What Is Molarity?
Molarity is a measure of the concentration of a solute in a solution. It tells you how many moles of a substance are dissolved in one liter of solution. It is the most commonly used concentration unit in chemistry because it directly relates to the number of molecules involved in a reaction.
Molarity is expressed in units of mol/L, also written as M (molar). A 1 M solution contains exactly 1 mole of solute per liter of solution. A 0.5 M solution contains half a mole per liter.
What This Calculator Does
This calculator solves the molarity equation for any of its three variables:
- Molarity (M): given moles and volume
- Moles (n): given molarity and volume
- Volume (V): given molarity and moles
An optional tool converts mass and molar mass to moles, which you can then use as input for the main calculation.
How the Calculation Works
Molarity (M) = Moles of solute (n) / Volume of solution (V in liters)
Moles (n) = Molarity (M) × Volume (V)
Volume (V) = Moles (n) / Molarity (M)
When you know the mass of a substance and its molar mass, you can find the number of moles using: Moles = Mass (g) / Molar Mass (g/mol). For example, sodium chloride (NaCl) has a molar mass of 58.44 g/mol. Dissolving 29.22 g of NaCl in enough water to make 1 liter gives a 0.5 M NaCl solution.
How to Use the Calculator
- Select what you want to solve for using the tabs
- Enter the two known values with appropriate units
- Optionally enter mass and molar mass at the bottom to find moles
- The result appears instantly in the right panel
Example Calculations
Example 1: Preparing 1 L of 0.5 M NaCl solution
Molar mass of NaCl = 58.44 g/mol. Moles needed = 0.5 M × 1 L = 0.5 mol. Mass needed = 0.5 × 58.44 = 29.22 g. Dissolve 29.22 g of NaCl in water and make up to 1 liter.
Example 2: Finding the molarity of a glucose solution
Glucose (C6H12O6) has a molar mass of 180.16 g/mol. You have 36.03 g dissolved in 500 mL (0.5 L) of solution. Moles = 36.03 / 180.16 = 0.2 mol. Molarity = 0.2 / 0.5 = 0.4 M.
Real-World Scenarios
Laboratory Solutions
Scientists prepare buffer solutions, reagents, and standard solutions using molarity. A phosphate-buffered saline (PBS) solution used in biology labs is typically prepared at a specific molar concentration of each salt component.
Pharmaceutical Formulation
Drug concentrations in intravenous medications are expressed in molar terms. Knowing the molarity helps pharmacists calculate the exact dosage in milligrams from the volume drawn from a vial.
Dilution Calculations
The dilution formula M1V1 = M2V2 relies on molarity. If you have a 2 M stock solution and need 100 mL of 0.1 M, you need 5 mL of stock diluted to 100 mL. Molarity calculations are the foundation of all dilution work.
Why This Calculation Matters
Incorrect molarity in a laboratory or medical setting can invalidate experimental results, ruin chemical reactions, or harm patients. Accurate molarity calculations are a basic competency for chemists, pharmacists, and laboratory technicians.
Common Mistakes to Avoid
- Using mL instead of L: Molarity requires volume in liters. If your volume is in mL, divide by 1000 before calculating
- Confusing molarity and molality: Molarity (M) uses volume of solution in liters. Molality (m) uses mass of solvent in kilograms. They are not interchangeable
- Using molar mass of the wrong form: Molar mass must match the actual substance being dissolved. Hydrated salts like CuSO4·5H2O have a higher molar mass than anhydrous CuSO4