Molarity — Definition & Calculation

What is Molarity?

Molarity, also known as molar concentration, is a unit of measurement that describes how much of a substance in a solution The substance dissolved in a solution is known as a solute. The substance the solute is dissolved in is called the solvent. The solute amount can be measured and converted to moles. The number of solute moles per liter (volume) of solvent is called molarity (M). $$Molarity\ (M) = \frac{moles\ (mol)}{liter\ (L)}$$

Calculating Molarity

Determining molarity begins with making sure your units of solute are in moles and units of solvent is in liters. This is done by converting from one unit to another.

Let's start with the solute. Typically the amount of solute is given as a mass (g) or volume (L). If grams are known moles can be converted to through molecular weight (g/mol): $$g\ \times\ \frac{1\ mol}{g}\ =\ mol$$ If the volume is known moles can be converted to through the density (g/mL) and molecular weight of the substance: $$mL\ \times\ \frac{g}{mL}\ \times\ \frac{1\ mol}{g}\ =\ mol$$ Keep in mind that if the initial volume is in liters, the volume will need to be converted to milliliters first.

Next, let's consider the solvent. The solvent is typically given as a volume (L) or mass (g). If the volume is given in milliliters (mL) it is converted by: $$mL\ \times\ \frac{1\ L}{1000\ mL}\ =\ L$$ If the mass of the solvent is given, it is converted to L by using the density: $$g\ \times\ \frac{mL}{g}\ \times\ \frac{1\ L}{1000\ ml}\ =\ L$$

Example Molarity Calculation

1.60 g of sodium acetate or NaOAc ($NaC_2H_3O_2$) is added to 25.50 g of water ($H_2O$). What is the molar concentration of the solution?

Moles of Sodium Acetate

The molecular weight of NaOAc is 82.03 g/mol $$1.60\ g\ NaOAc\ \times\ \frac{1\ mol}{82.03\ g}NaOAc\ = 0.0195\ mol\ NaOAc$$

Liters of Water

The density of water is 1.00g/mL $$25.50\ g\ H_2O\ \times\ \frac{1\ mL}{1.00\ g}H_2O\ \times\ \frac{1\ L}{1000\ mL}\ =\ 0.02550\ L\ H_2O$$

Molarity

$$\frac{0.0195\ mol\ NaOAc}{0.02550\ L\ H_2O}\ =\ 0.765\ M\ NaOAc\ solution$$

However, molarity is not the only concentration measurement! Molality is another concentration measurement where instead of calculating concentration in terms of volume, concentration is measured in terms of mass.

(Video) Molarity Made Easy: How to Calculate Molarity and Make Solutions

by ketzbook

Figuring out molarity problems can be tough. But don't worry, ketzbook is here to help! Watch this video to find out what molarity is and how to calculate it in a variety of problems.

Molarity: What is molarity, and how to calculate it?