AP CHEM

posted by .

Hello, I'm working these homework questions out; however it is a series of questions and without one step correctly completed, it's nearly impossible to get the others. I posted my work and commented where I need assistance. Thank you so much in advance!!!

Potassium hydrogen phthalate is a weak solid acide w/ the formula KHc8H4O4 (often abbrv. KHP) and a formula weight of 204.23 g/ mole. It is often used to standardize a solution of a strong acid with an unknown concentration. When 1.328 g of KHp is dissolved in 50 mL of H20, phenolphthalein indicator is added, and then titrated w/ a solution of NaOH (conc unknown), it takes 42.85 mL to complete the titration. Answer the following questions-The hydrogen phthalate ion HC8H4O4^- (a) acts as a weak acid in water.

a) write out full molecular/net ionic reaction btw KHP and the NaOH solution

NaOH(aq) + KHC8H4O4(aq) ----> H20 (l)+NaKC8H4O4 (aq)

Net.I.E. OH^- (aq) + HC8H4O4^- ---->H20 (l) + C8H4O4

b)Calculate the concentration of the NaOH solution

1.328 g KHP(1 mol KHP/204.23 g) (1 mol Naoh/ 1 mol KHP) = 0.006502 mols NaOH

M= 0.006502 mols/ 0.04285 L = 0.1517 M NaOH

c)It is found that after 16.50 Ml of the NaOH has been added that the pH is 4.972. Calculate the Ka of HC8H4O4.

pH = pka + log (A-/HA)

% completion = 16.5 mL/42.85 mL = 0.3851= 38.51 %

4.972 = pka + log (38.51/61.49)
pka = 4.972-log (38.51/61.49)
pka = 5.175
Ka = 10^-5.175
Ka = 6.67 X 10 ^ -6

the next part (below) is where i'm having some trouble. I don't understand how the ratio works for the Kb is it still the same % completion or does it change???
d) calculate the Kb for the pthalate ion using the Ka from your answer above

pOH = 14-4.972
pOH = 9.028

pOH = pkb + log (HB+/B)
9.028 = pkb + log (what goes here??)

  • AP CHEM -

    sorry for part c that should read 16.50 mL not MI

  • AP CHEM -

    Potassium hydrogen phthalate is a weak solid acide w/ the formula KHc8H4O4 (often abbrv. KHP) and a formula weight of 204.23 g/ mole. It is often used to standardize a solution of a strong acid with an unknown concentration(Actually KHP is used to standardize a base and that is how it is being used here). When 1.328 g of KHp is dissolved in 50 mL of H20, phenolphthalein indicator is added, and then titrated w/ a solution of NaOH (conc unknown), it takes 42.85 mL to complete the titration. Answer the following questions-The hydrogen phthalate ion HC8H4O4^- (a) acts as a weak acid in water.

    a) write out full molecular/net ionic reaction btw KHP and the NaOH solution

    NaOH(aq) + KHC8H4O4(aq) ----> H20 (l)+NaKC8H4O4 (aq)This is OK

    Net.I.E. OH^- (aq) + HC8H4O4^- ---->H20 (l) + C8H4O4 This tartrate ion should have a double minus charge.

    b)Calculate the concentration of the NaOH solution

    1.328 g KHP(1 mol KHP/204.23 g) (1 mol Naoh/ 1 mol KHP) = 0.006502 mols NaOH

    M= 0.006502 mols/ 0.04285 L = 0.1517 M NaOH

    c)It is found that after 16.50 Ml of the NaOH has been added that the pH is 4.972. Calculate the Ka of HC8H4O4.

    pH = pka + log (A-/HA)

    % completion = 16.5 mL/42.85 mL = 0.3851= 38.51 %

    4.972 = pka + log (38.51/61.49)
    pka = 4.972-log (38.51/61.49)
    pka = 5.175
    Ka = 10^-5.175
    Ka = 6.67 X 10 ^ -6 I get this same number but since the pH in the problem is to three significant figures and all your other work is to four, I wonder if you shouldn't allow yourself another place.?

    the next part (below) is where i'm having some trouble. I don't understand how the ratio works for the Kb is it still the same % completion or does it change???
    d) calculate the Kb for the pthalate ion using the Ka from your answer above

    pOH = 14-4.972
    pOH = 9.028

    pOH = pkb + log (HB+/B)
    9.028 = pkb + log (what goes here??)
    The easy way to to Kb is to remember that KaKb = Kw. You have Ka and Kw. OR you can remember that pKa + pKb = 14 and do it that way. You've done good work here.

  • AP CHEM -

    Where did you get the 61.49 from?

Respond to this Question

First Name
School Subject
Your Answer

Similar Questions

  1. to bobpursley

    My classmates friends want me to post their questions as well.Thats why there are multiple names is there a problem. Yes. You are posting duplicate or even triplicate questions. That is silly. If we have answered one, the answers to …
  2. Chem Help!

    I really need help for my homework on balancing equations please show all the work including the coeffients if needed. I need help with 3 questions, 1) NaN3 -> Na + N2 2) NO2 + O2 -> NO + 03 3) LiOH + CO2 -> Li2CO3 + H2O
  3. gnozahs

    I removed about ten questions you posted this morning. There was no indication of what you had done, what your thinking was, or what you wanted from us. We are not going to do homework for you. If you want hints on these calculus questions, …
  4. Algebra

    please help me solve my homework with all the work involved. i have trouble paying attention in class so it's hard for me to do my homework. here are a few of the questions. I need to find out what the letters equal. 1.) 10(-4+y)=2y …
  5. 3rd grade math

    on one test, chi answered 76 out of 100 questions correctly. on another test, he answered test, he answered 7 out of 10 questions correctly. write these scores as decimals and compare them.
  6. business

    hello: i am considering going back to school to get a degree in business. i have borrowed a book from a friend who is already in the program. meanwhile: i sometimes have trouble working my way through the chapter questions. he said …
  7. Jiskha

    I typed several questions and answers regarding the subject of Geography, and suddenly, as I was waiting for a reply from Ms. Sue, it seemed as though my questions were deleted from the webpage where all of the other questions are …
  8. Math

    I need help solving the problem below. Chris answered 19 out of 25 questions correctly on a math test. What percent of the questions did she answer correctly?
  9. probabilities

    A multiple choice test has 16 questions. Each question has 4 possible answers with only one is correct. Assume that all questions are answered and the probability to answer any question correctly is 0.25. Let X be the number of questions …
  10. statistics

    Bill has completed a 10-question multiple-choice test on which he answered 7 questions correctly. Each question had one correct answer to be chosen from five alternatives. Bill says that he answered the test by randomly guessing the …

More Similar Questions