Molarity & Solution Preparation
Relate moles of solute, solution volume, and molar concentration (mol/L). Weigh the calculated mass, dissolve it, then bring the solution to the final volume.
Result
Sodium chloride (NaCl) · Anhydrous
0.5844 g
Mass of the selected form to weigh.
- Weigh 0.5844 g of the solute.
- Dissolve it in less than the final volume of solvent.
- Add solvent until the solution reaches the required final volume.
Show full instructions
mass (g) = concentration (mol/L) × final volume (L) × molar mass (g/mol)
mass (g) = 0.1 mol/L × 0.1 L × 58.44 g/mol = 0.5844 g
- Convert 100 mM to 0.1 mol/L.
- Convert the final volume, 100 mL, to 0.1 L.
- mass (g) = 0.1 mol/L × 0.1 L × 58.44 g/mol = 0.5844 g.
- Weigh 0.5844 g of the chemical.
- Dissolve that mass in a suitable solvent, using less solvent than the final volume.
- After it dissolves, add solvent until the solution reaches the final volume of 100 mL. Do not add that full volume of solvent to the dry chemical.
- Mix. The final volume, not the solvent volume, is what the calculation uses.
- This assumes the solute can dissolve and that the solution is brought to the stated final volume. It does not check solubility or chemical hazard.
- No purity correction is applied. An impure or hydrated sample can change the required mass.
Use eye protection, gloves, and a fume hood when the substance requires them. These calculations do not describe hazards, compatibility, or whether the solute will dissolve.
Worked examples
Example 1
100 mL of 100 mM sodium chloride
Sodium chloride is anhydrous NaCl at 58.44 g/mol. Mass = 0.100 mol/L × 0.100 L × 58.44 g/mol = 0.5844 g. Weigh that mass, dissolve it in less than 100 mL of water, then bring the solution to 100 mL.
Example 2
Dilute 1 M stock to 100 mM
For 100 mL of 100 mM solution, transfer 10 mL of 1 M stock and add solvent until the total volume is 100 mL (90 mL of solvent). V2 is the final volume, not the solvent volume.
Example 3
1 L of 1 M NaCl at 98% purity
The pure-salt mass is 58.44 g. Dividing by 0.98 gives 59.6327 g of the 98% material. The extra mass is treated as inert; the correction does not detect a hydrate.
Molar masses are calculated from conventional IUPAC atomic weights and rounded to 0.01 g/mol. They are not a live database lookup. Use a custom molar mass for a supplier assay or a more precise value.
How the calculations work
Molarity is moles of solute per liter of solution. The primary tab converts the requested concentration and final volume into moles, then multiplies by the molar mass of the selected form. Dilution uses C1 × V1 = C2 × V2. If the target is stronger than the stock, or the stock volume would exceed the final volume, the result is rejected.
Mass concentration and molarity describe different quantities. One gram per liter equals one milligram per milliliter. Dividing grams per liter by the molar mass in g/mol gives mol/L. The library distinguishes hydrates, anhydrous salts, and different substances that share a formula, including the common C6H12O6 sugars.
Common questions
How is the required mass calculated?
Mass in grams equals concentration in mol/L, times final volume in liters, times molar mass in g/mol. A 100 mM solution is 0.100 mol/L, and 100 mL is 0.100 L.
Why should I not add the full volume of solvent to the dry chemical?
The solid and the dissolved solute occupy volume. The concentration is defined by the final solution volume. Dissolve the weighed mass in a smaller portion of solvent, then add solvent until the solution reaches the mark.
What is the difference between an anhydrous salt and a hydrate?
A hydrate includes water in the crystal, so its molar mass is larger. Copper(II) sulfate and copper(II) sulfate pentahydrate are different entries. Using the wrong form changes the number of moles you actually weigh.
What does a purity adjustment assume?
The stated percent is treated as the active fraction, and the remainder is treated as inert mass. It does not identify an unknown hydrate, and it does not convert a weight-percent liquid that also needs density.
Where do the molar masses come from?
Molar masses are calculated from conventional IUPAC atomic weights and rounded to 0.01 g/mol. They are not a live database lookup. Use a custom molar mass for a supplier assay or a more precise value. The library contains 424 verified entries. Substances that are not listed can be entered with a manual molar mass.