Solution Preparation Calculator
Solute grams from molarity, volume, and molar mass.
What it does
Making a solution of exact molarity is a two step calculation. First you find how many moles the final volume must hold, then you convert those moles to grams with the molar mass. The tool above does both steps at once: type molarity, final volume, and molar mass, and it prints the grams to weigh, the moles, and the volume in liters.
Molarity means moles per liter of finished solution. That last phrase matters. You do not add 250 mL of water to the powder; you dissolve the powder in part of the water and then add water until the total reaches 250 mL. Volumetric flasks exist for this reason, and graduated cylinders work for routine work where the last drop of accuracy is not needed.
The worked example uses table salt, NaCl, at 0.5 M and 250 mL. The math gives 0.125 mol and 7.305 g, a comfortable amount for a common lab balance. Smaller targets, such as a few milliliters of a millimolar solution, can call for single milligrams, which is where beginners go wrong. In that case make ten times more solution or prepare a concentrated stock and dilute it.
This page covers simple molarity from a solid. It does not cover dilutions from a liquid stock, percent solutions, or normality for acids with more than one proton. For those jobs use a dilution calculator with C1V1 = C2V2, or convert percent to molarity first with the molar mass.
How to use
- Enter the target molarity in moles per liter.
- Enter the final solution volume in mL.
- Enter the molar mass of the exact solid, with hydration water included.
- Read the solute mass in grams, the moles, and the volume in liters.
- Weigh the mass, dissolve in about two thirds of the water, then fill to the mark.
- Label with name, concentration, date, and preparer before storage.
Formula
- M
- Target concentration in moles per liter, 0.0001 to 20.
- V
- Final solution volume in mL, converted to liters by dividing by 1000.
- MW
- Mass of one mole of the exact solid weighed, in grams per mole.
Moles come first, then grams. At rate zero there is no special case here, but very small masses need a larger batch or a stock solution because ordinary scales cannot weigh a few milligrams well.
Worked example
Example: 0.5 M NaCl, 250 mL
Defaults: 0.5 M NaCl, 250 mL, molar mass 58.44. Lab glassware is metric only, so no imperial mode is needed.
- Inputs: 0.5 mol per L, 250 mL final volume, molar mass 58.44 g per mol (NaCl).
- Convert volume: 250 mL is 0.25 L.
- Moles needed: 0.5 times 0.25 is 0.125 mol.
- Mass needed: 0.125 times 58.44 is 7.305 g.
- Weigh 7.305 g NaCl, dissolve in part of the water, then add water to 0.25 L. Tool shows 7.305 g, 0.125 mol, 0.25 L.
Reference table
| Target | Math | Mass |
|---|---|---|
| 0.5 M, 0.25 L, 58.44 | 0.125 mol | 7.305 g |
| 1 M, 0.5 L, 40 | 0.5 mol | 20 g |
| 0.1 M, 1 L, 58.44 | 0.1 mol | 5.844 g |
| 2 M, 0.1 L, 100 | 0.2 mol | 20 g |
| 0.25 M, 2 L, 58.44 | 0.5 mol | 29.22 g |
Limits
- Single solute in one solvent. Mixtures need one line per solute.
- Dilute aqueous solutions near room temperature. Density shifts matter when concentrated.
- Accuracy depends on the balance and the volumetric glass, not just the math.
- Hydrates, purity, and assay are on you. Technical grade needs a purity correction.
Common mistakes
- Adding the full water volume instead of filling to the mark.
- Using anhydrous mass for a hydrated salt, or the reverse.
- Mixing mL with liters and missing a factor of 1000.
- Trusting milligram results from a kitchen scale.
- Skipping the label, then guessing the bottle a week later.
Try it yourself
Quick checks. Answers sit tucked under each question.
1 M NaOH, 500 mL, molar mass 40?
Moles are 1 times 0.5 is 0.5 mol. Mass is 0.5 times 40 is 20 g. Tool shows 20 g, 0.5 mol, 0.5 L.
0.1 M NaCl, 1000 mL, molar mass 58.44?
Moles are 0.1 times 1 is 0.1 mol. Mass is 0.1 times 58.44 is 5.844 g. Tool shows 5.844 g, 0.1 mol, 1 L.
Questions
How do I find the mass for a target molarity?
Multiply molarity by volume in liters to get moles, then multiply by molar mass. Here 0.5 times 0.25 times 58.44 is 7.305 g.
Do I add 250 mL of water or fill to 250 mL?
Molarity is moles per liter of final solution, not per liter of water added. Dissolve first, then fill to the mark in a volumetric flask.
Which molar mass do I use for hydrated salts?
Use the molar mass of the exact form you weigh, including waters of hydration. Anhydrous and hydrated salts differ a lot.
Can I use mL directly without converting to liters?
Yes for most dilute aqueous work at room temperature. For very concentrated or nonaqueous solutions, check density tables.
Is 1% the same as 0.1 M?
No. Percent means grams per 100 mL, molarity means moles per liter. Convert with molar mass before comparing.
What if the result is only a few milligrams?
Weighing error dominates below about 10 mg on hobby scales. Make a larger batch or use a stock solution and dilute.