# chemistry

posted by .

Calculate the pH and percentage protonation of the solute in an aqueous
solution of 0.39 M quinine if the pKa of its
conjugate acid is 8.52.

• chemistry -

Let's call quinine, BN
.........BN + HOH ==> BNH^+ + OH^-
initial.0.39M.........0........0
change...-x...........x.........x
equil...0.39-x........x........x

Kb = (BNH^+)(OH^-)/(BN)
You have pKa of the conjugate acid, convert to pKb for the base, substitute into the Kb expression and solve for (OH^-) then convert that to pH.
%protonation = [(OH^-)/(BN)]*100 = ?

• chemistry -

-log ka=8.52
ka= 3*10^-09
Kb= kw/ka
= 10^-14/ 3*10^-09 = 3.4 *10^-06
kb= x^2/0.39
x= squre root of ( 3.4*10^-6*0.39)
=1.15*10^-3
POH= -log[OH-]= -logx= -log 1.15*10^-3
=2.939
PH= 14 - 2.939
=11.06

IS THIS RIGHT?

• chemistry -

nvm, it IS right, ty xD

• chemistry -

I calculate 3.31E-6 for Kb which runs through the rest of the problem and ends with 11.055 which I would round to 11.06.

• chemistry -

for the %protonation = [(OH^-)/(BN)]*100
i tried doing 1.15*10^-3 / 0.39 * 100
= .295, but the answer is incorrect.
can you tell me what i did wrong?

• chemistry -

I think it goes back to the incorrect conversion of pKa to pKb to Kb. Then for OH^- I obtained 1.136E-3.
0.001136/0.39 = 0.00291 and that times 100 = 0.291% which rounds to 0.29 to 2 significant figures.
Your problem may very well be that you are reporting too many s.f. I'll bet 0.29% will work. These data bases are "good" about that and catch a lot of trouble for it. Let me know.

• chemistry -

YES! it was the OH^- that messed up my calculation. The answer you provided me was correct, thanks for all the help!

## Similar Questions

1. ### Chemistry

A buffer containing 1.2169 M of acid, HA, and 0.1431 M of its conjugate base, A-, has a pH of 3.96. What is the pH after 0.0017 mol NaOH is added to 0.5000 L of this solution?
2. ### Chemistry

what are the other chemistry forms present in the dichloromethane solution besides the normal neutral forms for benzoic acid and quinine?
3. ### Chemistry

Calculate the pOH, pH, and percent protonation of solute in the following aqueous solution. 0.043 M quinine, given that the pKa of its conjugate acid is 8.52
4. ### chemistry

Benzoic acid and quinine are dissolved in a solution of dichloromethane. Besides the normal neutral forms for benzoic acid and quinine, will there be any other chemical form(s) present in the dichloromethane solution?
5. ### Chemistry

Calculate the pOH, pH, and percent protonation of solute in the following aqueous solution. 0.059 M quinine, given that the pKa of its conjugate acid is 8.52. Please show me how to do it and not just the answers. Thanks!
6. ### science

100 ml of 0.02 M acetic acid (pKa= 4.76) is titrated with 0.02 N KOH. After adding some base to the acid solution, the observed pH is 2.76. At this pH degree of protonation is
7. ### chemistry

Calculate the pH of the solute in an aqueous solution of 0.0047 M codeine, if the pKa of itsconjugate acid is 8.21. What is the percentage protonation of the solute?
8. ### chemistry

A 2.5 g sample of NaOH (Mw = 40.00) was dissolved in water to give a solution of final volume 250 cm3. (i) With reasons, state whether NaOH is a strong or a weak base. Give the conjugate acid of NaOH and decide whether this conjugate …
9. ### Chemistry

Benzoic acid and quinine are dissolved in a solution of dichloromethane. (benzoic acid pKa = 4.2; quinine pKa = 8.5) a) Besides the normal neutral forms for benzoic acid and quinine, will there be any other chemical form(s) present …
10. ### Chemistry

The pH of a buffer can be predicted using the Hendersen-Hasselbach equation: pH=pKa+ log([conjugate base][conjugate acid]) The choice of the conjugate acid-base pair (as you did in the previous questions) determines the pKa value to …

More Similar Questions