Asked by Val

Suppose that 0.483 g of an unknown monoprotic weak acid, HA, is dissolved in water. Titration of the solution with 0.250 M NaOH(aq) required 42.0 mL to reach the stoichiometric point. After the addition of 21.0 mL, the pH of the solution was found to be 3.75.
(a) What is the molar mass of the acid?
g/mol

(b) What is the value of pKa for the acid?

I see how I need to work backwards here...but I am confused on how to set this up. I know the concentration of H3O+ is 1.778E-4M and I tried dividing .483g by that to get the molar mass, but that was incorrect. I also know that the given pH is the half stoichiometric point, but I am unsure on how to use this to find the amount of Acid in moles that is being used. Any guidance? Thanks!
Answered by Val
oops so nevermind I realized i made a mistake when calculating the molar mass. I found it to be 46 g/mol but I am still unsure how to find the pKA value. Thanks!
Answered by DrBob222
You have the Ka for the weak acid is
Ka = (H^+)(A^-)/(HA)
If we solve this for (H^+) we get
((H^+) = Ka*(HA)/(A^-)
At the exact half-way mark to the equivalence point, the acid that is left (not yet neutralized) exactly equals the salt formed; therefore, (HA) = (A^-). Thus,
(H^+) = Ka
and pH = pKa. Neat, eh?
Answered by Val
that simple and I've been working on this stuff for 5 hours! Thanks DrBob...I really wish my professor was able to break things down so clearly. Nice lady but she has a way of making me more and more confused...

as for a PUG chart..maybe that's her own thing? It's similar to an ICE chart but it uses moles. so it'll be set up like this (using arbitrary numbers in moles)

HA + OH- ---> H2O + A-
"P"ut In .05 .03 0 0
"U"se up -.03 -.03 +.03 +.03
"G"et Out .02 0 .03 .03

Then we find the concentration using the "G" values by dividing the HA's .02 by the total Liters and the A- .03 by the total liter and inserting those values into an ICE chart (with expression say HA+H20 <--> H3O^+ + A^-
Answered by Val

HA + OH- ---> H2O + A-
.05 .03 0 0 "P"ut in
-.03 -.03 +.03 +.03 "U"se up
.02 0 .03 .03 "G"et Out

It didn't line up well the first time. It makes sense to me...just a little tedious. If you have never seen this, what way have you taught your students do solve/set up these kinds of problems?
Answered by DrBob222
I must be honest and say I just wrote the numbers on the board under the appropriate reagent and I almost ALWAYS worked from moles. But this is the first time I have heard of the PUG chart. We can't draw on this board, however, and spaces don't work worth anything (for more than one space). I saw students using the ICE chart several years ago and it is so easy to say ICE than it is to fight with the **&^%$ spaces and I now use that phrase to save time. Did you get the CH3NH2 problem ok with my suggestion?
Answered by Anonymous
What is the acid?
Answered by Juma
The question need solving skills I need help
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