Topic 11: Measurement and Data Processing

11.1 Uncertainties and Error in Measurement and Results

Qualitative analysis

  • Based on chemical or physical properties

  • reactivity

  • solubility

  • melting point

  • mass spectra

  • etc. 

Quantitative analysis

  • Amount or concentration expressed as numerical values and appropriate units. 

Precision

  • the closeness of agreement between independent test results 

  • lower random error= higher precision

Accuracy

  • the closeness of the agreement between results and the true

Significant Figures

  • Number of figures reflecting the precision of a given measurement

Systematic errors

  • Flaw in the design of the experiment or misuse of equipment

    • Instrumental errors

      • Example:​ Faulty calibration of equipment

    • Experimental methodology errors

      • Example:​ Calorimeter insulation

    • Personal errors

      • Example:​ Reading of graduated cylinder

  • Affect accuracy

  • improve: adopt greater care to experimental design

Random erros

  • Cannot be eliminated, but reduced--> repeated measurement 

  • affect precision

  • Example: estimating quamtity between graduations

Absolute and relative uncertainty

  • Absolute: related to measurement

  • Relative: related to size of measurement (%)

11.2 Graphical Techniques

Slope of a line

  • tangent at a certain point

Intercept

  • extrapolation

  • y=ax+b

Best fit line

  • Trendline

  • Need R squared

Sketch

  • Labelled but unscaled axis

  • show proportionality

Graph

  • Labelled and scaled axis 

  • quantitative measurement

11.3 Spectroscopic Identification of Organic Compounds

​Index of hydrogen deficiency (IHD)

  • used to determine number of rings or multiple bonds in a molecule

  • degree of unsaturation is used to calculate the number of pi bonds and number of rings where:

    • Double bond​: one degree

    • Triple bond: two degree

    • Ring:one degree

    • Aromatic ring: four degree

  • OR: IHD = 0.5(2c+2-h-x+n)​

Infrared spectroscopy

  • Identifies various functional groups

  • based on spring model

    • symmetric stretch ​

    • symmetric bend

    • asymmetric stretch

  • Fingerprint region should not be taken into consideration​

Proton nuclear magnetic resonance (1HNMR)

  • Information between different chemical environments of hydrogen atoms

  • Table 27 of data booklet

  • shows the relative number of hydrogen atoms present

Mass spectrometry (MS)

  • Highly energetic ions, may break in smaller fragments, some of which may be ions.

  • Section 28 of data booklet shows fragmentation pattern

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