Need help with pH problem!

[Hello2011]

---

Members do not see this ad.

Calculate the pH of 0.01M NH4Cl Kb= 1.8 x 10^-5

The answer is supposed to be 5.13, but i can't figure it out for the life of me!

 

[kehlsh]

Calculate the pH of 0.01M NH4Cl Kb= 1.8 x 10^-5

The answer is supposed to be 5.13, but i can't figure it out for the life of me!

Ka of (NH4+) is: 10^-14 / 1.8 x 10^-5 = 5 x 10^-10.
Ka of NH4+ = [H+][NH3] / [NH4+],
where [NH4+] = 0.01 M and [H+] = [NH3].
[H+] = sq rt (5 x 10^-10 x 0.01)
= sq rt (5 x 10^-12)
= 2.23 x 10^-6
pH = 5-6, 5.6ish

 

[PingPongPro]

Ka of (NH4+) is: 10^-14 / 1.8 x 10^-5 = 5 x 10^-10.
Ka of NH4+ = [H+][NH3] / [NH4+],
where [NH4+] = 0.01 M and [H+] = [NH3].
[H+] = sq rt (5 x 10^-10 x 0.01)
= sq rt (5 x 10^-12)
= 2.23 x 10^-6
pH = 5-6, low 5s

Damn this guy is so smart.

Its kind of annoying when the question writers are purposfully abnoxious and give you pKb's when solving for pH.

 

[Hello2011]

Ka of (NH4+) is: 10^-14 / 1.8 x 10^-5 = 5 x 10^-10.
Ka of NH4+ = [H+][NH3] / [NH4+],
where [NH4+] = 0.01 M and [H+] = [NH3].
[H+] = sq rt (5 x 10^-10 x 0.01)
= sq rt (5 x 10^-12)
= 2.23 x 10^-6
pH = 5-6, low 5s

Thanks, do you just do -log of(2.23 x 10^-6) to get the pH? What's the exact #? seems closer to 6 than 5...

 

[Hello2011]

Damn this guy is so smart.

Its kind of annoying when the question writers are purposfully abnoxious and give you pKb's when solving for pH.

we had another questionwhere OH was on top of the equation so having Kb made it shorter problem to do. I have trouble drawing the equation though.

 

[kehlsh]

Thanks, do you just do -log of(2.23 x 10^-6) to get the pH? What's the exact #? seems closer to 6 than 5...

ya, that's correct
answer should be 5.6 something to be exact, but u should be gud with just figuring out it's in betweent 5-6 (unless ur taking TBR exams ofc)

 

[PhilIvey]

Calculate the pH of 0.01M NH4Cl Kb= 1.8 x 10^-5

The answer is supposed to be 5.13, but i can't figure it out for the life of me!

1e-14 = 1 e-9
1.8e-5 1.8

10 5= .55 so ka= .55e-9= 5.5e-10
18 9

Using TBR equation for weak acid with pka between 2 and 12 the following is done: .5pka - .5log[HA]
=.5*log(5.5e-10) - .5log(.01)

=.5 * 10 -log 5.5. log 5.5 is the average of the log of 5 and 6 which are .7 and .78 so it's .74.

so, .5*(10-.74)= 4.63 - .5* log -2= 5.63

Obviously you would never do this on the test but in using TBR, the above takes roughly 30 seconds. So yes they are wrong

 

[PhilIvey]

Calculate the pH of 0.01M NH4Cl Kb= 1.8 x 10^-5

The answer is supposed to be 5.13, but i can't figure it out for the life of me!

Are you sure it's .01? because according to my math if it were .1 then the answer would indeed be 5.13.

.5(10-.74) -.5(1)= 5.13

 

[kehlsh]

Are you sure it's .01? because according to my math if it were .1 then the answer would indeed be 5.13.

.5(10-.74) -.5(1)= 5.13

ya I think it's .1M if answer was to be 5.13

 

[SN2ed]

Thread moved. These types of questions belong in the Study Q&A forum.