dilutioncalculator.co.uk§ Verified 20 Jun 2026 · Reviewed by A. Hargreaves, MChem (Hons)

Molarity from a stock solution

Most lab dilutions go from a 1 M or 100 mM stock down to a working concentration. Method, worked example, hydrate trap.

§ 1Worked example

  1. Stock = 1 M Tris-HCl pH 7.4. Need 250 ml of 50 mM working buffer.
  2. C₁V₁=C₂V₂: V₁ = (0.05 × 250) ÷ 1 = 12.5 ml of stock.
  3. Add 12.5 ml of stock into a 250 ml volumetric flask. Top up to mark with deionised water.
  4. Mix by inversion. pH should not shift if the stock was correctly pH-adjusted.

§ 2Hydrate trap

Many lab salts are hydrates. CuSO₄·5H₂O is 249.7 g/mol; CuSO₄ anhydrous is 159.6 g/mol. A label that says "CuSO₄ 1 M" must be read in light of which form was weighed. SDS will name the form. If it does not, weigh fresh and verify by hand.[7]

§ 3When to weigh fresh instead

  • Stock concentration is below 1 mM · pipette error budget is too high after a single dilution.
  • The compound degrades in solution. Most cell-culture additives, many antibiotics.
  • You need a different counter-ion or pH than the stock provides.

§ 4Related