8.2.2Additional lessons
Processing Data in Chemistry — IB Chemistry HL Revision Notes & Topical Questions
5 min read 6 parts Lesson 171 of 172
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 6Calculations from Raw Data
Processing converts a table of measurements into evidence for a conclusion.
Mean
- Mean Add the repeat values and divide by their number to reduce the effect of random variation.
Derived quantities
- Derived quantities Use the correct equation to find rates, moles, enthalpy changes or concentrations from the measured values.
Worked example
- Worked example Show one full sample calculation with units, and then state that the rest were done the same way.
Significant figures
- Significant figures Final values keep the same number of significant figures as the least precise measurement.
Part 2 of 6Uncertainty in Processed Data
Uncertainty in Processed Data
Spread of repeats
- Spread of repeats Half the range of the repeat values gives a simple estimate of random uncertainty.
Instrument uncertainty
- Instrument uncertainty Compare the spread of repeats with the instrument uncertainty and use whichever is larger.
Propagation
- Propagation Combine uncertainties through each calculation step using the addition rules for absolute and percentage values.
Error bars
- Error bars Plot error bars on graphs to show how reliable each point is.
Part 3 of 6Presenting Processed Data
1
Choose a graph type that shows the relationship, usually a scatter plot of dependent against independent variable.
2
Label axes with quantity and unit, plot points accurately and draw a suitable line of best fit.
3
Add error bars where they are significant compared with the scale of the graph.
4
Find the gradient or intercept if it relates to a chemical quantity such as a rate or an activation energy.
5
Compare calculated values with literature data and calculate the percentage error.
Part 4 of 6Good Practice
Traceability
- Traceability Each processed number can be traced back to the raw data so that the work can be checked.
Consistent units
- Consistent units Convert all quantities into the units required by an equation before calculating.
Do not round early
- Do not round early Keep extra digits during calculations and round at the end.
Software
- Software Spreadsheets can speed up repeated calculations, but formulae should still be checked on one example by hand.
Part 5 of 6Worked Example
Question
- Three repeat times for a reaction are 12.4 s, 12.5 s and 12.6 s.
- Find the mean time, its uncertainty from the range, and the rate taken as 1/t with its uncertainty.
- Mean time = (12.4 + 12.5 + 12.6) ÷ 3 = 12.5 s.
- Uncertainty = half the range = (12.6 − 12.4) ÷ 2 = ±0.1 s, which is 0.1 ÷ 12.5 × 100 = 0.8 %.
- Rate = 1 ÷ 12.5 = 0.0800 s⁻¹, and 0.8 % of 0.0800 is 0.0006 s⁻¹.
Answer
- t = 12.5 ± 0.1 s;
- rate = 0.0800 ± 0.0006 s⁻¹
Part 6 of 6Exam Checklist & Tips
8.2.2 – Key Points for the Exam
- Mean The sum of repeat values divided by the number of values.
- Range The difference between the largest and smallest values in a data set.
- Sample calculation A fully worked example showing how every processed value was obtained.
- Error bar A line on a graph showing the uncertainty in a plotted point.
- Percentage error The difference from an accepted value as a percentage of that value.
Common Mistakes
- Calculating a mean that includes an obvious anomaly without comment.
- Quoting a mean to more decimal places than the instrument can justify.
- Plotting raw data when the processed data, such as rate, is needed to answer the question.
- Giving a processed value without any statement of its uncertainty.
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