Molarity
Concentration tells how much solute is in a given amount of solution. The most common unit in chemistry is molarity (M).
molarity = moles of solute ÷ liters of solution, or M = n / V
A 1.0 M solution contains 1.0 mol of solute in every liter of solution. M is read as “molar” and means mol/L.
Worked example: find the molarity
0.50 mol of NaCl is dissolved to make 2.0 L of solution.
M = 0.50 mol ÷ 2.0 L = 0.25 M
Worked example: starting from grams
What is the molarity of 10.0 g of NaOH in 500 mL of solution?
Moles: 10.0 g ÷ 39.997 g/mol = 0.250 mol
Volume: 500 mL = 0.500 L
M = 0.250 mol ÷ 0.500 L = 0.500 M
Worked example: find the moles
How many moles of solute are in 0.250 L of a 2.00 M solution? Rearrange to n = M × V.
n = 2.00 mol/L × 0.250 L = 0.500 mol
Common mistake
Dividing by milliliters. Molarity uses liters. Also, the volume is that of the final solution, not the water you started with.