Molarity

1
Molarity
Molarity concentration or ratio
Molarity of moles of solute
1 L of solution
moles grams
molecular weight
Molarity grams _
g_ molecular weight
m.wt. 1 L
solution L
2
K2SO4 has a molar mass of K 39.0983
x 2 S 32.06 x 1 O
15.999 x 4 174.3 g
mole
Lets say that we had 4.47 g of potassium
sulfate and wanted to make 25 mL of solution --
what concentration (molarity) would that be?
3
Molarity 4.47 g K2SO4
174.3 g/mole 1.0 M
.025 L
?the unit for molarity is abbreviated as M
?25 mL of solution must be converted into L
first before you use the equation
25 mL 1 L .025
L 103 ml
4
Needed 100 mL of a .227 M solution of NaCl
M g / Mol.Wt. / L
(M) (L) (Mol. Wt.) g
5
(.227 moles) (.1 L) (58.5 __g__)
L mole
Before we look at the final arithmetic
calculation, examine the units in this equation
to make sure that they cancel to give you grams.
.227 moles .1 L 58.5 g
L
mole
1 g NaCl (1 SF)
Please also note that if you are given mL in the
problem, youve got to first convert it to L to
use in the problem.
100 mL 1 L .1 L
103 mL
6
Try these problems on your own
1. Determine the molarity if 35 g of
potassium chloride has been dissolved in a
total of 3.0 L of solution.
1. g Mol. Wt.
L
35 g 74.4 g/mole 3.0
L
.16 M
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2. If a 2.5 M solution of magnesium
nitrate is used in an experiment, what
volume (in L) is required to provide 3.00
moles of magnesium nitrate?
M moles so. L
M L moles
3.00 moles 1.2 L 2.5 moles/L
L moles / M
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3. If 85.0 g of sodium nitrate is used
to make a 0.5 M solution, find the volume
of solution used in liters.
g 85.0 g
V Mol. Wt. 84.99 g/mole 2 L
M .5 mole/L
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4. A student wants to prepare 275 mL of
magnesium sulfate which is 0.80M. How
many grams of magnesium sulfate should she
weigh out?
g mole x L x _ g__
L mole

(.80 mole) (.275L) (120.365 g) L
mole 26 g