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Mar 17th, 2015

1)  Solvation is the process by which the solvent molecules (often water) orient themselves around the solute (often a salt). During the process, the solvent molecules have sufficient attraction for the individual solute ions to break the ionic bond of the compound. The solvent particles then surround the individual ions leading to the solid eventually becoming dissolved.

***NOTE: As you will no doubt be required to explain the process in your own words, perhaps that becomes much easier when you have a visual in mind.  Hopefully this video helps.***

2)  First divide by 10^6 (to get rid of the ppm)….

3.5ppm / 10^6 = 3.5x10-6

This is the mass of Pb in grams per 1 g of soln.  We want the mass of Pb per 25.0 grams of soln. So multiply by 25.0…

3.5x10^-6 g x  25.0 = 8.75 x 10^-5 g  or 8.8 x 10^-5 g Pb  if sigfigs  are important

3)  There are more than one type of percentage designations for concentration (weight/weight, vol/vol, weight/vol). We’ll assume weight per volume for this problem.

First use the def. of weight per vol. (w/v) %...

 (w/v) = mass solute (g) / vol soln (mL) x 100

Rearrange to solve for mass solute (g)…

(w/v) /100 x vol soln (mL) = mass solute (g)

Plug n’ chug…

2.5 / 100 x 500 =  12.5 g NO3-

Finally we convert grams to mols. The molar mass of nitrate is 62.004 g/mol…

12.5 g x (1 mol / 62.004 g) = 0.2015998 mols or if sigfigs are important = 0.2 mols NO3-

4)  Convert 3.0 micro grams to grams…

3.0 x 10 -6 g

Divide by 20 g…

3.0 x 10^-6 g / 20 g = 1.5 x 10 -7

Multiply by 10^9 to get ppb…

1.5 X 10^-7  x  10^9

= 150 ppb 

or IF YOU USE THE FIVE-EVEN RULE for rounding sigfigs… 200 ppb

Mar 17th, 2015

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