Posted by **Holly** on Tuesday, October 4, 2011 at 9:24pm.

Calculate the pH at the point in the titration of 25.00 mL 0.108 M HNO2 at which 10.00 mL 0.162 M NaOH have been added. For HNO2, Ka=5.1 x 10^-4 and:

HNO2 + OH- -----> H2O + NO2-

I know that HNO2 is a weak acid and NaOH is a strong base.

Help given to me so far from Dr. Bob222:

HNO2 + NaOH ==> NaNO2 + H2O

moles HNO2 = M x L = 0.025 x 0.108 = 0.00270 moles HNO2 initially.

moles NaOH added = 10mL x 0.162 = 0.00162

moles HNO2 remaining after reaction = 0.00270-0.00162 =0.00108.

M HNO2 = moles/L (L = 25 mL + 10 mL)

So this problem becomes determine the pH of a 0.00108/0.035)M soln of HNO2. Set up an ICE chart, solve for H^+ and convert to pH……

I set up the ICE chart:

HNO2 + H20 H3O^+ + NO2^- Ka=5.1 x10^-4

I: 3.085x10^-2 M --- ----

C: -X +X +X

E: 3.085x10^-2 –X M XM XM

Since the Ka and the Molarity of the acid are within 10^2, I cannot drop the –X. So, 5.1x10^-4= [X^2]/[3.085x10^-2 –X]. I get a quadratic equation of X^2+5.1x10^-4X -1.5734x10^-5=0. Therefore X=[H3O^+]=0.003719. pH=-log [X]. pH=2.43. My book says the answer is 3.47. I am confused. Please tell me what I am doing wrong. Thank you!

- chemistry -
**DrBob222**, Tuesday, October 4, 2011 at 10:12pm
First, I divided 2.70 mmoles (0.00270 moles) by 35 = 0.03086M which isn't enough difference to worry about. Besides we don't have that many sig digits, anyway. The main thing is that you didn't include the NaNO2 that is formed. That is the salt of the weak acid so it is a buffered solution. (NaNO2) = 2.70mmoles/35mL = 0.0771M

Ka = 5.1E-4 = ((H^)(NO2^-)/(HNO2)

5.1E-4 = (x)(x+0.0771)/(0.03086-x).

I worked it both with a quadratic and without and it doesn't make much difference; however, I did NOT obtain 3.47. I found 2.02E-4 for (H^+) for pH = 3.69. I also used the Henderson-Hasselbalch equation which is

pH = pKa + log (NO2^-)/(HNO2)

pH = 3.29 + log(2.70mmols/1.08 mmols)

pH = 3.687 which would round to 3.69

I don't believe the 3.47 answer is correct. Check my work carefully.

- chemistry -
**Holly**, Tuesday, October 4, 2011 at 11:30pm
I got it! You took the wrong mol of No2- from the above neutralization rxn....it should be 0.00162 mol NO2-/0.035L=0.0463M NO2-....I did all the calcs and I get 3.47 for pH which is book answer! Thank you for pointing me in the right direction! :-)

## Answer This Question

## Related Questions

- chemistry - Calculate the pH at the point in the titration of 25.00 mL 0.108 M ...
- Chemistry - At 298 K, nitrous acid (HNO2) dissociates in water with a Ka of 0....
- Chemistry - 500 ml of a buffer solution with ph=2.10 must be prepared using .4 M...
- chem- acids and bases - sorry to take up your time again, acids and bases just ...
- A.P. Chemistry - You have solutions of 0.200 M HNO2 and 0.200 M KNO2 (Ka for ...
- Chemistry - What is the (H30) concentration when 40 mL of 0.10 M nitrous acis, ...
- Chemistry - 15 ml of 0.5 M of HCl is added to a 100 ml sample of 0.2 M HNO2 (Ka ...
- A.P. Chemistry - You have solutions of 0.200 M HNO2 and 0.200 M KNO2 (Ka for ...
- Chemistry - Buffers- Common Ion effect IS this Correct Calculate the pH of a ...
- Chemistry-Repost for DrBob222 - 500 ml of a buffer solution with ph=2.10 must be...

More Related Questions