How to Calculate Molarity of a Solution: Formula & Solute Mass Guide

In chemistry laboratories and biomedical research, Molarity (M) is the standard unit of concentration, expressing the number of moles of solute dissolved per liter of solution.

This guide explains the fundamental molarity equations, shows how to weigh dry reagents accurately on a balance, and connects to the CalculatorAll Molarity Calculator.

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The Core Molarity Formulas

1. Solution Molarity Formula

Molarity (M) = Moles of Solute / Volume of Solution (Liters) = mol / L

2. Moles of Solute from Mass

Moles = Mass of Solute (Grams) / Molar Mass (g/mol)

3. Grams of Solute Needed for Lab Preparation

Grams Needed = Target Molarity (mol/L) × Volume (L) × Molar Mass (g/mol)

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Worked Example: Preparing 500 mL of 0.5 M NaCl Solution

Suppose you need to prepare 500 mL (0.50 Liters) of a 0.50 M Sodium Chloride (NaCl) solution:

• Molar Mass of NaCl: Na (22.99) + Cl (35.45) = 58.44 g/mol.

1. Calculate Required Solute Mass: Grams = 0.50 mol/L × 0.50 L × 58.44 g/mol = 14.61 grams of NaCl.
2. Laboratory Preparation Protocol:
   • Weigh out 14.61 grams of pure analytical-grade NaCl on a precision balance.
   • Transfer to a 500 mL volumetric flask containing ~300 mL of distilled water.
   • Swirl until completely dissolved.
   • Add distilled water until the bottom of the meniscus touches the 500 mL calibration line (quantity sufficient / q.s.).

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Molarity (M) vs. Molality (m) Comparison