Molarity
Ck12 Science
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Printed: May 14, 2015
AUTHOR
Ck12 Science
www.ck12.org
C HAPTER
Chapter 1. Molarity
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Molarity
• Define molarity.
• Perform calculations involving molarity.
How many molecules can be found in a reaction?
Chemists deal with amounts of molecules every day. Our reactions are described as so many molecules of compound
A reacting with so many molecules of compound B to form so many molecules of compound C. When we determine
how much reagent to use, we need to know the number of molecules in a given volume of the reagent. Percent
solutions only tell us the number of grams, not molecules. A 100 mL solution of 2% NaCl will have a very different
number of molecules than a 2% solution of CsCl. So we need another way to talk about numbers of molecules.
Molarity
Chemists primarily need the concentration of solutions to be expressed in a way that accounts for the number of
particles that react according to a particular chemical equation. Since percentage measurements are based on either
mass or volume, they are generally not useful for chemical reactions. A concentration unit based on moles is
preferable. The molarity (M) of a solution is the number of moles of solute dissolved in one liter of solution. To
calculate the molarity of a solution, you divide the moles of solute by the volume of the solution expressed in liters.
Molarity (M) =
moles of solute
mol
=
liters of solution
L
Note that the volume is in liters of solution and not liters of solvent. When a molarity is reported, the unit is the
symbol M and is read as “molar”. For example a solution labeled as 1.5 M NH3 is read as “1.5 molar ammonia
solution”.
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Sample Problem: Calculating Molarity
A solution is prepared by dissolving 42.23 g of NH4 Cl into enough water to make 500.0 mL of solution. Calculate
its molarity.
Step 1: List the known quantities and plan the problem.
Known
Unknown
mass = 42.23 g NH4Cl
molarity = ? M
molar mass NH4Cl = 53.50 g/mol
volume solution = 500.0 mL = 0.5000 L
The mass of the ammonium chloride is first converted to moles. Then the molarity is calculated by dividing by liters.
Note the given volume has been converted to liters.
Step 2: Solve.
1 mol NH4Cl
= 0.7893 mol NH4Cl
53.50 g NH4Cl
0.7893 mol NH4Cl
= 1.579 M
0.5000 L
42.23 g NH4Cl ×
Step 3: Think about your result.
The molarity is 1.579 M, meaning that a liter of the solution would contain 1.579 mol NH4 Cl. Four significant
figures are appropriate.
In a laboratory situation, a chemist must frequently prepare a given volume of solutions of a known molarity. The
task is to calculate the mass of the solute that is necessary. The molarity equation can be rearranged to solve for
moles, which can then be converted to grams. See sample problem 16.3.
Sample Problem:
A chemist needs to prepare 3.00 L of a 0.250 M solution of potassium permanganate (KMnO4 ). What mass of
KMnO4 does she need to make the solution?
Step 1: List the known quantities and plan the problem.
Known
• molarity = 0.250 M
• volume = 3.00 L
• molar mass KMnO4 = 158.04 g/mol
Unknown
• mass KMnO4 = ? g
Moles of solute is calculated by multiplying molarity by liters. Then, moles is converted to grams.
Step 2: Solve.
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Chapter 1. Molarity
mol KMnO4 = 0.250 M KMnO4 × 3.00 L = 0.750 mol KMnO4
158.04 g KMnO4
= 119 g KMnO4
0.750 mol KMnO4 ×
1 mol KMnO4
Step 3: Think about your result.
When 119 g of potassium permanganate is dissolved into water to make 3.00 L of solution, the molarity is 0.250 M.
Watch a video of molarity calculations:
MEDIA
Click image to the left or use the URL below.
URL: http://www.ck12.org/flx/render/embeddedobject/63361
http://www.youtube.com/watch?v=8oTqwBAvbnY
Summary
• Calculations using the concept of molarity are described.
Practice
Read the material and work the problems at the site below:
http://www.occc.edu/kmbailey/Chem1115Tutorials/Molarity.htm
Review
Questions
1. What does M stand for?
2. What does molarity tell us that percent solution information does not tell us?
3. What do we need to know about a molecule in order to carry out molarity calculations?
• molarity (M): The number of moles of solute dissolved in one liter of solution.
References
1. Image copyright Creativa, 2014. http://www.shutterstock.com .
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