Concentration in g/dm³
- Many reactions happen in solutions. The concentration of a solution can be measured in mass per given volume.
- Concentration (g/dm³) = mass of solute (g) ÷ volume of solution (dm³).
- 1 dm³ = 1000 cm³, so divide a volume in cm³ by 1000 to get dm³.
- Example: 10 g of salt dissolved to make 250 cm³ of solution: 10 ÷ 0.25 = 40 g/dm³.
- (Higher) Using more solute in the same volume, or the same solute in a smaller volume, gives a higher concentration.
Concentration in mol/dm³ (Higher, separate sciences)
- Concentration (mol/dm³) = moles of solute ÷ volume of solution (dm³).
- To convert mol/dm³ into g/dm³, multiply by the Mr of the solute.
- Example: 0.1 mol dissolved in 500 cm³ gives 0.1 ÷ 0.5 = 0.2 mol/dm³.
Titrations (separate sciences)
- A titration finds the exact volume of one solution needed to react with a known volume of another, for example an acid and an alkali.
- Use a pipette to put a measured volume (such as 25.0 cm³) of alkali in a conical flask, and add a few drops of indicator such as phenolphthalein or methyl orange.
- Add acid from a burette, swirling, until the indicator just changes colour (the end point). Add acid drop by drop near the end.
- Record the volume used (the titre). Repeat until you have concordant results (within 0.10 cm³ of each other) and calculate the mean.
Titration calculations (Higher, separate sciences)
- Find moles of the solution with known concentration: moles = concentration × volume (dm³).
- Use the equation to find moles of the other substance, then divide by its volume in dm³.
- Example: 20.0 cm³ of 0.10 mol/dm³ HCl neutralises 25.0 cm³ of NaOH. Moles of HCl = 0.10 × 0.0200 = 0.00200 mol, so moles of NaOH = 0.00200 mol and its concentration = 0.00200 ÷ 0.0250 = 0.080 mol/dm³.
Key terms
- Solute
- The substance that dissolves in a solvent.
- Solution
- A mixture formed when a solute dissolves in a solvent.
- Concentration
- The amount of solute in a given volume of solution.
- dm³
- A cubic decimetre, equal to 1000 cm³ or one litre.
- Titration
- A method for finding the exact volume of one solution that reacts with another.
- Burette
- A long, graduated tube with a tap used to add a solution accurately.
- Pipette
- Apparatus used to measure a fixed volume of liquid accurately.
- End point
- The point in a titration when the indicator changes colour.
- Titre
- The volume of solution added from the burette to reach the end point.
- Concordant results
- Titres that are within 0.10 cm³ of each other.