go to the AUS-e-TUTE homepage
home game Join AUS-e-TUTE test contact
 

 

Experimental Errors

Key Concepts

  • Experimental error does NOT refer to mistakes made in calculations, nor to mistakes made when reading an instrument.
    The experimenter is always assumed to be careful and competent so that mistakes do not happen.

  • Experimental error DOES refer to the uncertainty about the accuracy of the results of an experiment.

  • There are two types of experimental errors in chemistry:

        (a) random errors (or indeterminate errors)

        (b) systematic errors (or determinate errors, or inherent errors)

  • Random errors result from random events which cannot be eliminated during the experiment.

  • Systematic errors are errors inherent in the experiment and which can be determined and therefore compensated for.

  • The goal in a chemistry experiment is to eliminate systematic error and minimize random error to obtain a high degree of certainty.
    Removal of uncertainty results in accuracy and precision.

Mistakes

Mistakes are NOT considered to be experimental errors.
It is assumed that if an experimenter has made a mistake then he/she will discard the results of the experiment or calculation and start again, that is, results from an experiment that included mistakes would NOT be reported.
Mistakes occur if the experimenter is careless, or, if the experimenter is incompetent.
When the results of an experiment are reported, it is assumed that the experimenter was both careful and competent.

Would you like to see this example?
Click this link to go to the complete tutorial if you are an AUS-e-TUTE member.
Not an AUS-e-TUTE Member?

  • Find out how an AUS-e-TUTE Membership can help you here.
  • Become an AUS-e-TUTE member here.

Remember, if you make a mistake during an experiment or calculation, you should discard what you have done so far and start again.
You should not report the results of an experiment that includes mistakes.

Mistakes are NOT the same as experimental errors. Experimental errors are either random or systematic errors as described below.

Random Errors

Random errors result from random events which cannot be eliminated during the experiment.
Random errors usually result from the experimenter's inability to take exactly the same measurement in exactly the same way any number of times and get the exactly the same number.

Examples of the sources of random errors are:

  • fluctuation of the power supply during the use of electronic equipment such as an electronic balance

  • using a contaminated reagent in a particular experiment

  • experimenter being distracted while taking a measurement

  • locating the bottom of the meniscus for volume measurements using pipettes, burettes, measuring cylinders, etc

Since random errors cannnot be eliminated from the experiment, they are minimised by repeating the experiment a number of times until the uncertainty has been reduced to an acceptable level.

The arithmetic mean (or average) value of the measurements is then calculated, and is the number that is used as the final result, or, in further calculations.

arithmetic mean (average) = sum of all the results divided by the number of results

If one of the results in a set of results is enormously different to the other results, this result is rejected before the arithmetic mean (average) is calculated. (Technically, this result is known as a grossly-deviant result).

Would you like to see this example?
Click this link to go to the complete tutorial if you are an AUS-e-TUTE member.
Not an AUS-e-TUTE Member?

  • Find out how an AUS-e-TUTE Membership can help you here.
  • Become an AUS-e-TUTE member here.

Systematic Errors

Systematic errors are errors inherent in the experiment and which can be determined and therefore compensated for.
Systematic errors are reproducible inaccuracies that are inherent in the procedure, operator, instrumentation, and, treatment of results.
The source and magnitude of systematic errors can, in principle, be determined.

Would you like to see this example?
Click this link to go to the complete tutorial if you are an AUS-e-TUTE member.
Not an AUS-e-TUTE Member?

  • Find out how an AUS-e-TUTE Membership can help you here.
  • Become an AUS-e-TUTE member here.

Because the source and magnitude of the systematic errors is known, they can be compensated for individually, or, as a set by calibrating the equipment used.
When you look at the labels on glassware used for volumetric analysis such as volumetric flasks, you will see the label includes a capital letter (A or B), a temperature, usually 20oC, as well as the volume. The label is telling you that the volumetric flask will only measure 250 mL of solution at 20oC, at any other temperature the volume will not be 250 mL. Volumetric analysis should therefore be carried at in a laboratory with a constant temperature of 20oC.

Many substances absorb moisture from the atmosphere, sodium hydroxide pellets are an excellent example. To eliminate this as a source of systematic error, the substance (the sodium hydroxide pellets for instance) is placed in a sealed vessel known as a dessicator. In the bottom of the dessicator is placed a substance, known as the dessicant, that is used to absorb moisture from the air. Silica gel is often used as the dessicant (also known as the drying agent). The dessicant absorbs the moisture in the dessicator so that our substance does not absorb water from the air.
Other dessicants (drying agents) used to keep the air in a dessicator free of moisture are granules of fused calcium chloride, anhydrous calcium sulfate and activated alumina.

To prevent liquids from evaporating before weighing, they could be kept in a sealed vessel saturated with the liquid's vapor.

Systematic errors from faulty instrumentation can be eliminated by calibrating the instrument before using it. This often involves using the instrument to measure substances with accurately known values and then constructing a calibration curve as a reference for the experiment. The experimenter uses the instrument to measure the unknown sample, and then uses the calibration curve to obtain an accurate value.


What would you like to do now?

Would you like to:
advertise on the AUS-e-TUTE website and newsletters
 
 

Search this Site

You can search this site using a key term or a concept to find tutorials, tests, exams and learning activities (games).
 

Become an AUS-e-TUTE Member

 

AUS-e-TUTE's Blog

Recent AUS-e-BLOG Posts:

 

Subscribe to our Free Newsletter

Email email us to
subscribe to AUS-e-TUTE's free quarterly newsletter, AUS-e-NEWS.

AUS-e-NEWS quarterly newsletter

AUS-e-NEWS is emailed out in
December, March, June, and September.

 

Ask Chris, the Chemist, a Question

The quickest way to find the definition of a term is to ask Chris, the AUS-e-TUTE Chemist.

Chris can also send you to the relevant
AUS-e-TUTE tutorial topic page.

 
 
 

Share this Page

Bookmark and Share

 
 

© AUS-e-TUTE