Solutions and dilutions
Weigh for a molar solution, dilute a stock, or make 1 M acid from the concentrated bottle, with the steps written out for your notebook.
Make a solution
Recipe
- Weigh 42.28 g of squalane.
- Dissolve it in about 70 mL of water in a 100 mL volumetric flask.
- Make up to the mark with water and mix.
- To weigh
- 42.28 g
- Amount
- 100 mmol
- Molarity
- 1.00 M
- Mass per litre
- 423 g/L
How the amounts are worked out
From a solid, the mass is concentration × volume × molar mass, divided by the purity. Choose the hydrate you actually have: sodium thiosulfate pentahydrate weighs 248.18 g/mol, the anhydrous salt 158.11 g/mol.
A dilution keeps the amount of solute: C₁V₁ = C₂V₂, so the stock volume is the final volume times the ratio of the two concentrations.
A concentrated reagent's molarity is its strength by mass times its density divided by the molar mass: 37 % hydrochloric acid at 1.184 g/mL is 0.37 × 1184 / 36.46 = 12.0 M.
Why the recipe says “make up to the mark”
Volumes do not simply add: a litre of 96 % sulfuric acid and a litre of water give less than two litres, and hot. So the recipe dissolves or adds first, lets the solution reach room temperature, and only then brings it to the final volume. The solvent volume shown for dilutions is a guide for dilute stocks only.
Where the reagent data come from
Strengths are the usual commercial grades. Densities at 20 °C are from the CRC Handbook of Chemistry and Physics, section “Concentrative properties of aqueous solutions”. Molar masses are calculated from IUPAC standard atomic weights.