The Nature of Chemistry
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    • Observation and inference
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    • Accuracy, validity and reliability
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    • Bias and the role of humans in science
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    • (IB TOK outline)
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    • Stoichiometry
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    • Equilibrium
    • Acids and bases
    • Redox processes
    • Organic chemistry
    • Measurement and data processing
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    • A - Materials
    • B - Biochemistry
    • C - Energy
    • D - Medicinal chemistry
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Accuracy, validity and precision

Suggested answers can be found at the bottom of the NOC ANSWERS​ page.
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Activity 1 - Precision in science

Time​: 30 min.
Task​: Much of the work we do in IB chemistry is based on using constants such as Avogadro's constant (NA), the gas constant (R) and the speed of light (c), but we rarely consider the precision or uncertainties in the values we use. Try tasks A and B below.
Task  A - Precision in a calculated value
​i. Consider the three values to the right and identify which of them is the most precise. How do you know?
ii. Why has the IB chosen a value with only 2 decimal places when there are more precise values available? (you might consider the precision of lab measurements to which we then utilise Avogadro's constant)
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Task B - Precision in measurement

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Experimentally defining an accurate and precise value of Avogadro's constant is a very challenging process as all experimental measurements have some degree of uncertainty. Two of the measurements required in this process are measuring the diameter of a silicon sphere and also its mass.
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i. What would be the absolute uncertainty (+/-) in measurements made using the ruler and balance shown in the images?
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ii. Use your answer to part i to calculate the % uncertainty of a 3.65 cm measurement and a 4.81 g measurement.
iii. Assuming that the piece of silicone was a cube (volume = width x depth x height). Calculate the density of the silicon using the two values in part ii.
iv. Calculate the total % uncertainty in the density of the cube by propagating the % uncertainty of each measurement through your calculations.
Task C - How did the National Institute of Standards and Technology calculate Avogadro's constant in 2018?
  • Read this article about the level of precision required - Redefining the mole.
  • Can you suggest examples of how they minimised uncertainty in their calculated values?
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  • Home
  • Recent news
  • NOC
    • The scientific method
    • Observation and inference
    • Evidence, modelling and prediction
    • Theories and paradigm shifts
    • Cause and effect
    • Accuracy, validity and reliability
    • Reason and imagination
    • Bias and the role of humans in science
    • Communication of science
    • Contrasting with other areas of knowledge
    • (IB TOK outline)
  • Topics
    • Stoichiometry
    • Atomic structure
    • Periodicity
    • Chemical bonding and structure
    • Energetics
    • Chemical kinetics
    • Equilibrium
    • Acids and bases
    • Redox processes
    • Organic chemistry
    • Measurement and data processing
  • IB Options
    • A - Materials
    • B - Biochemistry
    • C - Energy
    • D - Medicinal chemistry
  • People
  • Answers
    • NOC answers
    • Topics answers