IFAP - Molarity (Lesson)
Molarity
A solute is a component of a solution that is typically present at a much lower concentration than the solvent. Solute concentrations are often described with qualitative terms such as dilute (of relatively low concentration) and concentrated (of relatively high concentration).
Concentrations may be expressed using a wide variety of measurement units, each convenient for particular applications. Molarity (M) is a useful concentration unit for many applications in chemistry. Molarity is defined as the number of moles of solute per 1 liter (1 L) of solution:
EXAMPLE: 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 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.
Examples of Different Types of Molarity Calculations
The following examples show different types of molarity calculation problems that you may encounter.
Example One:
Example Two:
Example Three:
Preparing Solutions
Solutions of known concentration can be prepared either by dissolving a known mass of solute in a solvent and diluting to a desired final volume or by diluting the appropriate volume of a more concentrated solution (a stock solution) to the desired final volume. A container known as a volumetric flask is often used to prepare solutions of known molarity. There are no graduations of measurement on such a flask, only one, highly precise volume marking.
You might be asked on the AP exam how to prepare a solution. You will need to respond with appropriate calculations, procedural steps, and note needed equipment.
EXAMPLE: How do you prepare 500 mL of a 0.50 M solution of KOH (aq)?
Step 1: Determine the moles of KOH in 500 mL of a 0.50 M solution and convert to mass.
500 mL x 0.50 moles/1000 mL = 0.250 moles
0.250 moles KOH x 56.11 g KOH/1 mole = 14.03 g KOH
Step 2: Weigh 14.03 g KOH on a mass scale. Add solid to a 500 mL volumetric flask. Add about half full with water and mix to dissolve.
Step 3: When all solid is fully dissolved, dilute with additional water to the 500 mL mark.
Because the solute occupies space in the solution, the volume of the solvent needed is almost always less than the desired volume of solution.
You Try It! Molarity
Beer-Lambert Law
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