Making a Solution

Solutions used in the laboratory are usually made from either solid solutes (often salts) or stock solutions.

To make a solution from solid solutes, first calculate how many moles of solute are in the desired solutions (using the molarity).  Calculate the amount of solid you need in grams using the moles needed and the molar mass of the solute and weight out the needed amount.  Transfer the solute to a container (preferably a volumetric flask, which most accurately measures volume of solution labeled on the flask) and add a small amount of solvent. Mix thoroughly to dissolve the solute. Once the solute has dissolved, add the remaining solvent to make the solution of the desired volume and mix thoroughly.

For example, to make 0.5 Liters of 0.5 molar NaCl:

1. Multiply the concentration (0.5 mols/Liters) by the volume of solution you want (0.5 Liters) to find the moles of NaCl you need.

0.5 moles/Liter * 0.5 Liters = 0.25 moles of NaCl

2. Multiply the moles of NaCl by its molar mass (58.44 g/mol) to find the grams of solute needed.

(0.25 moles NaCl)*(58.44 grams/mole) = 14.61 grams of NaCl

Making a solution of a certain concentration from a stock solution is called a dilution. When diluting a solution, keep in mind that adding a solvent to a solution changes the concentration of the solution, but not the amount of solute already present.

To dilute a solution with known concentration, first determine the number of moles of solute are in the solution by multiplying the molarity by the volume (in Liters). Then, divide by the desired molarity or volume to find the volume or concentration needed.

The equation to use is simply

M₁V₁ = M₂V₂

M₁ and V₁ are the concentration and volume of the original (stock) solution to dilute; M₂ and V₂ are the desired concentration and volume of the final solution.