Experimental Techniques

Apparatus for Measurements

• Time is measured with a stopwatch or stopclock, accurate to 1–2 decimal places.
• Temperature is measured with a thermometer $(precision \pm 0.5 ^{\circ}C)$ or digital probe $(\pm 0.1 ^{\circ}C)$.
• Mass is measured using a digital balance (usually to 2 decimal places).
• Volume of liquid: burettes (most accurate for variable volumes), volumetric pipettes (fixed volume), measuring cylinders (approximate).

Which piece of apparatus is most accurate for measuring a variable volume of liquid between $0 cm^{3}$ and $50 cm^{3}$?

• Measuring cylinder
• Volumetric pipette
• Burette
• Gas syringe

Measuring Gas Volumes

• Gas volume can be measured using a gas syringe or by downward displacement of water.
• A gas syringe is more precise and accurate than downward displacement.
• Downward displacement of water does not work if the gas is soluble in water.

A gas syringe is more ____ and accurate than downward displacement of water.

Solutions Terminology

• Solvent: the liquid in which a solute dissolves (e.g., water in seawater).
• Solute: the substance that dissolves (e.g., salt in seawater).
• Solution: the mixture formed when a solute dissolves in a solvent.
• Saturated solution: contains the maximum concentration of solute at a given temperature.

Which of these statements about solutions are true?

• A solute is the liquid that dissolves a substance.
• A saturated solution contains the maximum possible amount of solute at that temperature.
• Filtrate is the liquid that passes through a filter.
• Residue is the substance that dissolves in a solvent.

Acid-Base Titrations

• Titrations determine the concentration of an acid or alkali by neutralisation.
• Apparatus: volumetric pipette $(25 cm^{3})$, burette $(50 cm^{3})$, conical flask, white tile, indicator.
• The endpoint is when one drop causes a sharp colour change.
• Concordant results are within $0.1 cm^{3}$ of each other.

What is the purpose of a white tile in a titration?

• To hold the conical flask steady
• To provide a white background to see the colour change clearly
• To prevent the flask from breaking
• To measure the volume of acid

Titration Method

• 1. Use a pipette to transfer exactly $25 cm^{3}$ of alkali into a conical flask.
• 2. Fill the burette with acid, remove air bubbles, and record initial reading.
• 3. Add a few drops of indicator to the flask.
• 4. Add acid from burette while swirling, until the indicator changes colour.
• 5. Record final reading and repeat for concordant results.

The endpoint of a titration is when one ____ causes a sharp colour change.

Common Indicators

• Methyl orange: red in acid, yellow in alkali, orange at endpoint.
• Phenolphthalein: colourless in acid, pink in alkali.
• Litmus is not suitable for titrations because it gives a gradual colour change.
• Thymolphthalein: colourless in acid, blue in alkali.

Which indicator is colourless in acid and pink in alkali?

• Methyl orange
• Litmus
• Phenolphthalein
• Thymolphthalein

Common Titration Errors

• Not removing the funnel from the burette can cause extra liquid to drip in, giving false high readings.
• Not filling the jet space (below the tap) leads to false low readings.
• Reading the burette incorrectly: always read from the bottom of the meniscus.
• The burette scale has $0.00 cm^{3}$ at the top and $50.00 cm^{3}$ at the bottom.

Which of these are common errors in a titration?

• Leaving the funnel in the burette during titration.
• Reading the burette from the top of the meniscus.
• Using a volumetric pipette to measure the acid in the burette.
• Not filling the jet space of the burette before starting.

Recording Titre Volumes

• Record initial and final burette readings to the nearest $0.05 cm^{3}$.
• The titre is the volume added (final – initial).
• Repeat until you have two concordant results $(within 0.1 cm^{3}$ of each other).
• Use a results table with columns: Rough, Titre 1, Titre 2, Titre 3.

Concordant results in a titration are those within ____ cm^{3} of each other.