Posted by david on Friday, February 12, 2010 at 8:53pm.
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)
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.
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 = 2.25E-6 = 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.
what is 0.00225 in two sigs? i hate sig figs -.-. thank you though.
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.