Molarity cheat sheet

Everything you need to compute molarity by hand, in one page. Bookmark it or paste it into your lab notebook.

The three equations

The core relation:

$M = \dfrac{n}{V}$

where $n$ is moles of solute and $V$ is the volume of the final solution (not the solvent) in liters. Moles are recovered from mass via:

$n = \dfrac{m}{MW}$

Combined, the working form of the molarity equation is:

$M = \dfrac{m}{MW \cdot V}$

Unit conversions

Volume ladder is the same shape:

Mass ladder:

Common lab reagent MWs

All MWs from PubChem (see sources at the bottom of this page). If a bottle label disagrees, trust the bottle — atomic weight ranges evolve slightly between IUPAC revisions.

Three most common mistakes

1. Solution volume ≠ solvent volume. Molarity is per liter of final solution. Weigh out solute, add water in a volumetric flask, and top up to the mark. Do not add solute to a pre-measured volume of water.
2. Hydrates. A “MW” from a bottle of CuSO₄·5H₂O is 249.68 g/mol, not 159.61 g/mol. The five waters count toward the mass you weigh.
3. Sig figs. 500 (no decimal) is ambiguous; 500. (with decimal) is three sig figs. Round only at the display, never during arithmetic.

Quick worked example

Prepare 250 mL of 100 mM Tris buffer (as free base).

• $M = 0.100\ \text{mol/L}$
• $MW_\text{Tris} = 121.14\ \text{g/mol}$
• $V = 0.250\ \text{L}$
• $m = M \cdot MW \cdot V = 0.100 \cdot 121.14 \cdot 0.250 = 3.028\ \text{g}$

Weigh out 3.03 g of Tris base, dissolve in ~200 mL water, adjust pH with HCl, then top up to 250 mL. Store at 4 °C.