Molarity is a measure of the concentration of a solute in a solution. This molarity example problem shows the steps needed to calculate the molarity of a solution given the amount of solute and the desired volume of solution.
Problem
Calculate the molarity of a solution created by pouring 7.62 grams of MgCl2 into enough water to create 400 mL of solution.
Solution
The formula to calculate molarity is
In this case, the solute is 7.62 grams of MgCl2. The formula needs the number of moles.
Using a periodic table, the molecular mass of MgCl2 is 95.21 grams/mol. Find the number of moles in 7.62 grams in MgCl2.
moles MgCl2 = 0.08 moles
Molarity also needs to have the volume in liters, not milliliters.
The final volume of our solution is 400 mL, which is 0.4 L.
Plug this information into the formula
M = 0.2 moles/L or 0.2 M
Answer
The concentration of MgCl2 in the solution is 0.2 M.