# 280Exp3 - Experiment 3 CHM 280L Quantitative Analysis...

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Experiment 3 CHM 280L “Quantitative Analysis” 1 Experiment 3. Potentiometric Titration and the Gran Plot In this experiment, you will monitor the extent of a titration reaction by measuring the pH of the solution. You will also determine the acid dissociation constant (K a ) of a weak acid by constructing a Gran Plot. ALWAYS WERE EYE PROTECTION IN THE LABORATORY BE SURE YOUR LEGS AND FEET ARE COVERED Titration of a Weak Acid with a Strong Base HA H + + A - Consider the dissociation of a weak acid (HA) in water to from H + and A - . By definition, a weak acid dissociates very slightly, therefore, an equilibrium is reached with most of the acid in the HA form. The extent of the reaction is described by the equilibrium constant, which in this case is called the acid dissociation constant K a , and K a id typically very small. Eq. 1 ] [ ] ][ [ HA A H K a + = This equation may be rearranged so that: Eq. 2 ] [ ] [ ] [ + = A HA K H a Next, if we consider the case where we titrate the weak acid with a strong base, NaOH, the acid is neutralized upon the addition of base as follows: NaOH + HA → H 2 O + Na + + A - For this case, Gran showed in 1950 that [HA] and [A - ] could be replaced with the volumes of base added as follows: Eq. 3 ( ) add add eq a V V V K H = + ] [ where V add is simply the total volume of NaOH added in mL at any point during the titration, and V eq is the volume of NaOH added to reach the endpoint or equivalence point. K a

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Experiment 3 CHM 280L “Quantitative Analysis” 2 Multiplying both sides by V add gives the following equation: Eq. 4 eq a add a add V K V K V H + = + ] [ Notice that this is the equation for a line! If we prepare a plot with V add (x-axis) versus [H + ]V add (y-axis) we should get a straight line with slope = -K a . At the extrapolated x-intercept of this plot, where [H + ]V add = zero, V add = V eq . So we can find the amount of NaOH needed to reach the endpoint without detecting the endpoint precisely with an indicator. Part I. Standardization of NaOH
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280Exp3 - Experiment 3 CHM 280L Quantitative Analysis...

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