Question

The base ethylamine, C2H5NH2
, has a pKb
equal to 3.30. Calculate the hydroxide concentration, the
pH, and the % ionization for a 0.42 M solution

Answer 1

I figured out Kb was 0.000501187, and I drew an incomplete ICE TAle. That's as far as I got.

Answer 2

okay, thats good. have you written an equilibrium expression for the dissociation?

Answer 3

C2H5NH2--->HC2H5NH2+OH

Answer 4

ignored water, but the expression I think is [HC2H5NH]*[OH]/[C2H5NH2]

Answer 5

don't forget Kb.

Kb=[HC2H5NH]*[OH]/[C2H5NH2]

so you said you had an ice table?


you're gonna end up with: Kb=x^2/[C2H5NH2-x]

so: 0.000501187=x^2/(0.42-x)

Answer 6

Why is it 0.42?

Answer 7

it's given in the question, it's the concentration of ethyl amine before it abstracts a proton from water.

Answer 8

Oh gosh forgot to write that down in my work.

Answer 9

thats probably why you were stuck :S. does it make sense now?

Answer 10

Just one more thing until I continue, can the 0.42 be ignored? Someone told me that before, but I don't know if they are right.

Answer 11

the 0.42 cannot be ignored but the x in "0.42-x" could be.

it depends on the "5% rule", because the Kb is so small, the amount ionized will be insignificant on 0.42

you can check this after by verifying that x is less than 5% of 0.42

Answer 12

Thanks!

Answer 13

no problem, dude !