help with calculations!

I'm doing a past paper for Salters OCR A2 and am completely stuck on a calculation. I have the mark scheme but I don't understand why each stage of the calculation occurs - can anyone explain it to me?

"A 25.0cm3 sample of stream water was titrated with 0.00500moldm-3 edta4- solution. 12.0cm3 of the edta4- solution were required to reach the endpoint.
[Cu(H2O)6]2+ + edta4- => [Cu(edta)]2- + 6H2O

Calculate the concentration of Cu2+ in the water sample in mg dm-3.

1mg = 1 x 10-3g"

I've worked out the number of moles of edta (6 x 10-5) and I understand that you use m=n x Mr to find the mass of Cu but then the mark scheme says to multiply the mass by 1000, divide by 25 then multiply by 1000 again.

I assume that the 25 is something to do with the volume of water but why the multiplications?!

Please help - I'm so stuck and the exam is so soon!

Thank you.

Reply 1

Beenom

They do this because when you work out the mass of Cu you work it out g cm^-3. However they want the units to be in mg dm^-3. So then you have to convert grams to milligrams. 1 g = 1000mg so thats why you multiply by 1000. (to change grams into milligrams).

Then you know that

X mg are in 25.0cm^3 of water so divide both sides by 25 to get:
X/25 mg in 1cm^3 of water. Then finally multiply both sides by 1000 to change cm^3 to dm^3
(X/25) x 1000 mg in 1dm^3 of water.