Phar Organic Lab: Molarity and Solution Preparation

Study Notes

Molarity is the measure of the number of moles of a substance in a solution
The formula to find the moles of a substance is: n = m/MW (where n is the number of moles, m is the mass of the substance, and MW is the molar mass)

Solutions can be prepared by: • Using a solid solute • Diluting a concentrated solution
To prepare a solution from a solid solute, the molarity equation is used to determine the mass of the solute required
Steps involved in preparing a solution from a solid solute: • Calculate the amount of solid required • Mass out the solid • Place the solid in an appropriate volumetric flask • Fill the flask about half full with water and mix • Fill to the mark with water and invert to mix

To prepare 250 mL of a 0.10 M NaCl solution: • Calculate the number of moles of NaCl required: n = 0.10 M x 0.25 L = 0.025 moles • Calculate the mass of NaCl required: m = 0.025 moles x 58.44 g/mol = 1.46 g • Steps to prepare the solution: mass out 1.46 g of NaCl, place in a volumetric flask, fill half full with water and mix, fill to the mark with water and invert to mix

To prepare 500 mL of a 0.04 M glucose solution: • Calculate the number of moles of glucose required: n = 0.04 M x 0.5 L = 0.02 moles • Calculate the molar mass of glucose: C₆H₁₂O₆ = 6(12) + 12(1) + 6(16) = 180 g/mol • Calculate the mass of glucose required: m = 0.02 moles x 180 g/mol = 3.6 g • Steps to prepare the solution: mass out 3.6 g of glucose, place in a volumetric flask, fill half full with water and mix, fill to the mark with water and invert to mix

A common method to create a solution of a specific concentration is to take a more concentrated solution and add water until the desired concentration is reached.
This process is known as dilution.