Views: 14
Molarity
The most common unit of solution concentration is molarity (M). The molarity of a solution is defined as the number of moles of solute per one liter of solution. Note that the unit of volume for molarity is liters, not milliliters or some other unit.
Also, note that one liter of solution contains both the solute and the solvent. Molarity, therefore, is a ratio between moles of solute and liters of solution. To prepare laboratory solutions, usually a given volume and molarity are required. To determine molarity, the formula weight or molar mass of the solute is needed. The following examples illustrate the calculations for preparing solutions.
If starting with a solid, use the following procedure
- Determine the mass in grams of one mole of solute, the molar mass,
- Decide volume of solution required, in liters, V.
- Decide molarity of solution required, M.
- Calculate grams of solute (g) required using equation 1.
Equation. 1.
g = MMs x M x V
- Example: Prepare 800 mL of 2 M sodium chloride.
(MMNaCl = 58.45 g/mol)
g NaCl = 58.45 g/mol x 2 mol/L x 0.8 L
g NaCl = 93.52 g NaCl
Dissolve 93.52 g of NaCl in about 400 mL of distilled water, and then add more water until final volume is 800 ml.
If starting with a solution or liquid reagent:
- When diluting a more concentrated solutions, decide what volume (V2) and molarity (M2) the final solution should be. Volume can be expressed in liters or milliliters.
Determine molarity (M1) of starting, more concentrated solution.
- Calculate volume of starting solution (V1) required using equation 2. Note: V1 must be in the same units as V2
Equation. 2.
M1V1 = M2V2
- Example: Prepare 100 mL of 1.0 M hydrochloric acid from concentrated (12.1 M) hydrochloric acid.
M1V1 = M2V2
(12.1 M)(V1) = (1.0 M)(100 mL
V1 = 8.26 mL conc. HCl
Add 8.26 mL of concentrated HCl to about 50 mL of distilled water, stir, and then add water up to 100mL.