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Molarity

Molarity is a measurement of concentration that expresses the number of moles of solute per liter of solution. It is calculated by taking the moles of solute and dividing by the total liters of solution. The document provides an example of calculating molarity by dissolving 15.0g of sodium hydroxide into 225mL of solution to obtain a concentration of 1.67M sodium hydroxide solution. Two methods are demonstrated - calculating moles and liters individually, or combining the calculations using dimensional analysis.

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223 views2 pages

Molarity

Molarity is a measurement of concentration that expresses the number of moles of solute per liter of solution. It is calculated by taking the moles of solute and dividing by the total liters of solution. The document provides an example of calculating molarity by dissolving 15.0g of sodium hydroxide into 225mL of solution to obtain a concentration of 1.67M sodium hydroxide solution. Two methods are demonstrated - calculating moles and liters individually, or combining the calculations using dimensional analysis.

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govardhan50
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1 of 2

http://www.occc.edu/kmbailey/chem1115tutorials/Molarity.htm

Calculating Molarity
Home

The properties and behavior of many solutions depend not only


on the nature of the solute and solvent but also on the
concentration of the solute in the solution. Chemists use many
different units when expressing concentration; however, one of
the most common units is molarity. Molarity (M) is the
concentration of a solution expressed as the number of moles of
solute per liter of solution:
Molarity (M) =

moles solute
liters solution

Chem 1115

For example, a 0.25 M NaOH solution (this is read as 0.25 molar)


contains 0.25 moles of sodium hydroxide in every liter of
solution. Anytime you see the abbreviation M you should
immediately think of it as mol/L.
In order to calculate the molarity of a solution, you need to know
the number of moles of solute and the total volume of the
solution.
To calculate molarity:

Chem 1215

1. Calculate the number of moles of solute present.


2. Calculate the number of liters of solution present.
3. Divide the number of moles of solute by the number of
liters of solution.
Instead of calculating the moles of solute and liters of solution
present individually, you can also string all the calculations
together in one problem:

Tutorial
List

1. Divide the amount of solute by the volume of solution


(regardless of the initial units given).
2. Use dimensional analysis to convert the amount of solute to
moles of solute.
3. Use dimensional analysis to convert the volume of solution
to liters of solution (if necessary).
The following example will illustrate both methods.
Example: What is the molarity of a solution prepared by
dissolving 15.0 g of sodium hydroxide in enough water to make a
total of 225 mL of solution?
Method 1:
Calculate the number of moles of solute present.
mol NaOH =

15.0g NaOH

1 mol NaOH
40.0 g NaOH

mol NaOH = 0.375 mol NaOH


Calculate the number of liters of solution present.

24-03-2015 10:20 AM

2 of 2

http://www.occc.edu/kmbailey/chem1115tutorials/Molarity.htm

L soln

= 225 mL

1L

0.225 L soln

1000 mL
Divide the number of moles of solute by the number of
liters of solution.
M =

0.375 mol NaOH = 1.67 M NaOH


0.225 L soln

Method 2:
Divide the amount of solute by the volume of solution
(regardless of the initial units given).
M =

15.0 g NaOH
225 mL soln

Use dimensional analysis to convert the amount of solute to


moles of solute.
M = 15.0 g NaOH x
225 mL soln

1 mol NaOH
40.0 g NaOH

Use dimensional analysis to convert the volume of solution


to liters of solution (if necessary).
M = 15.0 g NaOH
225 mL soln

x 1 mol NaOH
40.0 g NaOH

1000 mL soln
1 L soln

M = 1.67 mol NaOH = 1.67 M NaOH


L soln
As you can see, both methods give exactly the same result.
Choose whichever method is most comfortable for you. Just
remember that ultimately your units must be mol/L (= M).

Practice Problems
1. What is the molarity of a solution that contains 1.724 moles
of H2SO4 in 2.50 L of solution?
2. What is the molarity of a solution prepared by dissolving
25.0 g of HCl (g) in enough water to make 150.0 mL of
solution?

Check Answers

24-03-2015 10:20 AM

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