IB CHEMISTRY:
TOPIC 11 - MEASUREMENT AND DATA
PROCESSING NOTES
11.1 Uncertainties and errors
Quantitative and Qualitative Data
• Quantitative data is data taken from measurements made in the laboratory and is
associated with random errors
• Qualitative data includes non-numerical data obtained from observations, not from
measurements
• A results table should include quantitative data with units and uncertainties
• Quantitative data should be recorded to the appropriate precision
• Qualitative data should also be recorded
Absolute and Percentage uncertainties
• The absolute uncertainty of digital apparatus is ± the smallest scale division
𝑨𝒃𝒔𝒐𝒍𝒖𝒕𝒆 𝒖𝒏𝒄𝒆𝒓𝒕𝒂𝒊𝒏𝒕𝒚
𝑷𝒆𝒓𝒄𝒆𝒏𝒕𝒂𝒈𝒆 𝑼𝒏𝒄𝒆𝒓𝒕𝒂𝒊𝒏𝒕𝒚 = × 𝟏𝟎𝟎%
𝑴𝒆𝒂𝒔𝒖𝒓𝒆𝒎𝒆𝒏𝒕
• Absolute uncertainty of mass balance is ±0.01g
• Absolute uncertainty of analog apparatus is ± half the smallest scale division
o Absolute uncertainty of measuring cylinder is ±0.5cm3
• Absolute uncertainty of burette is ±0.05cm3
Random Errors
• Random errors are caused by unpredictable changes in the experiment (in the
conditions or apparatus)
• With random errors, there is an equal probability of the measured value being too high or
too low
• Examples of random error:
o Changes in the environment during the experiment (such as a change in the
room temperature)
o Observer misinterpreting the reading
, o Insufficient data (not conducting repeat trials)
• Random errors cannot be eliminated but can be reduced by conducting repeat
trials
• They can also be reduced by using precise apparatus (such as a volumetric pipette
rather than a beaker to measure volume)
Systematic Errors
• Systematic errors occur as a result of a flaw in the experimental design of apparatus
• Systematic errors cause the measured value to be consistently higher or lower than the
actual value
• They cannot be reduced by conducting repeat trials
• Examples of systematic error:
o Heat loss in an experiment to measure enthal py change
o Losing a product (such as a gas) in a reaction
o Overshooting the endpoint in a titration
o Reading from the top of the meniscus when measuring volume
o Forgetting to zero a mass balance
Percentage error
• Percentage error is a measure of how close the experimental value is to the theoretical or
accepted value
𝑬𝒙𝒑𝒆𝒓𝒊𝒎𝒆𝒏𝒕𝒂𝒍 𝒗𝒂𝒍𝒖𝒆 − 𝑻𝒉𝒆𝒐𝒓𝒆𝒕𝒊𝒄𝒂𝒍 𝒗𝒂𝒍𝒖𝒆
𝑷𝒆𝒓𝒄𝒆𝒏𝒕𝒂𝒈𝒆 𝑬𝒓𝒓𝒐𝒓 = × 𝟏𝟎𝟎%
𝑻𝒉𝒐𝒓𝒆𝒕𝒊𝒄𝒂𝒍 𝒗𝒂𝒍𝒖𝒆
If the experimental value is less than the theoretical value, the percentage error will be negative
11.3 Spectroscopic Identification of Organic Compounds
Index of Hydrogen Deficiency (IHD)
• The index of hydrogen deficiency (IHD) is a count of how many molecules of H2 need to
be added to convert the molecule to the corresponding, saturated, non-cyclic molecule.
In other words a degree of unsaturation
• The IHD for a hydrocarbon with 𝐶 carbon atoms, 𝐻 hydrogen atoms, N nitrogen atoms
and X halogen atoms:
TOPIC 11 - MEASUREMENT AND DATA
PROCESSING NOTES
11.1 Uncertainties and errors
Quantitative and Qualitative Data
• Quantitative data is data taken from measurements made in the laboratory and is
associated with random errors
• Qualitative data includes non-numerical data obtained from observations, not from
measurements
• A results table should include quantitative data with units and uncertainties
• Quantitative data should be recorded to the appropriate precision
• Qualitative data should also be recorded
Absolute and Percentage uncertainties
• The absolute uncertainty of digital apparatus is ± the smallest scale division
𝑨𝒃𝒔𝒐𝒍𝒖𝒕𝒆 𝒖𝒏𝒄𝒆𝒓𝒕𝒂𝒊𝒏𝒕𝒚
𝑷𝒆𝒓𝒄𝒆𝒏𝒕𝒂𝒈𝒆 𝑼𝒏𝒄𝒆𝒓𝒕𝒂𝒊𝒏𝒕𝒚 = × 𝟏𝟎𝟎%
𝑴𝒆𝒂𝒔𝒖𝒓𝒆𝒎𝒆𝒏𝒕
• Absolute uncertainty of mass balance is ±0.01g
• Absolute uncertainty of analog apparatus is ± half the smallest scale division
o Absolute uncertainty of measuring cylinder is ±0.5cm3
• Absolute uncertainty of burette is ±0.05cm3
Random Errors
• Random errors are caused by unpredictable changes in the experiment (in the
conditions or apparatus)
• With random errors, there is an equal probability of the measured value being too high or
too low
• Examples of random error:
o Changes in the environment during the experiment (such as a change in the
room temperature)
o Observer misinterpreting the reading
, o Insufficient data (not conducting repeat trials)
• Random errors cannot be eliminated but can be reduced by conducting repeat
trials
• They can also be reduced by using precise apparatus (such as a volumetric pipette
rather than a beaker to measure volume)
Systematic Errors
• Systematic errors occur as a result of a flaw in the experimental design of apparatus
• Systematic errors cause the measured value to be consistently higher or lower than the
actual value
• They cannot be reduced by conducting repeat trials
• Examples of systematic error:
o Heat loss in an experiment to measure enthal py change
o Losing a product (such as a gas) in a reaction
o Overshooting the endpoint in a titration
o Reading from the top of the meniscus when measuring volume
o Forgetting to zero a mass balance
Percentage error
• Percentage error is a measure of how close the experimental value is to the theoretical or
accepted value
𝑬𝒙𝒑𝒆𝒓𝒊𝒎𝒆𝒏𝒕𝒂𝒍 𝒗𝒂𝒍𝒖𝒆 − 𝑻𝒉𝒆𝒐𝒓𝒆𝒕𝒊𝒄𝒂𝒍 𝒗𝒂𝒍𝒖𝒆
𝑷𝒆𝒓𝒄𝒆𝒏𝒕𝒂𝒈𝒆 𝑬𝒓𝒓𝒐𝒓 = × 𝟏𝟎𝟎%
𝑻𝒉𝒐𝒓𝒆𝒕𝒊𝒄𝒂𝒍 𝒗𝒂𝒍𝒖𝒆
If the experimental value is less than the theoretical value, the percentage error will be negative
11.3 Spectroscopic Identification of Organic Compounds
Index of Hydrogen Deficiency (IHD)
• The index of hydrogen deficiency (IHD) is a count of how many molecules of H2 need to
be added to convert the molecule to the corresponding, saturated, non-cyclic molecule.
In other words a degree of unsaturation
• The IHD for a hydrocarbon with 𝐶 carbon atoms, 𝐻 hydrogen atoms, N nitrogen atoms
and X halogen atoms:
