Molarity

The concentration of a solution is a measure of the amount of solute dissolved in a given quantity of solvent. A dilute solution contains only a small amount of solute. A concentrated solution contains a large amount of solute. Although these terms are frequently used, they are rather vague when it comes to the actual amount of solute in solution. Since the solubilities for different substances vary, dissolving 1.0 gram of one substance in 100.0 grams of water would be dilute whereas dissolving 1.0 gram of a different substance in 100.0 grams of water would be concentrated.

To make clearer the concentration of a solution, chemists calculate molarity (M). Molarity is the number of moles of solute per liter of solution.

For example: Calculate the molarity of a solution where 38.0 grams of sodium chloride is dissolved in water to make 900. mL of solution.

You must first convert grams to moles by dividing the compound's molar mass.
(38.0 grams) / (58.5 grams/mole) = 0.650 moles

Next convert mL of solution to liters of solution.
900. / 1000. = 0.900 liters

Now, plug it in the formula and solve.
Molarity = 0.650 moles / 0.900 liter= 0.722 Molar

You may also find these related tutorials helpful

Compounds in Crust of Earth

Density

Elements in Crust of Earth

Significant Figure Calculations

Significant Figures