Enrichment · Passion Projects · Chemistry

Grams on the balance,
moles in the flask.

Choose the solute and fill in any two of molarity, volume and mass. The calculator works out the third, allows for purity, and tells you how to make the solution up.

Make a solutionWhat you get

The molarity calculator

The worked example is 250 mL of 0.5 M saline from 99.5% sodium chloride. Nothing you type leaves this browser.

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What you get

Enrichment · Passion ProjectsFree

What you finish with
The missing quantity (grams to weigh, molarity or volume), the moles and grams per litre, how to make the solution, and grams for other flask sizes. It prints as a summary and downloads as a spreadsheet.
Who it is for
Chemistry and biology students, teachers preparing lab solutions, and science fair students.
Format
Browser calculator for a phone, tablet or computer, by touch or keyboard. Prints a summary; downloads a spreadsheet (.csv); export and import a file.
Access
Free. No account, no sign-in and nothing to install. Everything you type stays in this browser; nothing is sent to Caveat.
Where it lives
The Passion Projects space in Caveat Enrichment.
Support
Questions, a correction or a request: team@caveatacademic.com.

How to use it

  1. Choose the solute, or Other and type its molar mass.
  2. Set the purity from the label.
  3. Fill in two of molarity, volume and mass, leaving the one you want blank.
  4. Read the answer and the steps to make the solution.
  5. Print the summary for the lab bench.

Make a solution

How the solution is worked out

Moles connect grams to litres.

The worked example: 0.5 M salineMoles needed0.1250 mol0.5 mol/L × 0.250 L× molar mass7.305 g0.125 × 58.44 g/mol÷ purity7.342 g÷ 0.995Make up to250 mLin a volumetric flaskThe worked example: 0.5 M salineMoles needed0.1250 mol0.5 mol/L × 0.250 L× molar mass7.305 g0.125 × 58.44 g/mol÷ purity7.342 g÷ 0.995Make up to250 mLin a volumetric flask
Molarity counts particles, not grams: moles per litre of finished solution. The molar mass turns moles into something a balance can weigh.

The arithmetic

Molarity is moles of solute per litre of finished solution. Moles = molarity × litres; grams = moles × molar mass. A reagent that is 99.5% pure needs its mass divided by 0.995. Solving the other way, molarity = (grams × purity ÷ molar mass) ÷ litres. The molar mass of a hydrate includes its water, so copper(II) sulfate pentahydrate is 249.69 g/mol, not 159.61.

Making it up

Dissolve the solid in less water than the final volume, then add water to the mark of a volumetric flask, because a dissolved solid takes up space of its own. Label the bottle with the name, molarity, date and your initials. Sodium hydroxide releases heat and is corrosive; wear goggles and gloves.

Sources

  1. IUPAC. Compendium of Chemical Terminology (the Gold Book): amount concentration (molarity) (goldbook.iupac.org).
  2. IUPAC Commission on Isotopic Abundances and Atomic Weights. Standard atomic weights (ciaaw.org).
  3. National Institute of Standards and Technology. NIST Chemistry WebBook: molecular weights of the listed compounds (webbook.nist.gov).
  4. American Chemical Society. Guidelines for Chemical Laboratory Safety in Secondary Schools (acs.org).

The worked example is made up. Follow your school's lab safety rules. Your figures stay in this browser. Support: team@caveatacademic.com.