⚖️ Molar Mass and Solution Preparation

Written and maintained by the PhDino author · Last reviewed 21 September 2026 · Checked against 1 independent reference calculation · how PhDino checks its numbers

Converting a target molarity and volume into the actual mass of solid to weigh out.

Preparing a solution from a solid compound starts from the same M₁V₁-style relationship as dilution, but in reverse: instead of starting from an existing solution, you're starting from moles you need to dissolve. The concentration you want, times the volume you want, gives the moles of solute required — and multiplying moles by the compound's molar mass converts that into the number that actually matters at the balance: grams.

Molar mass itself is just the sum of a compound's atomic masses (from the periodic table) in the ratio its formula specifies — a fixed, calculable property of the compound, not something that depends on how much of it you're using.

Key formula

Moles needed = target concentration × target volume
Mass to weigh out = moles needed × molar mass

Notes & limitations

• The standard lab procedure is to weigh the solid, dissolve it in less than the final volume, then add water up to the calibration mark of a volumetric flask — not to dissolve it directly in the full target volume, since the solid itself displaces some volume as it dissolves.
• This assumes the compound is anhydrous (contains no water of crystallization) — many common lab solids are hydrates (e.g. copper sulfate pentahydrate) whose molar mass includes the water, and using the anhydrous molar mass for a hydrate salt gives a solution that's too dilute.