Posted by Kat on Monday, March 2, 2009 at 11:21am.

A solution of volume 80.0 mL contains 16.0 mmol HCHO2 and 9.00 mmol NaCHO2.
If 1.00 ml of 12 M HCl is added to this, what will be the resulting pH?


* CHEM HELP...PLEASE!! - DrBob222, Monday, March 2, 2009 at 1:25pm

Use the Henderson-Hasselbalch equation.
pH = pKa + log (base/acid)
First, however, you need to make some adjustments to the acid/base because you have added some HCl to it.
HCOOH = 16.0 mmole initially.
HCOONa = 9.00 mmol initially. Adding 1.00 mL of 12 M HCl adds 12 x 1 = 12 mmole (is that 12 or 12.0 or 12.00 mmole?) to this base. Therefore, 9.00 + 12.0 = 21.0 mmole base.
pH = pKa + log[(21.0)/(16.0)] = ??
If your prof is picky, s/he will not like this BECAUSE (base) and (acid) are concentrations and not mmols. Technically, then, the concn is mmols/mL = 21.0/81.0 for (base) and 16.00/81.0 for (acid) so the equation is
pH = pKa + log[(21.0/81.0)/(16.0/81.0)] = ??. You will note that the 81.0 mL volume cancels and some profs just don't put it there. I ALWAYS counted off for not including the volume; however, since it ALWAYS cancels, I would allow the student to use V as in
pH = pKa + log [(21.0/v)/(16.0/v)] = ??. That way the student let me know that a volume went there but it would cancel and never entered into the calculation.

* CHEM HELP...PLEASE!! - Kat, Monday, March 2, 2009 at 10:00pm

Sorry, there's one thing I don't understand. If an acid is being added to a buffer solution, shouldn't the pH decrease? By following your method, the answer I'm getting is greater than the original pH.

  1. 👍 0
  2. 👎 0
  3. 👁 105
asked by Kat
  1. Well, you are correct. The pH SHOULD become more acid but it doesn't because I've led you astray. Check my thinking on this.
    The 12 mmoles HCl added uses ALL of the HCOONa and leaves 3 mmoles HCl in excess.
    The 9 mmoles HCOONa has changed to HCOOH and there is now 16 + 9 = 25 mmoles HCOOH (and no HCOONa). So the pH is that of 25 mmoles HCOOH in 81 mL + 3 mmoles HCl in 81 mL. I would use the excess HCl as a common ion to determine the (H^+) contributed by HCOOH to the solution in the presence of HCl, then add it to the (H^+) from the excess HCl. The H^+ contributed by HCOOH may be small enough to neglect but I didn't work it out. In other words, I don't think you have a buffer after the HCl is added.

    1. 👍 0
    2. 👎 0
    posted by DrBob222
  2. This is how I did it:

    HCOOH + H2O <---> HCOO- + H3O+
    I 16.00 9.00
    I 12.00
    C +9.00 -9.00 -9.00
    E 25.00 0 3.00

    HCOOH + H2O <---> HCOO- + H3O+
    I 25.00 0 0
    I 3.00
    C -x +x +x
    E 25 -x x 3+x

    Ka = ------- = 1.8*10^-4

    Assume x is very small, then x = 0.0015

    So, [H30+] = (3+x)mmol / 81ml
    = (3 + 0.0015)mmol / 81 ml
    = 3.0015mol / 81 ml
    = 0.03706 M

    And pH = -log[H30+]
    = 1.43

    I know the technicalities are pretty badly used here, but does my answer make sense?

    1. 👍 0
    2. 👎 0
    posted by Kat
  3. yes, it makes sense.
    My answer was similar but slightly different.
    3 mmol H^+ from HCl.
    Ka = 1.77 x 10^-4 (note slightly differnt Ka) = (H^+)(formate)/(HFormate)
    Substitute 3/81 for (H^+) and (25/81 for (Hformate; i.e., formic acid) and solve for formate ion. I get 0.00147 M which is essentially your answer if we used the same Ka value I think.
    Then I converted to moles.
    moles H^+ from formic acid = 0.00147 x 81 mL = 0.119 mmoles.
    moles from HCl = 3 mmoles.
    Add to get 3.119 mmoles
    Then 3.119 mmoles/81 mL = 0.0385 M
    pH = 1.41 which is very close to your answer. Both of us neglected the quadratic if we had solved the Ka with formic acid exactly. The numerator would have been 0.00147 + 0.03704 (we just used 0.03704) and the denominator would have been (0.003086-0.00147)(I used 0.03086 and that could make a difference. A better solution would be to solve that quadratic. In any case, watch the significant figures in the final result.

    1. 👍 0
    2. 👎 0
    posted by DrBob222

Respond to this Question

First Name

Your Response

Similar Questions

  1. physics

    Posted by norm on Monday, March 15, 2010 at 9:21am. What is the minimum downward force on the box in the figure that will keep it from slipping? The coefficients of static and kinetic friction between the box and the floor are

    asked by norm on March 15, 2010
  2. Math(Geometry)

    To Lauren - Ms. Sue, Monday, September 28, 2009 at 10:24pm What post? What is your question? In addition, your "name" can not be HELP ME NOW!!!! No one appreciates being screamed at to help you now! Using all caps online is like

    asked by Mr.Sonyericson on September 28, 2009
  3. Plz -- Chem Help

    Posted by Rushi on Sunday, March 15, 2009 at 3:35pm. Question.. Nicotinic acid (niacin) is a monoprotic acid with the formula HC6H4NO2. A solution that is 0.012M in the nicotinic acid has a pH of 3.39 at 25C. what is the

    asked by A on March 18, 2009
  4. Physics

    Posted by Adam on Thursday, August 13, 2009 at 11:08am. In the figure take q1 = 22uC and q2 = 20u. The force on q_1 points in the - x direction, What is the magnitude of the force on q_1? I don't know where to start, can someone

    asked by Adam R on August 14, 2009
  5. TRIG!

    Posted by hayden on Monday, February 23, 2009 at 4:05pm. sin^6 x + cos^6 x=1 - (3/4)sin^2 2x work on one side only! Responses Trig please help! - Reiny, Monday, February 23, 2009 at 4:27pm LS looks like the sum of cubes sin^6 x +

    asked by hayden on February 23, 2009
  6. Chemistry-pls help

    I refer to the following question. Is my explanation correct, that step two C6H14(g)--> 6CO2(g) + 7H2O(g) can be considered combustion reaction? Thank you! Posted by jan on Tuesday, March 31, 2009 at 10:28pm. C6H14(l)->C6H14(g)

    asked by jan on April 1, 2009
  7. English

    Posted by rfvv on Monday, March 24, 2014 at 6:36am. 1. I haven't read Harry Potter yet. 2. I don't read Harry Potter yet. 3. I didn't read Harry Potter yet. ------------------------------ Which expressions are not grammatical? Is

    asked by rfvv on July 5, 2014
  8. Chemistry

    Posted by Angeline on Monday, April 20, 2009 at 5:35pm. A few many grams are in 48.7x10^24? I don't know how to convert atoms to grams. I can do moles to grams or moles to atoms, but not atoms to grams. My other

    asked by Angeline on April 20, 2009
  9. socials

    Posted by Sara on Monday, September 14, 2009 at 8:40pm. The puritans had a long-lasting effect on english society and , to some extent , our own. with someone, brainstorm aspects of society today that seem "puritan" in origin.

    asked by Sara on September 14, 2009
  10. English

    Homework Help: English Posted by rfvv on Tuesday, March 12, 2013 at 6:29pm. Posted by rfvv on Tuesday, March 12, 2013 at 5:35pm. Posted by rfvv on Monday, March 5, 2012 at 7:39pm. Try hard to succeed. (What is the part of speech

    asked by rfvv on March 13, 2013

More Similar Questions