LSS_Molarity Lesson
Molarity
In this section, we learn about molarity and colligative properties. The following video is approximately 23 minutes long. While viewing the video, take notes about important concepts presented. Pause the video if necessary to write down observations. Be sure to answer the video quiz questions at the end of the video.
Molarity Tutorial
Mark Rosengarten explains what molarity is, how to calculate it, and how to calculate the grams needed to make a solution of desired molarity and volume.
Scientific Method Steps
- Observation: Using our senses (sight, hearing, taste, touch, and smell) to collect information about the problem we are trying to solve.
How to Solve Molarity Problems
Molarity is a quantitative description of solution concentration. It is abbreviated M.
Molarity=molesofsolutelitersofsolution
Using the formula above and your basic knowledge of algebra, solve the following problems:
Example 1
To make a 4.00 M solution, how many moles of solute will be needed if 12.0 liters of solution are required?
Step 1:
Identify each piece of information given in the problem and identify what the problem is asking for.
M = 4.00 M
mol = ?
L = 12.0 L
Step 2:
Rearrange the molarity equation to solve it for the variable that the question is asking for.
moles of solute = Molarity x liters of solution
(Multiply both sides of the equation by liters of solution. On the right hand side of the equation, they cancel out, on the other side, you are left with Molarity x liters of solution.)
Step 3:
Plug in your data from step 1 into the rearranged equation from step 2.
moles of solute = 4.00 M x 12.0 L
moles of solute = 48.0 mol
Remember that M stands for mol / L. If we substitute this into the equation, we can see that L will cancel, leaving units of mol.
You try this one:
Example 2
What is the molarity of a solution of HNO3 that contains 12.6 grams HNO3 in 1.0 L of solution?
Step 1:
Identify each piece of information given in the problem and identify what the problem is asking for.
M = ?
mol = ?
L = 1.0 LLiq
In this equation moles are not given, but grams are. You must use your knowledge of molar mass to convert grams of HNO3 to moles.
molHNO3=12.6gHNO3(1moleHNO363g)
mol HNO3 = 0.200 mol
Step 2:
Rearrange the molarity equation to solve it for the variable that the question is asking for.
Molarity=molesofsolutelitersofsolution
Step 3:
Plug in your data from step 1 into the rearranged equation from step 2.
M=0.200molHNO31.0L
M = 0.200 M
You try this one:
Example 3
125 cm3 of solution contains 3.5 moles of solute. What is the molarity of the solution?
Step 1:
Identify each piece of information given in the problem and identify what the problem is asking for.
M = ?
mol = 3.5 mol
L = ?
In this equation, the volume is given in cm3. We know from a previous module that 1 cm3 = 1 mL. You must use your knowledge of metric prefixes to complete this conversion.
L=125cm3×(1mL1cm3)×(1L1000mL)=0.125L
Step 2:
Rearrange the molarity equation to solve it for the variable that the question is asking for.
Molarity=molesofsolutelitersofsolution
Step 3:
Plug in your data from step 1 into the rearranged equation from step 2.
M=3.5mol0.125L
M = 28 M
Remember to work on the module practice problems as you complete each section of content.
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