Scientific Tutor
Chem – Molarity
Author: Andrew
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?
Answer: 0.13 M
A 9M solution was made by filling a container up with 7L of solution. How many moles did it take?
Answer: 63 mol
If you used 2.8 moles to make a 6.3 M solution. What is the volume of that solution?
Answer: 0.44 L
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?
Answer:
Molarity = 15M
Volume = 4.2L
What does the question ask for?
Answer: moles = ?
How do we set up the problem?
Answer: Start with the equation
| 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 |
COMPLETE ANSWER: 63 mol
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?
Answer:
moles = 7.6 mol
molarity = 1.2 M
The BaCl2 does not matter.
What does the question ask for?
Answer: Volume = ? L
How do we set up the problem?
Answer: Start with the equation
| 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 |
COMPLETE ANSWER: 6.3 L
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?
Answer: 0.55 M
A 0.58 M solution was made by filling a container up with 1.3 L of solution. How many moles did it take?
Answer: 0.75 mol
If you used 3.4 moles to make a 23 M solution. What is the volume of that solution?
Answer: 0.15 L
If you used 6.5 moles to make a 4.2 M solution. What is the volume of that solution in mL?
Answer: 155 mL
Posted in Chemistry
