How to Calculate the Molarity of a Solution Step by Step

Knowing how to calculate the molarity of a solution is an essential chemistry skill. After reading this lesson, you'll be able to find the molarity (concentration) of any solution if you know the amount of solute and the total volume of solution.

What is Molarity and Why Does It Matter?

Molarity (symbol: $M$) tells you how many moles of a substance (solute) are dissolved in one liter of solution. It's written as “moles per liter” ($\text{mol/L}$) and is used whenever you need to know how concentrated a solution is, such as in reactions or when preparing solutions in a lab.

The Molarity Formula

The formula for molarity is:

$$M = \frac{n}{V}$$

Where:

• $M$ = molarity (mol/L)
• $n$ = number of moles of solute (mol)
• $V$ = volume of solution (liters)

Worked Example: Calculating Molarity

Question: What is the molarity of a solution made by dissolving 5.00 grams of NaCl (sodium chloride) in enough water to make 250 mL of solution?

Step 1: Calculate moles of solute.

• Molar mass of NaCl = $22.99 + 35.45 = 58.44$ g/mol
• $n = \frac{\text{mass}}{\text{molar mass}} = \frac{5.00\ \text{g}}{58.44\ \text{g/mol}} = 0.0856$ mol

Step 2: Convert volume to liters.

• $250\ \text{mL} = 0.250\ \text{L}$

Step 3: Plug into the formula.

• $M = \frac{0.0856\ \text{mol}}{0.250\ \text{L}} = 0.342\ \text{mol/L}$

Final Answer: The molarity is $0.342\ M$.

Where You’ll See This in Real Life

Pharmacists, chemists, and lab technicians all use molarity to make medicines, do experiments, and check water quality. For example, making saline for medical use requires a precise molarity.

Common Mistakes and How to Avoid Them

• Forgetting to convert mL to L: Always use liters in the formula, not milliliters.
• Using the wrong molar mass: Double-check the periodic table for correct atomic masses.
• Mixing up solute and solvent: Remember, molarity is about the solute (what’s dissolved), not the total mass.

Try It Yourself

What is the molarity of a solution made by dissolving 2.00 g of KCl (molar mass = 74.55 g/mol) in enough water to make 100 mL of solution?

Key Takeaway

If you know the amount of substance and the total volume, you can always calculate molarity. This skill is vital for almost every chemistry experiment you’ll do in school and beyond.