[alextu]

I'm having trouble solving pH problems...

I have an example that needs to be worked through, any help would be great, Thank you!
 
The example is as follows:
What is the pH of a 0.030 M pyridine solution that has a Kb= 1.9 X 10-9? The equation for the dissociation of pyridine is
C5H5N(aq) + H20(l) <---> C5H5NH+(aq) + OH-(aq)

Another example I need to be worked through is:
Para-Aminobenzoic acid (PABA), p-H2NC6H4, is used in some sunscreens and hair conditioning products.  Calculate the pH of an aqueous solution with [PABA] = .30 M and Ka=2.2 X 10-5.

[enahs]

What it help you to point out that K_w = K_a ? K_b, and so K_a = K_w/K_b ?

Do you know what K_a and K_b? They are the acid dissociation constant and base hydrolysis constant.

K_a = [H⁺] [A^-]
             [HA]

K_b = [BH⁺][OH^-]
               

Where HA is the concentration of the un-dissociated acid and B is the concentration of the base. That is what you have in your cases, the concentration of the original solutions (HA and/or B).
You know the HA -> H⁺ and A^-. There for Ka = [X][X]
                                                                           [HA – X]
Simple solving for x, once you know x you know the concentration of the H⁺ and therefore the pH (once you "-log it) .

Using the one with Kb (base hydrolysis) you get the concentration of the OH^-, so use the Kw=Ka*Kb and convert to Ka to  H+ concentration. Or pH + pOH = 14 (you have to be careful with this one though, because very dilute or very concentrated solutions can throw this off)


Note: The calculations will change some if it is a polyprotic acid.

[alextu]

I figured out the pH for the two I asked about but now I have been working on for a few hours and haven't been able to figure it out, please *delete me*

Calculate the pH of a .300M solution of methylammonium chloride, CH3NH3Cl, The Kb for methylamine, CH3NH2 is 3.7X10-4.

[Borek]

CH₃NH₃Cl is methylammonium hydrochloride, salt of weak base and strong acid.

http://www.chembuddy.com/?left=pH-calculation&right=toc

[alextu]

Can anyone calculate the methyammonium chloride problem through??  I keep coming up with an answer of 12.02, which is wrong, any help would be appreciated. Thanks

[enahs]

Can anyone calculate the methyammonium chloride problem through??  I keep coming up with an answer of 12.02, which is wrong, any help would be appreciated. Thanks

I get 5.5₄

Convert K_b to K_a, then it is just like the other problems.

[Borek]

Salt of a weak base (methylammonium) contains this base protonated - CH₃NH₃⁺. This protonated base is a weak acid. So what you have to do is to calculate pH of the weak acid solution.

http://www.chembuddy.com/?left=pH-calculation-questions&right=pH-weak-base-hydrolysis-q1