More Concentrations
(ppm/ppb, molarity and dilutions)
Learning Goals:
I will be able define ppm and ppb units and apply
knowledge of them to perform the necessary
calculations to solve for very small concentrations
I will be able to define what molar concentration
(molarity) means, and apply it to solve for the molar
concentration of a given solution, as well as
unknowns in diluted solutions
Very Small Concentrations
(ppm and ppb)
Very dilute solutions have concentrations
much less than 1% (m/m)
Use ppm (parts per million) or ppb (parts
per billion) for these
6
ppm= mass of solute x 10
mass of solution
9
mass
of
solute
x
10
ppb=
mass of solution
Example: ppb problem
Cadmium is a highly toxic metal. The average level of
cadmium in the blood of Canadians is about 0.35ppb. At this
level, what mass of cadmium would be present in 1.5 kg of
blood? (Convert units to grams first!)
Example # 2: ppm problem
Health Canada’s guideline for max mercury levels in fish is
0.5 ppm. When a 1.6kg salmon was tested, it was found to
contain 0.6 mg of Hg. Would this salmon be safe to eat?
(Convert units to grams first!)
You Try: ppb/ppm problem
House paint produced in Canada must contain less than 600
ppm of lead. What is the maximum mass of lead permitted
in a can that contains 7.0 kg of paint? (Convert units to g!)
Molar Concentrations
Molar Concentrations
Also known as Molarity:the number of
moles of solute that can dissolve in 1 L of
solution.
Molar concentration (mol/L) = Amount of solute (mol)
Volume of solution (L)
c= n/v
n
c
V
Example #1: Calculating Molarity
What is the concentration, in mol/L, of a
solution formed by dissolving 28.0g of calcium
chloride in enough water to make 225 mL of
solution? (Find nCaCl2 first using M, then c)
Example #2: Calculating Molarity
How many grams of sodium nitrate would be
needed to make 425 mL of 6.00 mol/L solution?
(Find n first using n=cV, then m using m=nxM)
You Try #1: Calculating Molarity
What final solution volume would be required to prepare
A 0.100 mol/L solution of AgNO3(aq) if 1.20 g of the solid
salt will be used? (Find nAgNO3 first, then use V=n/c to
find V)
You Try #2: Calculating Molarity
A solution of sodium chloride in water contains
14.0 g of NaCl dissolved in 250 mL of solution. What is
the molarity of the solution?
Preparing Solutions in a Lab
We can prepare standard solutions (a
solution of an accurate, known
concentration) from stock solutions (a
concentrated solution) by using a
volumetric flask (glassware that is used to
make a liquid solution with an accurate
volume)
Watch video on preparing standard solutions using volumetric
flasks: http://www.youtube.com/watch?v=A2YyIo8vSCA
Preparing Diluted Solutions
If dilute solutions are prepared accurately,
no mass will be lost, and therefore the
amount of moles will stay the same.
Using c=n/v, we can
rearrange to get n=cV,
and since n is not
changing if we dilute a
solution, the formula
below arises:
c1V1=c2V2
Where c1 = initial concentration
of solution before dilution
v1 = volume of solution before
dilution
c2 = final concentration of
solution after dilution
v2 = volume of solution after
dilution
Example #1: Dilution Calculations
Your teacher has a stock solution of 12 mol/L HCl (aq). A
class experiment requires 2.0L of 0.10 mol/L of
hydrochloric acid. What volume of concentrated solution
should be used to make the dilute solution for the
experiment?
You Try: Dilution Calculations
What volume of 1.25 mol/L KI solution can you make with
125 mL of 3.00 mol/L potassium iodide solution?
How did we do?
Learning Goals:
I will be able define ppm and ppb units and apply
knowledge of them to perform the necessary
calculations to solve for very small concentrations
I will be able to define what molar concentration
(molarity) means, and apply it to solve for the molar
concentration of a given solution, as well as
unknowns in diluted solutions