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All PAG/Practical skills needed for OCR Alevel chemistry
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All experimental ideas that may come up in the OCR alevel chemistry exam - ALL PAGS
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CHEMISTRYTHE EXPERIMENTAL TECHNIQUES
The following experimental areas can appear on any of the papers:
Planning Implementing
● Experimental design to include ● Knowledge of a wide range of
the selection of suitable practical apparatus and
apparatus, and techniques techniques
● Identification of variables that ● Selection of appropriate units for
must be controlled measurements
● Evaluation of experimental ● Presentation of observations
methods to assess their and data in an appropriate
appropriateness format
Analysis Evaluation
● Processing, analysing and ● Drawing conclusions from
interpreting qualitative and evaluation of results
quantitative experimental results ● Identification of anomalies in
● Use of appropriate mathematical experimental measurements
skills for analysis of quantitative ● Evaluation of the limitations in
data experimental procedures
● Appropriate use of significant ● Evaluation of the precision and
figures accuracy of data
● Plotting and interpreting suitable ● Improvements to experimental
graphs design
General experimental knowledge
● Define independent variable.
o The variable that is changed in an experiment
● Define dependent variable.
o The variable that is measured in an experiment
● Describe how to measure mass
o Mass is measured using a mass balance
o It is important to minimise mass loss when transferring the
substance once measured
▪ If making a solution then the substance can be washed into
the equipment using the solvent
▪ If the solid is not being dissolved then measure the mass of
the container first and then add the substance and take a
second mass measurement. The amount of solid added can
then be calculated
● This uses two measurements when calculating
percentage uncertainty
● Describe the purpose of a beaker
, o A beaker is used to mix, stir or heat substances
o A beaker should NEVER be used to measure volume
● Describe how to measure volumes of liquids
o Pipettes
▪ Note – pipette here refers to ‘glass pipettes’
▪ Allows the accurate measurement of a specific small
quantity such as 25cm3
▪ Often used to take a sample from a volumetric flask for a
titration
o Burettes
▪ Measure from top to bottom
▪ There is a tap at the bottom to release liquid
▪ An initial reading is taken before liquid is removed
▪ A final reading is then taken and the volume used calculated
o Volumetric flasks
▪ Allow the accurate measurement of a specific large volume
(e.g. 250cm3)
▪ Used to make accurate dilutions/concentrations
▪ The substance is measured and then added to the
volumetric flask
▪ Rinse out the measuring vessel into the volumetric flask
using the solvent
▪ Fill the flask with more solvent until the meniscus is level
with the line
● Describe how to measure gas volumes
o Gas syringe
o Measure at room temperature as the volume will change with
different temperatures
● Describe how to measure temperature
o Use a thermometer or temperature probe
o Always wait for the temperature to stabilise before taking an initial
reading
● Describe the difference between qualitative and quantitative data
o Quantitative tests measure numerical data (e.g. volume or mass)
o Qualitative tests measure physical qualitites (e.g. colour or
production of precipitate)
● Describe how calculate the mean from a set of data
o Remove any anomalous results
o Calculate the average
● Describe how to calculate percentage uncertainty (percentage error)
, o Percentage uncertainty = maximum error x 100
amount measured
o Most instruments will give the error per reading
▪ e.g. 0.05 cm3
o If the maximum error is not given it is calculated as half the
graduated division of the instrument
▪ e.g. a burette has divisions every 0.1cm3 so the error is
0.05 cm3
o If more than one reading is taken then the maximum error of the
instrument is multiplied by the number of readings
▪ You always take a reading twice for a burette and a
temperature change
● Describe how to carry out an acid and base titration
o Titrations allow you to find the concentration of an acid or an alkali
o The solution with a known concentration is usually added to the
burette
o The solution with the unknown concentration is put in a conical
flask with a small amount of indicator
o Rough titration
▪ An initial reading is taken from the burette before the solution
is added to the solution in the conical flask
▪ Stop adding from the burette when the indicator shows a
permanent colour change
▪ A final reading from the burette is taken and the titre
(volume added) worked out
o Accurate titrations
▪ Repeat the process until you get a minimum of two titres
within 0.1cm3 of each other
▪ Calculate the mean of the titres within 0.1 cm3 of each other
▪ Note – Concordant results are titres of exactly the same
volume
● Describe how to measure volumes from a burette
o Read from the bottom of the meniscus
o Readings should be taken to the nearest 0.05cm3
▪ e.g. 21.70cm3 or 23.45cm3
o The mean should be given to 1 decimal place
▪ e.g. 21.7cm3 or 17.5cm3
● Describe how to make a standard solution and explain the purpose
o A standard solution is a solution that has a precisely known
concentration
o Steps
▪ Measure the mass of the required solid onto a watch glass
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