[newbie!]

ok so yeah i;m 2nd year undergrad (UK). upon taking a look at pastpapers i seem to face a few problems.

1) if you dissolve 0.025g of potassium choloride (KCl) in water and make it up to 200ml, what would the conc of potassium ions be in ppm?

SOLUTION: ok i know 1ppm=1mg/kg
                 % of K in KCl = 39.1/ 74.6 x 100 = 52%
                 moles of KCl = mass/gfm
                                  = 0.025/74.6
                                  = 3.35x10^-4
                 c=n/v
                  = (3.35x10^-4)/0.2
                  0.001675
so that would be in mol l-1 right? how do i get to ppm?


2) caffine is a weak base with a pKa of 10.4. calculate the fraction of caffine that is ionised at pH1 (stomach pH), and at pH6 (intestine pH). predict where caffine is preferentially absorbed.

SOLUTION: ok so pH=-logpKa
                           = -log 10.4
                           -1.02
i really dont have a clue where to go from there.

if you can help pls do. i have ried my best to show my working and that i HAVE attempted these questions.

(i have an exam in 2 days for this, and there are quite a few questions i'm having trouble with, but to be honest i dont have the time to type up all my wrong workings(that seems to be strict rule on here) but if you'd like to help me (and i wuld be greatly appreciated) then pls email me: ll-jk@live.co.uk

thanks

[AWK]

1. moles KCl = moles K => mass K in grams = mass K in miligrams

2. read carefully this link
http://answers.yahoo.com/question/index?qid=20070211180125AATqHYN

[newbie!]

thanks that has helped.

but for the second one.
inthe example its 7.5-7.15 giving 0.35=log[A-]/[HA]
but in mines its 1-10.4 giving -9.4=log[A-]/[HA]
when i inverse log this i get 3.98x10^-10
that doesnt seem right?

[AWK]

Absorbtion needs unprotonized base. At which pH this happens. No calculations needed