## Chem – Molarity

What is Molarity?

Molarity is by far the most common way to measure a solution concentration. So whenever you see the word concentration in a problem always immediately think molarity. The definition of molarity is moles of solute per liters of solution. I have written the equation below.

 Molarity = Moles of Solute Liters of Solution

In order to think of it in a more simplistic way and help with problem solving I have simplified the formula into just units like below.

 Molarity = Moles Liters

You can further shorten this into the letter representations of the units.

 M = mol L

Capital M represents molarity. The mol represents moles. Capital L represents liters. When you see the M in a sentence it should usually be next to a number like 5 M. That means 5 molarity and it is a unit.

Examples: Solve for the molarity problems below.

If you put 0.4 mols of a solute in a beaker and then fill up with water to 3 L. What is your molarity?

A 9M solution was made by filling a container up with 7L of solution. How many moles did it take?

If you used 2.8 moles to make a 6.3 M solution. What is the volume of that solution?

VIDEO Molarity Demonstrated Example 1: A 15M solution was made by filling a container up with 4.2L of solution. How many moles did it take?

What information are we given?

Molarity = 15M

Volume = 4.2L

What does the question ask for?

How do we set up the problem?

 M = mol L

What can we fill in for the equation?

Answer: The information we are given

 15 M = mol 4.2 L

How do we solve?

Multiply both sides by 4.2 L

 4.2 L * 15 M = mol * 4.2 L 4.2 L

Cross out 4.2 L on the right side

 4.2 L * 15 M = mol * 4.2 L 4.2 L

Simplify

 4.2 L * 15 M = mol 11

Multiply the left side

 4.2 L * 15 M = mol 11

Result

 63 = mol 11

VIDEO Molarity Demonstrated Example 2: If you used 7.6 moles of BaCl2 to make a 1.2 M solution. What is the volume of that solution?

What information are we given?

moles = 7.6 mol

molarity = 1.2 M

The BaCl2 does not matter.

What does the question ask for?

How do we set up the problem?

 M = mol L

What can we fill in for the equation?

Answer: The information we are given

 1.2 M = 7.6 mol L

How do we solve?

Multiply both sides by L

 L * 1.2 M = 7.6 mol * L L

Cross out L on the right side

 L * 1.2 M = 7.6 mol * L L

Simplify

 L * 1.2 M = 7.6 mol 1

Divide both sides by 1.2 M

 L * 1.2 M = 7.6 mol 1.2 M 1.2 M

Cross out 1.2 M on the left side

 L * 1.2 M = 7.6 mol 1.2 M 1.2 M

Simplify

 L = 7.6 mol 1.2 M

Divide the right side

 L = 6.3 11

PRACTICE PROBLEMS: Solve for the MOLARITY problems below.

If you put 4.7 mols of a solute in a beaker and then fill up with water to 8.6 L. What is your molarity?

A 0.58 M solution was made by filling a container up with 1.3 L of solution. How many moles did it take?

If you used 3.4 moles to make a 23 M solution. What is the volume of that solution?  