Chemical Forums
Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: manbot on January 26, 2007, 12:03:09 AM
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Im starting to go insane because a simple lab procedure is not making sense and the only thing i can figure is that there is a mistake in the manual. Here is the question:
1. Start with 100 ml of .2M sodium acetate (MW = 82 g/mol).
(i made this with 1.64 g of sodium acetate in water)
2. Make 120 ml of .2M acetic acid (glacial acetic acid concentration = 17.45 M)
(i made this with 1.38 ml of acetic acid and water.)
3. Titrate the sodium acetate with the acid until the pH is 4.5.
Now by my calculations this is impossible. Given pKa of acetic acid to be about 4.75, it will obviously require more acid than base to lower the pH below the pKa. In lab, after mixing the entire amount of acid, the pH just barely reached 4.52 (even though through calculation with the hendersohn hesselbalch equation (pH = pKa + log(A- / HA)) the pH should have only reached 4.68). By my calculation it should have taken 178 ml of acid at this concentration to achieve that pH. So im either making some horribly obvious mistake, or the manual is in error. Can anyone see what is going on with this?
thanks
rick
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Yep, something is wrong. Not enough acid. After mixing all together I got 4.67 or 4.55 depending on whether ionic strength and activities were taken into account or not - in both cases it is above 4.50.
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yeah, i knew somethign was up. ill have to ask my TA about it.