Estimating Equilibrium Constants The comparative strengths of hydrochloric and acetic acids are usually demonstrated qualitatively by comparing their conductivity. When solutians of equal molarity are used in the standard classroom conductivity apparatus, the light bulb is much dimmer with acetic aeid than with hydrochloric aeid. The fallowing demonstration, using the same apparatus, allows the class to compare the acids quantitatively and actually determine the ionization constant of acetic acid: Start with equal concentrations of bath acids, and then dilute' the hydrochloric aeid hy known steps. Demonstrate the conductivity of each dilution as you prepare it (keeping constant the amount of electrode surface in contact with the solution), until the class decides that the light bulb is of the same intensity with dilute HCI as it is with the acetic acid. Assuming (1) that conductivity depends on the concentration of ions present in the solution and (2) that the HCI is eampletely ionized, you can now estimate the concentration of ions in the acetic acid. The following set of dilutions gives spectacularly accurate results: Start with 100 ml of each 1 M acetic and 1 M HCI, and use a 60- or 75-W bulb in the apparatus: (a) dilute 10 ml 1M HCI to 100 ml to get 0.1 M H C I ; (b) dilute 10 mlO.l M HCI to 100 ml to get 0.01 M HC1; (c) dilute 10 mlO.O1 M H C I to 100 ml t o get O.W1 M HCl (too dilute); (dl dilutC50 mlO.O1 MHCI to 100 ml t o get 0.005 MHCI (too concentrated); (e) dilute 40 mlO.O1 MHCI to 100 ml to get 0.004 MHCI. The last dilution has the same conductivity as the 1M acetic. The results show that 1M acetic acid is 0.4% ionized and K , =
[H +I [CzH&
-1
= 1.6 X
[HC&021 The published values are 0.43% ionization or K , = 1.'8X Students are astounded by these results. Even if they choose the 0.005 M HCI as the proper concentration, K . is found to be 2.5 x which is not too bad for the supposedly crude light bulb apparatus. Weston High School ~ r n e s t c~. a r r i & o n Weston. Connecticut 06880
Volume 50, Numbers, August 1973
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549
