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Mar 13th, 2015

1)  1.50 mols / 0.50L = 3.0 M (with sigfigs in mind)

2)  First convert grams to mols. The molar mass of KI is 165.998 g/mol. Converting...

      58.5 g  *   (1 mol) / (165.998 g) = 0.352413 mols

     the molarity is given by....

    0.352413 mols / 10.L = 0.35 M (with sigfigs in mind)

3) First convert 5 micro grams Pb to grams Pb....

 5ug  = 5 x10^-6 g

To find ppb divide mass of Pb by mass of solution and multiply by 10^9...

(5x10^-6) / (200g)  * 10^9 = 25 (if sigfigs are important, our answer should have only one sigfig.  So our answer should be 30 ppb or IF YOU USE THE FIVE-EVEN RULE: 20 ppb )

4) given ppm, we work backwards to find the ratio of arsenic in the sample. First divide by 10^6...

5 / 10^6 = 5x10^-6

We know the amount of arsenic, which we'll denote A, in 50.0 g of sample has the ratio 5x10^-6. So we can write...

5x10^-6 = A / 50.0g

solving for A....

A = (5x10^-6) (50.0g) = 2.5 x10-4 g or 0.25 mg (again, if sigfigs are important: 0.30 mg or with the five-even rule in effect: 0.20 mg)

Mar 13th, 2015

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