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Topic: mass percent questions  (Read 5462 times)

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Offline aaaalex

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mass percent questions
« on: February 27, 2010, 02:48:33 PM »
a pb-contaminated water sample contains .0011% pb by mass. how much of the water (in mL) contains 150mg of pb. the density of the solution is 1.0g/mL.

to solve that would you just start with 150mg of pb then convert that to grams, use the mass % as a conversion factor by making it 100g sample/ .0011g pb then use the remaining mass as the mass of the water so i end up with grams of water then use the density to figure out milliLiters of water?

150mg pb * 1gpb/10^3mgpb * 100g sample/.0011g Pb * 99.9989 g water/ 100g sample * 1ml / 1.0g = 13636.21364 mL

did i do that right or is that wrong?

another problem

how many  moles of potassium sulfate are present in a 240.0 g sample of a 35%  potassium sulfate sample by mass?

this is my calculation:

240.0g sample * 35g k2so4/100g sample * 1mole k2so4/ 174.26g k2so4 = .4766 moles k2so4

does that look right or did i do it wrong?

Offline JGK

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Re: mass percent questions
« Reply #1 on: March 01, 2010, 01:02:58 PM »
I got 13636.36364

I believe the key statement was "the density of the solution is 1.0g/mL". ie water containing the lead is 1 g/mL.

It isn't a very well worded question.  I think you're overcomplicating it by adjusting for density. the water volume you have calculated is the volume - the lead component.
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