[mana]

hi all
I have solved 4gr butanoic acid in 100 ml water and I want to extract it with 100 ml benzene. the distribution constant (k) btw benzene and water is 3. I want to calculate the residual  butanoic acid remained in water after 3rd extraction with benzen.
I don't know in the formula for calculation residual extract, do I have to substitute 3 for k or 1/3 (because I'm calculating residual butanoic acid in water not in benzene)?
can anyone help
and the formula is w_n= w₀(kv/kv+s/n)ⁿ
w_n: the residual butanoic acid in water after n times extarction
w₀: the amount of butanoic acid in water at first (4gr)
v: volume of water
s: volume of benzene

[Borek]

Don't make things more complicated than they are, calculate how much was extracted and subtract from the initial.

[mana]

Don't make things more complicated than they are, calculate how much was extracted and subtract from the initial.

yes, you are right but I want to know how to calculate with formula too, 

[Borek]

If you don't know how to follow the formula try to calculate the changes on the step by step basis - in each step you have known initial concentrations and initial volumes, that (and the fact total amount of the butanoic acid doesn't change) allows you to easily calculate final concentration ratio.