How to Make a Stock Solution

A stock solution is a concentrated solution you prepare once and dilute later. Making one comes down to a single relationship — convert your target concentration and volume into a mass to weigh (or a liquid volume to measure), dissolve it, and bring the solution to the exact final volume. This guide gives the formula, the bench technique that most people get wrong, the corrections for hydrates and purity, the liquid-solute path, and ready recipes for the stocks you make most often. The stock solution calculator computes the mass to weigh, with an optional purity correction, and the molarity calculator handles the concentration math in every direction.

What is a stock solution and why make one?

A stock is a concentrated master solution — 1 M Tris, 5 M NaCl, 10% SDS — stored for repeated use. You make one because weighing a small amount accurately for every experiment is slow and error-prone, whereas weighing once into a concentrated stock and diluting an aliquot each time is faster and more reproducible. The two routes are: from a solid (weigh it, dissolve, bring to volume) or from a concentrated liquid (measure a volume, dilute to volume).

How do I calculate the mass to weigh?

For a molar stock, the mass comes from the molarity equation rearranged for mass:

mass (g) = MW × M × V

where MW is the molecular weight (g/mol), M is the target molarity (mol/L), and V is the final volume (L). To make 500 mL of a 1 M NaCl stock (MW 58.44):

• V = 0.500 L, M = 1 mol/L
• mass = 58.44 × 1 × 0.500 = 29.22 g

If you do not yet know the MW, work it out from the formula with the molar mass calculator first. For a % w/v stock the math is even simpler — see the section below.

The 80% rule: how to actually prepare it

The technique matters as much as the arithmetic, and this is where most stocks go wrong:

1. Weigh the calculated mass into a clean vessel.
2. Add solvent to about 80% of the final volume and stir until fully dissolved.
3. Transfer to a volumetric flask (rinsing the vessel in), then add solvent up to the calibration mark.
4. Stopper and invert to mix.

You dissolve in less than the final volume because the solute itself occupies space and dissolution can change the volume — adding 500 mL of water to 29.22 g of salt gives slightly more than 500 mL of solution, and the true molarity comes out lower than 1 M. Bringing to the mark on the finished solution is what makes the concentration exact. Dissolution can also be endothermic or exothermic; let the solution reach room temperature before final volume adjustment, since volume — and therefore molarity — is temperature-dependent.

How do I make a % w/v stock?

A percent weight/volume stock is grams of solute per 100 mL of final solution. The mass to weigh is:

mass (g) = (% w/v ÷ 100) × final volume (mL)

For 250 mL of a 10% w/v SDS solution:

• mass = (10 / 100) × 250 = 25 g
• Dissolve in ~200 mL of water, then bring to 250 mL.

Note that 10% w/v equals 100 g/L, which is also 100 mg/mL. The percent solution calculator handles % w/v, % v/v and % w/w and reports the g/L equivalent, and the percent solution guide explains when each convention applies.

Hydrate and purity corrections

Two corrections catch most real-world errors.

Hydrates. Many salts are supplied as hydrates, and the water of hydration counts toward the molecular weight. CuSO4 is 159.6 g/mol but CuSO4·5H2O is 249.7 g/mol. Use the MW of the form you actually have on the bottle, or you will weigh too little. The molar mass guide shows how to include hydrate waters.

Purity. If a reagent is not 100% pure, divide the calculated mass by the purity fraction:

mass to weigh = calculated mass ÷ purity

For a 95%-pure solid needing 29.22 g of actual solute: 29.22 / 0.95 = 30.76 g. The stock solution calculator applies this purity correction directly.

How do I make a stock from a concentrated liquid?

When the solute is a concentrated liquid — 37% HCl, glacial acetic acid, a 10x buffer — you measure a volume rather than weigh a mass. For a labelled concentrate the dilution equation C1V1 = C2V2 gives the volume to take:

V1 = (C2 × V2) / C1

To make 1 L of 1x from a 10x stock: V1 = (1 × 1000) / 10 = 100 mL of concentrate, brought to 1 L. For a liquid labelled in % and density (such as 37% w/w HCl, density ≈ 1.19 g/mL), convert the percent and density to a molar concentration first, then apply the same equation. The dilution calculator and C1V1 guide cover this in full.

Recipes for common stock solutions

Ready figures for 1 L of the stocks you make most (using the anhydrous MW; check your bottle’s form):

Stock	MW / basis	For 1 L
1 M Tris base	121.14 g/mol	121.14 g
5 M NaCl	58.44 g/mol	292.2 g
1 M NaCl	58.44 g/mol	58.44 g
0.5 M EDTA (Na2)	372.24 g/mol	186.12 g
10% w/v SDS	% w/v	100 g
1 M glucose	180.16 g/mol	180.16 g

Scale linearly for other volumes: 500 mL of 1 M Tris is 60.57 g. If your weighed mass is far from these magnitudes for the target molarity, re-check the MW (hydrate?) and the volume unit before dissolving.

Common mistakes to avoid

• Adding solvent to the full volume first. Always dissolve in ~80%, then bring to the mark.
• Using the wrong MW for a hydrate. Anhydrous and hydrated forms differ substantially.
• Ignoring purity. A 95% reagent needs about 5% more mass.
• mg entered as g, or mL as L. A factor-of-1000 slip; check the unit before weighing.
• Not letting the solution reach room temperature. Volume drifts with temperature, so molarity does too.

If a calculated mass seems absurdly large or small for the volume, suspect a unit error or a wrong molecular weight before you weigh anything.

Where does this fit in the wider workflow?

Making the stock is the first half; diluting it to a working strength is the second, covered in how to prepare a working solution and how to calculate a dilution. The concentration concept behind all of it is in what is molarity, and every tool here lives in the laboratory calculators collection.