chemistry
posted by alex on .
if the ration of water drops to EDTA is 6.7 for the known sample and 10 for the unknown sample what is the concentration of the unknown sample? (The concentration of the known sample is 315 ppm)
a. 150mg/L
b. 210mg/L
c. 280mg?L
d. 320mg/L

I would set up a proportion.
315ppm/6.7 = xppm/10
and solve for x. 
that proportion solving for X doesnt give me a number listed as an option..I got 470

I never liked the drops way of doing things. Having said that, I must take the blame and say I made a mistake.
If it takes 3 drops of the EDTA for the known of 315 ppm and it takes only 2 drops for the unknown, the unknown must be weaker. You can do it by drops of EDTA (which makes more sense to me)
315 x 2/3 = ??
or you can use the ratio set up to give a smaller number; i.e.,
315 x (6.7/10) = ?? ( you don't get the same number because the 6.7 was rounded off from 6.66667 to 6.7).
or you can set up a ratio/proportion but the reverse of what I have above.
I suppose it's obvious but I didn't even look at the options for answers; perhaps I should have looked.