Help with this partition coefficient question please!

[shenzys]

The question is:

I am given 2 antibiotics the partition coeffecient, P is in chloroform/water)
More polar antibiotic: P = 0.48
Less polar antibiotic: P= 30

5 mg of each antibiotic were dissolved in 20ml of water in a seperating funnel and then shaken with 40ml chloroform. For each antibiotic, calculate the concentration (mg/ml) remaining in the aqueous phase.

I have no idea how to even start this question- can anyone help please? Thanks...

[shenzys]

OKay so I have the answer, but still don't get it!:

[More polar antibiotic] = (0.5g - 0.1387g)/50ml = 0.00723g/ml = 7.23mg/ml
[less polar antibiotic] = (0.5g - 0.48g)/50ml = 0.0004g/ml = 0.4mg/ml

Errr my question is HOW??? where did the 0.3187g come from? is there a formula? or a specific technique to use??? Use Log p? Why?

[Nik]

Presumably in your first post you don't mean 5mg or how else can you get a concentration of 7.23mg/ml ? Do you mean 500mg, 5g ? Please make clear first.

[shenzys]

Presumably in your first post you don't mean 5mg or how else can you get a concentration of 7.23mg/ml ? Do you mean 500mg, 5g ? Please make clear first.

The question I have does say 5mg,but it must be a typo since the answers have it as 5g!

[Nik]

Whilst i'm on holidays I can't be arsed to answer in detail using calculator etc... i left that in the exam room two weeks ago. but i will try to help anwyay.
Firstly do you know what log P measures ? Simply it's the ratio of the concentrations of unionised drug/compound in two solvents, octanol and water i.e log p = [conc in octanol]/[conc in unionised water]

You've been given log p values so you need to work out conc in octanol then bob's your uncle you've got the conc in aq phase.