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Posted by on Sunday, February 12, 2012 at 3:17pm.

A student is given 3 beakers:

Beaker 1- 50.0 ml of a solution produced by dissolving 6.00 grams of a weak
monoprotic acid ,HX, in enough water to produce 1 liter of solution.
The empirical formula of HX is CH2O. The solution contains 3 drops
of phenolphthalein.

Beaker 2- A 0.07M solution of the salt NaX. It has a pH of 8.8

Beaker 3 – 50.0 ml of 0.250M KOH

The contents of beaker 3 is added drop-wise to beaker 1 until a pink color appears and remains for 30 seconds. This takes exactly 20.0 ml of the beaker 3 solution. Identify X and calculate the pH of beaker 1 after the addition of the 20 ml.

  • chemistry - , Sunday, February 12, 2012 at 3:59pm

    Beaker 2 solves for Ka.
    ...........X^- + HOH ==> HX + OH^-
    initial...0.07...........0......0
    change.....-y............y.....y
    equil.....0.07-y.........y......y

    Kb for X^- = (Kw/Ka for HX) = (y)(y)/(0.07-y)
    Kw, of course, is 1E-14 and you solve for Ka for HX. y = OH and you can obtain that from the pH of 8.8.

    Back to beaker #1.
    6.00g/molar mass CH2O = 0.2M.
    50 mL x 0.2M = 10 millimoles of HX
    20 mL of 0.250 KOH (beaker 3) = 5 mmol.
    Reaction of HX(beaker 1) and KOH (beaker 3).
    .........HX + KOH ==> KX + H2O
    initial..10.....0.......0....0
    add............5.0...............
    change...-5.0..-5.0....+5.0....+5.0
    equil....5.0.....0.......5.0....5.0

    Substitute from the above ICE chart into the Henderson-Hasselbalch equation and solve for pH.
    pH = pKa + log [(base)/(acid)]
    You know pKa from Ka you solved for from beaker 2.
    Look up Ka in a table of Ka. I suspect that is formic acid and X^- is formate ion.
    Post your work if you get stuck.

  • chemistry - , Sunday, February 10, 2013 at 12:34am

    So, if the X suppose to be HCH2O?

  • chemistry - , Sunday, February 10, 2013 at 12:54am

    how do find the pH of beaker 1 after the addition of the 20 mL

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