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General Chemistry I · Aqueous Reactions

Molarity

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On this page 7 sections
  1. In 30 seconds
  2. Why this matters
  3. The college version
  4. Eli explains
  5. Key takeaway
  6. Study tools
  7. Sources & references

In 30 seconds

Molarity (M) is the most common way to express solution concentration: the number of moles of solute dissolved in one liter of solution. M = moles of solute / liters of solution. The crucial subtlety is that the volume in the denominator is the final volume of the solution, not the volume of the solvent — dissolving a solute changes the total volume. Molarity lets chemists count solute particles by measuring a liquid volume, which is why it drives nearly all solution stoichiometry.

Why this matters

Molarity is the language of the lab and the clinic. IV fluids are specified in molarity (and related units), reagents are bought and diluted by molarity, and every titration and kinetic study depends on knowing moles per liter precisely. Confusing solution volume with solvent volume is one of the most common — and most costly — preparation errors.

The college version

Key Ideas

The definition

  • Molarity = moles of solute per liter of solution, M = mol/L.
  • Symbol M is read "molar"; 0.500 M means 0.500 mol of solute in 1 L of solution.

Solution vs. solvent volume

  • Solution volume = solvent + dissolved solute (final measured volume).
  • Because solutes occupy space, adding solute to a solvent changes the total volume — so you fill to the mark after dissolving.

Preparing a solution

  • Weigh the solute, dissolve it in less than the final volume of solvent, then dilute to the exact mark in a volumetric flask.

Equations and Variables

  • Molarity: M = n / V, where n = moles of solute, V = liters of solution.
  • Moles from M and V: n = M × V.
  • Volume from M and n: V = n / M.
  • Mass needed: m = M × V × (molar mass).

How It Works (Problem-Solving Method)

  1. Identify the two known quantities among M, n, and V.
  2. Convert volume to liters if given in mL (divide by 1000).
  3. Apply the rearranged equation (n = M × V, M = n / V, or V = n / M).
  4. If preparing from solid, convert moles to grams with molar mass.
  5. Report with correct units and significant figures.

Worked Example

What is the molarity of a solution made by dissolving 0.500 mol of NaCl in enough water to make 2.00 L of solution?

  • M = n / V = 0.500 mol / 2.00 L = 0.250 M NaCl.

Mass-to-prepare version: How many grams of NaOH (molar mass 40.00 g/mol) are needed to prepare 250.0 mL of 0.100 M NaOH?

  • V = 0.2500 L. n = M × V = 0.100 mol/L × 0.2500 L = 0.0250 mol.
  • m = 0.0250 mol × 40.00 g/mol = 1.00 g NaOH (dissolve, then dilute to 250.0 mL total).

Common Confusions

  • "Volume means volume of solvent (water)." — It means volume of solution. Adding solute changes total volume, so the two differ.
  • "0.500 M means 0.500 mol in 1 L of water." — It means 0.500 mol in 1 L of solution (water plus solute).
  • "Molarity and moles are interchangeable." — Molarity is a concentration (mol/L); moles is an amount. You need a volume to relate them.
  • "Doubling the volume halves the moles." — Doubling the volume by dilution halves the concentration, not the moles (moles are conserved).
Eli, the EliExplains learning guide

Eli explains

The same idea, in plain words

Explain it like I’m 10

Molarity is like a recipe's "strength": how many spoonfuls of drink mix per cup of finished drink. If you stir 2 scoops into water and end up with 1 cup of drink, that's "2 scoops per cup." The key trick: the cup is measured after you've added the powder — the powder takes up a little space too, so you can't just measure the water first and ignore the powder. The catch: chemists count in moles and measure in liters, but the idea — amount of stuff per volume of finished solution — is the same.

Key takeaways

  • M = moles of solute / liters of solution.
  • Volume is the final solution volume, not solvent volume.
  • n = M × V; M = n / V; V = n / M.
  • mL → L: divide by 1000.
  • Preparing a solution: dissolve solute, then dilute to the mark.
  • Molarity M = mol solute / L solution.
  • Solution volume ≠ solvent volume.
  • n = M × V; rearrange for M or V.
  • Dissolve solute, then dilute to the final mark.

Keep learning

Ready to build on this? Continue to the next lesson.

Practice General Chemistry I

This lesson has no separate scored set. Practice draws from the subject’s question bank.

Study tools & related lessonsYou’ll learn to · Related

You’ll learn to

  • Define molarity (M) and state its units (mol/L).
  • Distinguish moles of solute, liters of solution (not solvent), and molarity.
  • Calculate molarity from moles and volume, and moles or volume from molarity.
  • Prepare a solution of a given molarity from a solid solute.

Sources & references

  1. OpenStax, "3.3 Molarity." *Chemistry 2e*.
  2. Chemistry LibreTexts, "13.6: Specifying Solution Concentration — Molarity."
  3. OpenStax, "3.4 Other Units for Solution Concentrations." *Chemistry 2e*.

This lesson was adapted from the open educational references above; their licenses and attributions are preserved. See Copyright & Licensing.

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