CHEMISTRY

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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 -

    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 -

    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 -

    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.

  • CHEMISTRY -

    what is 0.00225 in two sigs? i hate sig figs -.-. thank you though.

  • CHEMISTRY -

    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.

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