Distinguish concentration, amount and volume
Molar concentration expresses the amount of dissolved substance per litre of solution. A solution of 0.20 mol/L contains 0.20 mol in each litre of that solution. A smaller sample contains less substance while retaining the same concentration.
In a virtual investigation, record both concentration and sample volume. A value of “25” is incomplete: 25 mL and 25 L describe very different samples. Before calculating moles, convert volume to litres.
Dilution: keep the amount of solute unchanged
For a dilution in which no solute is lost or reacts, the amount of dissolved substance stays the same as the final solution volume increases. This gives the relationship below.
C₁V₁ = C₂V₂
Consider 25 mL of a 0.20 mol/L solution diluted to a final volume of 100 mL. Use the same volume unit on both sides of the dilution equation.
C₂ = 0.20 × 25 ÷ 100 = 0.050 mol/L
| Sample volume | Final solution volume | Final concentration |
|---|---|---|
| 25 mL | 50 mL | 0.10 mol/L |
| 25 mL | 100 mL | 0.050 mol/L |
| 25 mL | 200 mL | 0.025 mol/L |
The phrase “to 100 mL” means the total solution volume is 100 mL. It does not mean adding 100 mL of solvent to the sample. This distinction is a common source of incorrect calculations.
Titration: use the reaction ratio before an equation shortcut
At equivalence, reactants have combined in the amounts required by the balanced reaction. For an ideal 1:1 acid–base reaction, the acid and base amounts are equal. Other reaction ratios require a different calculation.
Suppose an acid sample of 20.0 mL requires 25.0 mL of a 0.100 mol/L base at equivalence. The base amount is 0.100 × 0.0250 = 0.00250 mol. For this 1:1 example, the acid sample contains the same amount.
Acid concentration = 0.00250 mol ÷ 0.0200 L = 0.125 mol/L
For a burette reading, the delivered volume is the final reading minus the initial reading. Keep the two readings in your record rather than recording only a final volume.
An indicator’s endpoint is an observed signal; equivalence is the stoichiometric point. They are related but not identical concepts. The pH at equivalence also depends on the acid–base system, so it should not always be assumed to be 7.
Review your result before saving it
Check the units, the balanced reaction ratio, the distinction between added and total volume, and whether the result is theoretical or measured. A repeatable investigation explains how each value was obtained.
Check your understanding: does adding solvent add more solute?
In a simple dilution with no reaction or loss, it does not. The same amount of solute is distributed through a larger solution volume, so concentration decreases.
NEUYI Science’s foundational chemistry models include strong acid–base titration, dilution, ideal gas laws and calorimetry. Use the available guided experiment or independent bench to explore supported variables, then keep your observations with their units and model assumptions.
References and further reading
These calculations describe ideal educational examples. They are not instructions for handling chemicals; physical chemistry practicals require appropriate supervision and laboratory procedures.