You need 250 mL of 0.5 M sodium chloride solution for an experiment. Where do you start? Making molar solutions is one of the most common chemistry-lab tasks, and the procedure is the same regardless of the compound. Here's how to do it correctly.

The basic math

Molarity (M) = moles of solute / liters of solution.

To prepare a solution of known molarity:

  1. Calculate moles needed: M × V (in liters).
  2. Calculate mass needed: moles × molar mass.
  3. Weigh out that mass of solute.
  4. Dissolve in less water than the target volume.
  5. Add water to reach exact target volume.

Worked example: 250 mL of 0.5 M NaCl

Step 1: moles needed.

  • 0.5 M × 0.250 L = 0.125 mol

Step 2: mass needed.

  • NaCl molar mass = 58.44 g/mol
  • 0.125 mol × 58.44 g/mol = 7.305 g

Step 3: procedure.

  1. Weigh 7.305 g of NaCl on a balance (precision ±0.001 g).
  2. Transfer to a 250 mL volumetric flask.
  3. Add ~150 mL distilled water; swirl to dissolve.
  4. Add water to the calibration mark on the flask.
  5. Cap and invert several times to mix.

Required equipment

  • Analytical balance: 0.001 g precision for accurate moles.
  • Volumetric flask: graduated to a single accurate volume (25 mL, 50, 100, 250, 500, 1000 mL). Most accurate volume measurement.
  • Distilled or deionized water: tap water has dissolved minerals that affect concentration.
  • Stir bar or wash bottle for transfer.
  • Weighing paper or boat: for transferring solid to flask.

Common mistakes

Adding water first. If you fill the flask with water and then add the solute, you'll overflow the calibration mark. Always add solute to less water, then dilute to the mark.

Using a graduated cylinder for the final volume. Graduated cylinders are ±5% accuracy. Volumetric flasks are ±0.1%. For molar solutions, use volumetric flasks.

Not letting the solute dissolve before final dilution. Some compounds are slow to dissolve. Add the solute to water, swirl until fully dissolved, then dilute to the mark.

Reading the meniscus wrong. The volume mark is at the bottom of the meniscus (the curved water surface). Read at eye level.

Using the wrong molar mass. Molar mass depends on the exact compound. NaCl is 58.44; NaCl·H₂O (monohydrate) would be 76.46 because of the water of hydration.

Hydrated compounds

Many crystals come with water built into the structure (hydrates). The molar mass includes this water.

  • CuSO₄ (anhydrous): 159.6 g/mol
  • CuSO₄·5H₂O (pentahydrate): 249.6 g/mol

If the bottle says "CuSO₄·5H₂O," you must use 249.6 as the molar mass — even though only the CuSO₄ matters for the chemistry.

Acids and bases

Concentrated acids (HCl, H₂SO₄) are sold as percent solutions, not solid. The procedure is different:

  1. Calculate volume needed: target moles × molar mass / (concentration × density).
  2. Add water to volumetric flask first (about 1/3 to 1/2 capacity).
  3. Slowly add the calculated volume of concentrated acid (always acid to water — adding water to concentrated acid causes splashing).
  4. Cool if necessary; dilution releases heat.
  5. Dilute to mark.

Common: making 1 L of 1 M HCl from concentrated 12 M HCl: V = (1 × 1) / 12 = 0.0833 L = 83 mL. Add 83 mL conc. HCl to ~500 mL water, then dilute to 1 L.

Solubility limits

Some compounds can't reach the molarity you want — they max out (saturated solution).

Examples:

  • NaCl in water: max ~6 M (saturated).
  • Sugar (sucrose): max ~6 M.
  • Potassium bromide: max ~4 M.

If your target molarity exceeds saturation, you can't make it as a true solution. Some falls out as solid.

Temperature effects

Molarity depends on volume, which depends on temperature (volumes expand with heat). For precise work:

  • Volumetric flasks are calibrated at 20°C (or sometimes 25°C).
  • Make solutions at room temperature.
  • For long-term storage, account for temperature variation.

Most casual lab work doesn't need this precision; for analytical chemistry, it matters.

Storage and labeling

Label every solution with:

  • Compound and concentration (e.g., "0.5 M NaCl").
  • Date prepared.
  • Initials of preparer.
  • Special hazards if applicable.

Most aqueous solutions are stable for weeks if refrigerated. Some (light-sensitive, oxidizable) need amber bottles or refrigeration.

Acidic and basic solutions can attack glassware over time; store in plastic where appropriate.

Diluting from a stock solution

Often you make a concentrated "stock" solution and dilute as needed:

Dilution equation: M₁V₁ = M₂V₂

Where M₁ and V₁ are the stock concentration and volume; M₂ and V₂ are the target.

To make 100 mL of 0.1 M NaCl from 1 M stock: V₁ = (0.1 × 100) / 1 = 10 mL. Take 10 mL of stock, dilute to 100 mL with water.

Calculate amounts

Our molarity calculator handles the math: enter desired moles, volume, and (optional) molar mass to get required mass. Useful for quick lab calculations or sanity-checking solution prep.