Posted by david on .
1.) A water sample is found to have 9.0 ppb of chloroform,CHCL3.
How many grams of CHCL3 would be found in a glassful (250 ML) of this water?

CHEMISTRY 
DrBob222,
9 ppb is so small that you may consider the mass of it negligible; therefore, the volume of water = mass of the water sinc the density is 1.00.
1 ppb = 1 gram/1,000,000 mL.
I would use ratio and proportion to solve.
(1/1,000,000 mL)= (x g/250 mL) 
CHEMISTRY 
david,
I calculated using the proportion and got 0.00025, and it was wrong. Then I used 9 ppb times 1000000 ml/ 1 ppb = 9000000, and used the proportion setup and still got it wrong.

CHEMISTRY 
DrBob222,
The 0.00025 for the first one is correct math BUT I made a typo. Of course the 1 should have been a 9.
For (9/10^6)=(x/250)
x = 9*250/10^6 = 0.00225
I made another typo when I wrote 1,000,000 (million) instead of 1,000,000,000(billion).
So (9g/10^9)=(x/250)
x = 9*250/10^9 = 2.25E6 = 2.25 micrograms.
Technically, 9 ppb = 9 parts (we can use grams) in 1 billion parts BY MASS.
So that is 9 grams in (9 g + 999,999,991 g water) but that is so close to 10^9 that it doesn't matter much and 1 mL = 1 g so 10^9 g = 10^9 mL.
I hope this takes care of your problem. Sorry about the typos. I thought billion but typed million. Thought 9 and typed 1. You'll do that when your 81. 
CHEMISTRY 
david,
what is 0.00225 in two sigs? i hate sig figs .. thank you though.

CHEMISTRY 
DrBob222,
0.00225 to two s.f. is 0.0022 if you round to the even number. But 0.00225 is not the correct value.
The correct value you are looking for is
2.25 x 10^6 grams or 2.25 micrograms in 250 mL water. That would be 0.0000022 grams to two s.f. or 2.2 x 10^6 g to two s.f.