Homework Help: chemistry

Posted by Tom on Tuesday, May 27, 2008 at 9:36pm.

Captain Kirk, of the Starship Enterprise, has been told by his superiors that only a chemist can be trusted with the combination to the safe containing the dilithium crystals that power the ship. The combination is the pH of Solution A described below, followed by the pH of Solution C. (For example, if the pH of Solution A is 3.47 and the pH of Solution C is 8.15, then the combination to the safe is 3-47-8-15.) The chemist must find the combination using the information below. All solutions are at 25°C. (20 points)

Solution A is 50 mL of 0.1 M solution of the weak monoprotic acid HX.

Solution B is a 0.05 M solution of the salt NaX. It has a pH of 10.02.

Solution C is made by adding 15 mL of 0.25 M KOH to Solution A.

What are you suppose to do with Solution B to help solve for solution A.

My work:
10^-10.02 = 9.55x 10^-11= [H+]
[OH-]= 1.047 x 10^.4
Is 9.55 x 10^-11 the Ka of Solution B.
NaX--Na+ + X
X + H2O-- HX + OH

wat should i do to solve for B to solve for A.

what should I do after this

Responses

Chemistry - DrBob222, Tuesday, May 27, 2008 at 9:02pm
Use solution B to solve for the Ka of the weak acid.
Use Ka from above to solve for the pH of solution A. That gives you the first 3 numbers of the combination.
Use solution C and the Henderson-Hasselbalch equation to solve for the pH of that solution which will be last three numbers of the combination.

Chemistry - Tom, Tuesday, May 27, 2008 at 9:36pm
how do u use Solution B to solve for the Ka?

chemistry - DrBob222, Tuesday, May 27, 2008 at 10:09pm
Solution B is 0.05 M solution of NaX.
The hydrolysis of X^- is
X^- + HOH ==> HX + OH^-
Kb = Kw/Ka = (HX)(OH^-)/(X^-)
You know pH is 10.02. Subtract from 14 to get pOH and solve for (OH^-) remembering that pOH= -log(OH^-). You know (HX) = (OH^-). YOu know Kw and (X^-) so the only unknown is Ka. Solve for Ka.

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