Calculate mass, volume, or concentration for your chemical solutions
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Calculate Concentration
What is Molarity?
Molarity (M) is a measure of concentration that expresses the number of moles of solute per litre of solution. It is one of the most commonly used units in chemistry for describing solution concentrations.
Molarity (M) = moles of solute ÷ litres of solution
Molecular Weight: The sum of atomic masses of all atoms in a molecule, expressed in grams per mole (g/mol). This value is essential for converting between mass and moles.
Moles: A unit that measures the amount of substance. One mole contains 6.022 × 10²³ entities (Avogadro’s number).
Solution vs Solvent: The volume used in molarity calculations is the total volume of the final solution, not just the solvent volume.
Example Calculation
To prepare 500 mL of 0.1 M sodium chloride (NaCl) solution:
• Molecular weight of NaCl = 58.44 g/mol
• Volume = 0.5 L
• Concentration = 0.1 M
• Mass required = 0.1 × 0.5 × 58.44 = 2.92 g
How to Use This Calculator
Calculate Mass
Use this when you know the desired concentration and volume, and need to determine how much compound to weigh out. Enter the molecular weight, target concentration, and volume to find the required mass.
Calculate Volume
Use this when you have a specific mass of compound and want to know what volume of solvent to add to achieve a particular concentration. This is useful when working with limited quantities of expensive reagents.
Calculate Concentration
Use this to determine the concentration of a solution when you know the mass of compound dissolved and the final volume. This is helpful for characterising existing solutions or checking your preparations.
Frequently Asked Questions
What’s the difference between molarity and molality?
Molarity (M) is moles of solute per litre of solution, whilst molality (m) is moles of solute per kilogram of solvent. Molarity depends on temperature as solution volume changes, but molality doesn’t change with temperature.
How do I find the molecular weight of a compound?
Add up the atomic masses of all atoms in the molecular formula. Use a periodic table to find atomic masses, and multiply by the number of each type of atom present. For example, H₂SO₄ = (2×1.008) + (1×32.07) + (4×16.00) = 98.08 g/mol.
Why use the total solution volume rather than solvent volume?
Molarity is defined as moles per litre of final solution. When you dissolve a solute, the total volume may differ from the original solvent volume due to volume changes upon mixing. Always use the final solution volume.
How accurate should my measurements be?
For most laboratory work, measure masses to 0.1 mg (4 decimal places) and volumes to 0.1 mL or better. Use volumetric flasks for precise volume measurements rather than graduated cylinders when accuracy is critical.
Can I use this calculator for any type of solution?
This calculator works for any solution where you know the molecular weight of the solute. It’s particularly useful for preparing solutions of salts, acids, bases, and organic compounds in research and analytical work.
References
Atkins, P., & de Paula, J. (2018). Atkins’ Physical Chemistry (11th ed.). Oxford University Press. Chapter 5: Simple Mixtures.
Zumdahl, S. S., & Zumdahl, S. A. (2016). Chemistry: An Atoms First Approach (2nd ed.). Cengage Learning. Chapter 4: Solution Stoichiometry.
Harris, D. C. (2015). Quantitative Chemical Analysis (9th ed.). W. H. Freeman and Company. Chapter 7: Activity and the Systematic Treatment of Equilibrium.
Skoog, D. A., West, D. M., Holler, F. J., & Crouch, S. R. (2013). Fundamentals of Analytical Chemistry (9th ed.). Brooks/Cole. Chapter 3: Stoichiometric Calculations.
