The student is expected to calculate the concentration of solutions in units of molarity AND use molarity to calculate the dilutions of solutions.
Molarity (M) is a term and a unit used to express the concentration of a solution and is expressed as moles of a solute dissolved per liter of solution, or mol/L. This unit of mol/L is also called “molar.”
Diluted solutions may be created from concentrated solutions by merely adding more solvent. The molarity or required volume of the diluted solution can be calculated from the concentrated solution using the equation M1V1 = M2V2, where M1V1 stands for molarity and volume of the concentrated solution and M2V2 stands for the molarity and volume of the dilution solution.
Molarity Defines Solution Concentration
A solution is a homogenous mixture. In these types of mixtures, a sample taken from any part of the mixture will have the same composition as another sample from the same solution. The focus of this scope is aqueous solutions, including both solvents and solutes, and their concentrations.
Molarity (M) is the term used to describe the specific units to express the concentration of a solution. Molarity is expressed as moles of a solute dissolved per liter of solution, or mol/L. Molarity is not an equality expression, but a way to define concentration. The units for molarity are moles of solute perliter of solution. The volume unit cannot be shown as other volume units if it is to describe molarity. Any other volume unit must first be converted to liters in order to be considered the unit of molarity.
Molarity = moles of solute/liters of solution
The Unit of Molarity is the Molar
This unit of mol/L is also known as “molar concentration”. It is a convenient way to describe the concentration of a solution.
M = mol/L
Molarity and Mole are Different Terms
Even though the terms molarity and mole both describe a substance in terms of moles, they are not synonymous. The term molarity is used to describe the concentration of a solute in a solution in terms of moles per liter. The term mole is used to describe the quantity of a material in terms of mass per mole.
For instance, if 0.87 mol NaCl is dissolved in pure water creating 3.00 L of solution, what is its molarity of the resulting solution? In this example, 0.87 mol is the quantity of solute, in this case NaCl. The number of moles of solute is not the same as the resulting molarity of the solution. The molarity of the solution is the following:
Molarity = (0.87 mol NaCl) / (3.00 L solution) = 0.29 M NaCl
Creating Diluted Solutions
There are two common ways that diluted solutions may be created from concentrated solutions. In the first method, a known molarity and volume of solution can be diluted to a wanted final molarity. In the second method, a wanted molarity and volume may be found by calculating the amount of the concentrated solution that must be used to create the diluted solution. Both methods will use the following equation to solve for the unknown factor:
V1M1 = V2M2
V1M1 = molarity and volume of the original concentrated solution
V2M2 = molarity and volume of the diluted solution.
This equation is based on the law of conservation of mass. The term V1M1 gives the quantity of substance in the unit of a mole, as does V2M2. When a liter is multiplied by molarity, you get (L)(mol/L). The liters cancel, leaving only moles. During the process of diluting, it is important to understand that the number of moles of the solute remains unchanged. The amount of total solution is what is changing. For this reason, the equation can be used to solve these types of dilution problems.
Calculating Volume of Diluted Solution (V2): For example, in the first method, V1, M1, and M2are known. You can simply solve for V2 and increase the amount of solution to the calculated amount.
Calculating Volume of Concentrate Solution (V1)
The second method is not so straightforward. You know the final volume and molarity that is wanted, both V2 and M2. You also know the molarity of the concentrated solution, or M1. To create this final solution, calculate the unknown volume of concentrated solution that must be used, or V1, by using the formula above. Then, add this amount of concentrate to a volumetric flask and add water until the desired volume, V2, is reached.
Note, the solution will only be at the correct molarity, no matter which method is used, when the solution is thoroughly and properly mixed, as molarity is based on volume of solution, not the volume of the solvent alone. Let the students know that in order to create a properly diluted solution, they will be using either a volumetric pipette or burette in procedures to accurately measure the volumes of solutions accurately. Ideally, they should also use a volumetric flask to measure the correct volumes of solutions.