Tool 1: Experimental Techniques (DP IB Chemistry)

This hazard symbol means oxidising.

Precise relates to how close a series of results are to each other. For example:

• 46.0, 46.1 and 46.2 are precise results as they are close together

• 46.1, 47.9 and 48.8 are not precise as they are spread out over a large range.

Accurate relates to how close a result is to the true value / expected result. For example, if the expected value is 23.7:

• 23.8 is accurate as it is close to the expected value

• 21.1 is not accurate as it is not close to the expected value.

Valid relates to whether something measures what it is supposed to measure. For example, a pH probe must be calibrated to gather valid results.

Reliable is about whether an experiment gives the same / similar result, when repeated:

• Using the same method

• Using the same equipment

• By the same people.

Reproducible is about whether an experiment gives the same / similar result, when repeated:

• Using the same method

• Using the same equipment

• By different people.

This piece of apparatus is called a gas syringe and it is used to measure volumes of gas produced in chemical reactions.

This piece of apparatus is called a measuring cylinder and it is used to measure volumes of liquid, when an accurate volume is not required.

The piece of apparatus which could be used to measure out 0.5 g of magnesium powder is a balance.

The standard unit of mass in chemistry is grams.

Advantages of using temperature probes include:

• More precise readings.

• Easy to make multiple readings.

• Can be automated to run over long periods of time.

The piece of apparatus which could be used to time the length of a reaction is a stopwatch / stopclock / timer.

Two methods that could be used to measure the volume of gas produced when marble chips are added to hydrochloric acid are:

• Using a gas syringe

• Bubbling into an inverted measuring cylinder filled with water / downward displacement of water.

False.

A measuring cylinder is less accurate than a pipette for measuring a fixed volume of liquid.

A meniscus is the curve produced by a liquid inside a container.

When measuring liquids, the reading should be taken from the bottom of the meniscus.

The volume on the burette, to one decimal place, is 17.8 cm³.

A burette should be used to measure the volume of liquid added (titre) during a titration.

A volumetric / standard flask should be used to prepare a standard solution.

A measuring cylinder should be used to approximately measure 25 cm³ of a liquid.

A volumetric pipette should be used to accurately measure 25 cm³ of a liquid.

False.

pH meters offer higher precision, but indicators are suitable for use in titrations as their sharp color change gives a clear endpoint.

A standard solution is a solution of known concentration prepared as accurately and precisely as possible.

Using a 500 cm³ standard flask, to make a 0.5 mol dm^-3 sodium hydroxide solution:

• Weigh out 10.0 g of sodium hydroxide

• Transfer it to the 500 cm³ standard flask

• Wash the weighing boat into the standard flask

• Add distilled water until the bottom of the meniscus is level with the graduation mark

• Add the stopper and invert the flask to mix

A serial dilution is used to create a sequence of dilutions, typically diluting by a factor of 10 each time, to achieve a solution of desired concentration.

This equipment set up is used for refluxing.

Heating under reflux allows organic reactions to occur at higher temperatures without loss of reactants, products, or solvent.

This equipment set up is used for distillation.

Distillation separates a mixture by the different boiling points of the compounds in the mixture.

Techniques that can be used to separate a mixture of solid and liquid include:

• Filtration

• Crystallisation

• Distillation.

False.

Some redox titrations, such as the Fe²⁺/MnO₄⁻ titration, do not require an indicator because they are self-indicating.

Recrystallisation is a purification technique that involves dissolving an impure solid in a hot suitable solvent, filtering and then allowing the compound to crystallise out of the solution.

Pure solids have sharp melting points which agree with literature / database values for the pure solid.

False.

An impure solid melts over a range of temperatures lower than the literature value melting point for the pure solid.

Chromatography separates a mixture of compounds by their different solubilities in a solvent.

True.

Colorimetry and spectrophotometry use the same basic method of measuring the absorbance/transmittance of light at specific wavelengths.