Chemistry

posted by .

I actually posted this up before and Gk helped out but i don't understand the steps what do i enter for the pka for the first one?

__________________________________________

I did an experiment on Buffers:

In a polystyrene beaker, mix 20 ml of 0.1M Acetic acid ad 25 ml of 0.1 M sodium acetate and immediately measure the pH. Remove the electrode and add 5 ml of 0.1 M HCL to this buffer. Stir the solution and measure the pH.

This is my data:

Conc.of acetic acid 0.1 M- volume 20mL
Conc.of sodium acetate 0.1M-volume 25ml
Conc. of Hydrochloric acid 0.1M-volume 5 ml

Buffer Solution- pH measured- 4.65
pH calculated ?

Buffer solution + HCL -pH measured- 4.43
pH calculated-?

A. So basically i need to show calculation for the calculated pH before the addition of HCl

B. Show the calculations for the calculated pH after the addition of HCl
------------------------------------
Responses

* Chemistry - GK, Thursday, July 16, 2009 at 5:16pm

The equilibrium in both mixtures is:
HC2H3O2(aq) <=> H+(aq) + C2H3O2^-(aq)

For the first mixture use the Henderson-Hasselbalch Equation to get the pH:
pH = pKa + log{[C2H3O2^-]/[HC2H3O2]}
NOTE: [C2H3O2^-] = molarity of NaC2H3O2
pH = pKa + log{1}

In the second mixture, reaction between HCl and NaC2H3O2 converts half of the C2H3O2- ions to acid, HC2H3O2, so,
pH = pKa + log{1/2}

Look up the pKa of HC2H3O2 and complete the calculations

  • Chemistry -

    This is what i have so far...The Ka is 1.8*10^-5

    so pKa is= -log(1.8*10^-5)
    = 4.74

    pH= pKa+log ([C2H3O2^-]/[HC2H3O2])
    = 4.47+ log ([C2H3O2^-]/[HC2H3O2])

  • Chemistry -

    pKa is the pKa for acetic acid. Since Ka for acetic acid is about 1.8 x 10^-5, the pKa is -log Ka = about 4.75 or so. You need to look up Ka for acetic acid in your text or notes and take the negative log of that value.

  • Chemistry -

    I think you had two calculations to make; one of the original pH of the solution and the second one after the adition of HCl to the mixture. Look at the problem. It appears to me that the initial concentration of HC2H3O2 is 0.1 M and the initial concn of C2H3O2^- is 0.1 so that is the log (0.1/0/0.1) = log 1 and that is zero. GK did that part for you.

Respond to this Question

First Name
School Subject
Your Answer

Similar Questions

  1. chemistry

    An enzyme-catalyzed reaction is carried out in a 50-mL solution containing 0.1 M TRIS buffer. The pH of the reaction mixture at the start was 8.0. As a result of the reaction, 0.002 mol of H+ were produced. What is the ratio of TRIS …
  2. chemistry lab

    An enzyme-catalyzed reaction is carried out in a 50-mL solution containing 0.1 M TRIS buffer. The pH of the reaction mixture at the start was 8.0. As a result of the reaction, 0.002 mol of H+ were produced. What is the ratio of TRIS …
  3. chemistry

    This is my third time to ask this and no one has answered. Please help. An enzyme-catalyzed reaction is carried out in a 50-mL solution containing 0.1 M TRIS buffer. The pH of the reaction mixture at the start was 8.0. As a result …
  4. chemistry

    I have already posted this and never got a response, please help. here is my data from lab: Buffer1: HPO4- weight 3.412g; original pH Buffer 2: HEPES wt: 1.090g original pH 10.08 buffer1: pH of 0.1M: 7.5; pH of 0.01: 7.72; pH of 0.001M: …
  5. physics

    does anyone know who conducted the experiment or a credible description of the experiment that helped prove Einstein's thought experiment: the twin paradox?
  6. chemistry

    i posted this already but i got the answer for someone elses question. the pka of acetone is CH3COCH3 is 19.3 which of these bases is strong enough to deprotonate acetone?
  7. chemistry

    i posted this already but i got the answer for someone elses question. the pka of acetone is CH3COCH3 is 19.3 which of these bases is strong enough to deprotonate acetone?
  8. chemistry buffers

    What molar ratio of HPO4 2- to H2PO4 - in solution would produce a pH of 7.0?
  9. Algebra 2

    Two days ago "Steve" helped me with this problem. I am grateful to him. I didn't get it and asked for him to help me again. He showed it to me again, but I needed more steps shown. Then I think he was finished for tutoring for the …
  10. Chemistry

    Take into account correction factors for phosphate buffers, predict the pH when 0.50ml of 1M HCl is added to 40mL of .10M buffer at expected pH 6 and pH 6.5. The buffer was made with Na2HPO4 and NaH2PO4. pKa is 7.2. When I did this …

More Similar Questions