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MOLARITY
Study guide and self-test
“Molarity” (M) equals the number of moles of solute dissolved per liter
of solution.
The weight of a mole of any element is defined to be equal to its atomic
weight in grams.
“Formula weight” refers to the weight of a compound.
NOTE: Molarity is an expression of CONCENTRATION, not amount.
To make 1L of a 1 M solution of CuSO4:
You can calculate the FW from the periodic table:
1 copper = 1 X 63.55 g = 63.55 g
1 sulfur = 1 X 32.06 g = 32.06 g
4 oxygens=4 X 16.00g = 64.00 g
FW = 159.61 g
To make a 1 L solution, add 159.61 g to enough water to make 1 L
solution.
Let’s try some practice problems:
1. If the formula weight of Na2SO4 is 142.04, how much Na2SO4 do you need
to make 1 L of 3M Na2SO4?
2. If the FW of CuSO4 is 159.6, how much CuSO4 do you need to make 250
mL of 1.5M CuSO4?
3. How much NaCl do you need to make 225 mL of 5M NaCl? Note that the
atomic weight of Na is 22.99 and the atomic weight of Cl is 35.45.
THE SOLUTIONS WILL APPEAR AT THE END OF THIS STUDY
GUIDE.
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Millimolarity and Micromolarity
Millimolarity (mM) and micromolarity (M) are commonly used
expressions of concentration in the biotechnology laboratory. They are
convenient to use because biological solutions often have very small
concentrations of ions that are cumbersome to express in terms of molarity
(for instance, 0.000005 M; more concisely, this is 5 M).
Millimolarity is one-thousandth of molarity (10-3 M).
This can also be expressed as 1 mmole/L or 0.001 mole/L.
Micromolarity is one-millionth of molarity (10-6 M).
This can also be expressed as 1 mole/L or 10-6 mole/L.
Example:
To make 1L of a 1 mM solution of CuSO4:
You can calculate the FW from the periodic table:
1 copper = 1 X 63.55 g = 63.55 g
1 sulfur = 1 X 32.06 g = 32.06 g
4 oxygens=4 X 16.00g = 64.00 g
FW = 159.61 g
Remember that 1 mM is equal to 1 mmole/L, or 0.001 mole/L.
1 L(0.001 moles CuSO4/L)(159.1 grams/mole) = 0.159 grams
To make 1 L of solution, dissolve 0.159 grams in enough CuSO4 water to
make 1 L total.
Let’s try some practice problems:
1. If the formula weight of Na2SO4 is 142.04, how much Na2SO4 do you need
to make 0.25 L of 35 mM Na2SO4?
2. If the FW of CuSO4 is 159.6, how much CuSO4 do you need to make 250
mL of 10M CuSO4?
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SOLUTIONS:
Molarity problems
1. If the formula weight of Na2SO4 is 142.04, how much Na2SO4 do you need
to make 1 L of 3M Na2SO4?
Unit cancellation:
1 L (3 moles Na2SO4 /L)(142.04 g Na2SO4/mole Na2SO4) = 426.12
g Na2SO4
Proportions:
Step 1: How many moles do you need?
____y___ = 3 moles Na2SO4
1L
1L
y = 3 moles Na2SO4
Step 2: How many grams do you need?
___z___
3 moles
=
142.04 grams
1 mole
z = (142.04 grams)(3 moles) = 426.12 grams Na2SO4
1 mole
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2. If the FW of CuSO4 is 159.6, how much CuSO4 do you need to make 250
mL of 1.5M CuSO4?
Unit cancellation: First remember that 250 mL = 0.25 L!!
0.25 L (1.5 moles CuSO4 /L)(159.6 g CuSO4/mole CuSO4) = 59.85
g CuSO4
Proportions:
Step 1: How many moles do you need?
____y___ = 1.5 moles CuSO4
0.25 L
1L
y = 0.375 moles CuSO4
Step 2: How many grams do you need?
___z___
=
0.375 moles
159.6 grams
1 mole
z = (159.6 grams)(0.375 moles) = 59.85 grams CuSO4
1 mole
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3. How much NaCl do you need to make 225 mL of 5M NaCl? Note that the
atomic weight of Na is 22.99 and the atomic weight of Cl is 35.45.
First, calculate the formula weight: 22.99 + 35.45 = 58.44
Unit cancellation: First remember that 225 mL = 0.225 L!!
0.225 L (5 moles NaCl /L)(58.44 g NaCl4/mole NaCl) = 65.75 g
NaCl
Proportions:
Step 1: How many moles do you need?
____y___ = 5 moles NaCl
0.225 L
1L
y = 0.225L(5 moles NaCl) = 1.125 moles NaCl
1L
Step 2: How many grams do you need?
___z___
=
1.125 moles
58.44 grams
1 mole
z = (58.44 grams)(1.125 moles) = 65.75 grams NaCl
1 mole
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Millimolarity and Micromolarity problems
1. If the formula weight of Na2SO4 is 142.04, how much Na2SO4 do you
need to make 0.25 L of 35 mM Na2SO4?
Remember that 1 mM = 10-3 moles/L.
Therefore, 35 mM = 35 X 10-3 moles/L
Unit cancellation:
0.25 L (35 X 10-3 moles Na2SO4 /L)(142.04 g Na2SO4/mole Na2SO4)
= 1.25 g Na2SO4
Proportions:
Step 1: How many moles do you need?
____y___ = 35 X 10-3 moles Na2SO4
0.25 L
1L
y = 8.75 X 10-3 moles Na2SO4
Step 2: How many grams do you need?
___z___
8.75 X 10-3 moles Na2SO4
=
142.04 grams
1 mole
z = (142.04 grams)( 8.75 X 10-3 moles) = 1.25 grams Na2SO4
1 mole
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2. If the FW of CuSO4 is 159.6, how much CuSO4 do you need to make 250
mL of 10M CuSO4?
Remember that 1 M = 10-6 moles/L.
Therefore, 10 M = 10 X 10-6 moles/L
Unit cancellation: First remember that 250 mL = 0.25 L!!
0.25 L (10 X 10-6 moles CuSO4 /L)(159.6 g CuSO4/mole CuSO4) =
3.99 X 10-4 g CuSO4
Proportions:
Step 1: How many moles do you need?
____y___ = 10 X 10-6 moles CuSO4
0.25 L
1L
y = 2.5 X 10-6 moles CuSO4
Step 2: How many grams do you need?
___z___
=
-6
2.5 X 10 moles
159.6 grams
1 mole
z = (159.6 grams)(2.5 X 10-6 moles) = 3.99 X 10-4 grams CuSO4
1 mole