^{5}
HOAc  H^{+}  +  OAc^{}  
Before reaction  0.10 M  0  0  
@ Equilibrium 

x  x 
K_{a} = [H^{+}][OAc^{}] / [HOAc]
assume x << 0.1 since K_{a} =1.8 x 10^{5}, then [HOAc] = 0.10 M
Notice the significant figures. For a log function the number in front of the decimal is the exponent of ten, thus pH = 2.87 is a 2 significant figure number.
Buffer calculations: One of the most frequent calls for calculating acid equilibria is calculations involving buffers. What is a buffer?
With this in mind let's do some examples.
Example: Calculate the pH of a buffer made up by dissolving 0.0125 moles acetic acid (HOAc) and 0.0250 moles of sodium acetate (NaOAc) in enough water to make 1.000 L of solution. K_{a} = 1.8 x 10^{5}

pH = 5.046
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© R A Paselk
Last modified 27 April 2009