Prepare a Lab Solution from Scratch — Project Guide

🧑‍🔧 Prepare a Lab Solution from Scratch

Field: Chemistry

Written and maintained by the PhDino author · Last reviewed 21 September 2026 · Every calculator used here is tested against independent reference values · how PhDino checks its numbers

Weigh out a compound to make a concentrated stock, then dilute it down (in one stage or two) to the working concentration you actually need.

A common way to prepare a solution isn't to weigh out exactly the amount needed for the final working concentration. It's to make a more concentrated stock first (easier to weigh accurately, and useful for more than one dilution later), then dilute that stock down to whatever working concentration a specific step actually calls for.

This guide walks through both halves of that: how much solid to weigh out for the stock, and then how much of that stock (plus water) gets you to the final concentration you need, including when to dilute in two stages instead of one.

🧮 Molar Mass & Solution Prep 🧮 Molarity / Dilution

What molarity means, and why you make a stock

Molarity is moles of solute per liter of solution, not per liter of water. That distinction is why a solution is made by dissolving the solid in part of the water and then filling to a final volume mark, rather than by adding the solid to a measured liter of water: the dissolved solid takes up room, so the two procedures give different concentrations.

A stock solution is a concentrated solution you make once and dilute many times. It saves time, and it improves accuracy. Weighing twelve grams on a balance that reads to a milligram is a tiny relative error, while weighing a few hundredths of a gram of the same salt for a dilute solution is a much larger one, and pipetting a few milliliters of stock is easier to do well than weighing crumbs.

What to know before you weigh anything

Most concentration errors are decided before the balance is switched on.

The exact formula of what is in the bottle. Many salts are sold as hydrates, with water built into the crystal, and the label says so in the formula.
The purity of the reagent. A 98 percent reagent contains 2 percent something else, and for careful work you weigh a little more.
The final volume and the glassware that can measure it. A volumetric flask is made for filling to a mark; a beaker or cylinder is not.
The safety data sheet for the compound, which lists the protective equipment, the hazards and how to dispose of it.

Steps

1⚖️Molar Mass & Solution Prep
Decide the concentration and volume of the stock solution you want to make, and enter the compound's molar mass (from its formula, or a reference table). Use the molar mass of what is actually in the bottle, including any water of hydration. This returns the moles needed and the mass to weigh out on the balance. Hit "📋 Copy Values" once you have a result: the concentration you just made becomes the "stock" the next step dilutes from.
🧮 Open the Molar Mass & Solution Prep calculator
2🧫Molarity / Dilution
"📥 Paste Values" fills in Stock Concentration with the concentration you just made in Step 1. Enter the working concentration and the final volume you actually need, and this returns how much of your stock to combine with how much water. If the stock volume comes out very small (a millilitre or less), run this step twice: dilute the stock to an intermediate concentration first, then dilute that.
🧮 Open the Molarity / Dilution calculator

Worked example: a copper sulfate stock, diluted in two stages

A teaching lab needs 100 mL of 0.005 M copper(II) sulfate for a colorimetric experiment. The reagent bottle reads CuSO₄·5H₂O, the blue pentahydrate. The plan is to make 100 mL of a 0.5 M stock and then dilute it in two stages.

Step 1: Weighing the stock

You enter	Value
Target Concentration	0.5 M
Target Volume	100 mL
Molar Mass	249.68 g/mol

The calculator returns	Value
Moles Needed	0.05 mol
Mass to Weigh Out	12.484 g

The molar mass has to be the one for what is in the bottle. The pentahydrate carries five waters with every CuSO₄: 159.61 g/mol for the CuSO₄ plus 5 × 18.015 for the water comes to 249.68 g/mol. The calculator's arithmetic is then plain: 0.5 mol/L × 100 mL ÷ 1,000 = 0.050 mol, and 0.050 mol × 249.68 g/mol = 12.484 g to weigh out.

Dissolve it in about three quarters of the final volume of water in a volumetric flask, swirl until it has all dissolved, and only then fill to the mark. The calculator's volume is the final volume of solution, so the water you start with is less than 100 mL.

The hydrate is the classic mistake. Had we used the anhydrous molar mass of 159.61 g/mol, the calculator would have said 7.98 g. Weighing that much of the pentahydrate gives 0.32 M rather than 0.5 M, a solution 36 percent too weak with nothing about it that looks wrong.

Step 2: First dilution: 0.5 M to 0.05 M

You enter	Value
Stock Concentration	0.5 M
Target Concentration	0.05 M
Target Volume	250 mL

The calculator returns	Value
Stock Solution Needed	25.00 mL
Water to Add	225.00 mL

The handoff carries the 0.5 M from step 1 in as the stock concentration. Dilution conserves moles, so C₁V₁ = C₂V₂ and V₁ = 0.05 × 250 ÷ 0.5 = 25 mL of stock, with the remaining 225 mL being water.

In practice you pipette the stock into a 250 mL volumetric flask and fill to the mark rather than measuring the water separately. In a dilute aqueous solution that is the same thing, and it avoids adding a second measurement error to the first.

Step 3: Second dilution: 0.05 M to 0.005 M

You enter	Value
Stock Concentration	0.05 M
Target Concentration	0.005 M
Target Volume	100 mL

The calculator returns	Value
Stock Solution Needed	10.00 mL
Water to Add	90.00 mL

A single dilution from the 0.5 M stock straight to 0.005 M would need only 1.0 mL of stock, and a pipette good to ±0.1 mL would carry a 10 percent error in that volume. Diluting in two stages, from the 0.05 M solution we just made, needs 10 mL, where the same ±0.1 mL is 1 percent.

The calculator makes each stage its own run, and the price is one extra flask. With 90 mL of water making up the rest, the working solution is done.

What it adds up to

Three runs turned one weighing of 12.48 g of the pentahydrate into a working solution 100 times more dilute than the stock: 25 mL of stock made the intermediate, and 10 mL of that made the final 100 mL. Most of the stock is left over for other concentrations, which was the point of making it.

Notes & limitations

If you only need one working concentration and no stock advantage, you can skip straight to Molar Mass & Solution Prep with your target concentration set directly. The two-step stock-then-dilute approach mainly pays off when you need several different working concentrations from the same compound.
Molar mass here must be the one for the form you are weighing. Hydrate salts are the common gotcha: see the worked example, where the mistake changes the concentration by more than a third.

Mistakes that change the concentration

Every one of these gives a solution that looks fine and is not the concentration on the label.

Using the anhydrous molar mass for a hydrate, or the reverse, as the example shows. Read the formula on the bottle every time.
Skipping the purity correction. For a 98 percent reagent the 12.48 g becomes 12.74 g if you want the true amount of compound.
Adding the solid to a full volume of water, or filling to the mark before everything has dissolved. Make up to the mark last.
Measuring small volumes in a cylinder. Use a volumetric pipette or a calibrated pipettor for the stock, and dilute in stages rather than measuring a tiny volume.
Filling to the mark while the solution is warm. Volumetric glassware is calibrated at a stated temperature, commonly 20 °C, so cool the solution first.
Not mixing. Concentrated and dilute layers of different density sit apart until the flask is inverted many times.
Trusting the bottle. Solids that absorb moisture, such as sodium hydroxide, are never exactly what the label says, and a solution of one needs standardizing against a known reference.

Safety, labeling and disposal

Read the safety data sheet before you handle a new compound, wear the protective equipment it asks for, and label every container with the compound, the concentration, the date and your initials, plus any hazard. An unlabeled flask of clear blue liquid is a risk to the next person.

For acids, add the concentrated acid slowly to the water, never the other way round. Solutions of heavy metals such as copper are toxic to aquatic life, so many jurisdictions and institutions do not allow them down the drain: follow your local waste rules.

Frequently asked questions

Is molarity per liter of water or per liter of solution?

Per liter of solution. That is why the procedure is to dissolve the solid in less than the final volume of water and then fill to the mark, so that the total volume, not the volume of water, is what you have controlled.

The dilution calculator says how much water to add. Should I measure that water?

Not separately. Pipette the stock into a volumetric flask and fill to the mark. For a dilute aqueous solution the two are the same, and filling to the mark keeps you from adding a second measurement error. For concentrated solutions, volumes do not add exactly, which is another reason to fill to the mark.

How accurate does my balance need to be?

The error in the mass should be small next to the mass itself. A laboratory balance good to 0.5 mg on 12.48 g is a 0.004 percent error, while a kitchen scale that reads to the nearest gram could be off by half a gram, 4 percent. For a teaching solution the first is comfortable and the second is not.

How long can I keep the stock?

It depends on the compound. Many stable inorganic salts keep for months in a sealed container, while some solutions grow mold, react with air or come out of solution over time. Label the date, look for cloudiness or crystals before using it, and follow the guidance for that specific compound.

Can I use the same steps if my reagent is a liquid?

Skip the weighing and start at the dilution step, using the concentration on the bottle's label as the stock concentration. Concentrated acids are typically labeled by percent and density rather than molarity, so convert first, and always add the acid to water.