Quantitative Chemistry - Chemical measurements

What is an ‘error’ ’?

An error is the difference between a value or quantity obtained in an experiment and an accepted or literature value for an experiment

How many types of errors are there in experiments?

2

What are the 2 types of errors in experiments?

random errors and systematic errors

What is an uncertainty?

Uncertainties express the confidence to which the measurement can be taken. They are the same as random errors

What is a random error?

A random error is when you are reading an instrument and estimate the final digit, there is an equal chance that you may read it slightly too high or slightly too low

What can random errors be affected by?

• How easily the instrument or scale is to read

• The person reading the scale poorly

• Changes in the environment

What are two examples of changes in the environment?

• fluctuations in the temperature of the lab

• air currents in the room

What is the effect of random errors?

Random errors will pull a result away from an accepted value in either direction (either too high or too low).

What are systemic errors?

Systematic errors are errors that occur as a result of a faulty or poorly designed experimental procedure

What is the effect of a systematic error?

Systematic errors will always pull the result away from the accepted value in the same direction (always too high or always too low)

What are 3 examples of systematic errors?

• If you forget to zero an electronic balance (using the tare button) the mass weighings will always be higher than they should be

• If you don’t read the volume in a burette at eye level, the volumes will always be smaller than they should be due 

• If you fail to keep a cap on a spirit burner in a calorimetry experiment, the alcohol will evaporate and give you a larger mass loss

How do you calculate uncertainty in results?

For results that are obtained from a series of repeated experiments, the uncertainty is ± half of the range of resultsH

How can you estimate the uncertainty?

• Calculating the mean average and then determining the deviation of the highest and lowest results from the mean value

• An alternative method is to calculate the range of the results and then divide this value by 2W

What is another source of uncertainty?

Other sources of uncertainty can arise where the judgement of the experimenter is needed to determine a changing property

How do you calculate uncertainty with analogue instruments?

Any instruments that have an analogue scale, the uncertainty is taken as half the smallest division on the scale

How do you calculate uncertainty with digital instruments?

Any instruments that have a digital scale , the uncertainty is taken as the smallest division on the scale