7.1.2Additional lessons
Measuring Variables in Chemistry — IB Chemistry HL Revision Notes & Topical Questions
5 min read 6 parts Lesson 159 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.
Section 1 of 6Measuring Amounts and Volumes
Quantities such as mass and volume are the starting point for nearly every calculation in a practical.
Mass
- Mass A balance gives mass in grams;
- a digital reading carries an uncertainty equal to its last displayed digit, such as ±0.001 g.
Volume of liquids
- Volume of liquids Pipettes and volumetric flasks deliver one fixed volume very precisely,
- while burettes and measuring cylinders let you choose a volume.
Gas volume
- Gas volume Gas syringes and inverted measuring cylinders over water collect gas, and the volume read must be corrected if the gas is soluble.
Choice of tool
- Choice of tool Pick the instrument whose smallest division is small compared with the quantity measured,
- because this keeps the percentage uncertainty low.
| Instrument | Typical use | Usual uncertainty |
|---|---|---|
| Digital balance | Weighing solids | ± last digit |
| Burette | Titre volumes | ±0.05 cm³ per reading |
| Volumetric pipette | Fixed 25.00 cm³ | Marked on the glassware |
| Measuring cylinder | Approximate volumes | ± half smallest division |
Section 2 of 6Measuring Temperature, Time and pH
Measuring Temperature, Time and pH
Temperature
- Temperature Thermometers and temperature probes record energy changes,
- and a temperature change involves two readings,
- so its uncertainty is the sum of both.
Time
- Time A stopwatch has a small instrument uncertainty, but human reaction time of about 0.2 s is usually the larger source of error.
pH
- pH A pH meter must be calibrated with buffer solutions first, and universal indicator only gives an approximate whole-number pH.
Conductivity
- Conductivity A conductivity probe follows the concentration of ions, which makes it useful for reactions that produce or remove ions.
Section 3 of 6Reading Instruments Correctly
1
Look at the scale and identify the smallest division before taking any reading from an analogue instrument.
2
Read liquid volumes at eye level with the bottom of the meniscus on the mark to avoid parallax error.
3
Record every digit shown on a digital display, including trailing zeros, because they show the precision of the instrument.
4
Quote the uncertainty as half the smallest division for analogue devices and as the last digit for digital devices.
5
Repeat the measurement where possible and compare readings to judge how reliable the instrument and method are.
Section 4 of 6Uncertainty and Precision
Absolute uncertainty
- Absolute uncertainty This is the size of the possible error in the same unit as the reading, for example ±0.05 cm³.
Percentage uncertainty
- Percentage uncertainty Divide the absolute uncertainty by the measured value and multiply by 100 to compare how reliable different measurements are.
Precision
- Precision Precision describes how closely repeat readings agree, and it is set by the instrument and the technique together.
Accuracy
- Accuracy Accuracy describes how close a result is to the accepted value, which a precise instrument cannot guarantee if it is miscalibrated.
Section 5 of 6Worked Example
Question
- A titre is found from two burette readings, 2.10 cm³ and 24.60 cm³, each with uncertainty ±0.05 cm³.
- Find the titre and its percentage uncertainty.
- Titre = 24.60 − 2.10 = 22.50 cm³.
- Absolute uncertainty = 0.05 + 0.05 = ±0.10 cm³ because two readings were subtracted.
- Percentage uncertainty = 0.10 ÷ 22.50 × 100 = 0.44 %.
Answer Titre = 22.50 ± 0.10 cm³ (0.44 %)
Section 6 of 6Exam Checklist & Tips
7.1.2 – Key Points for the Exam
- Meniscus
- The curved surface of a liquid in a tube;
- read the lowest point for most solutions.
- The curved surface of a liquid in a tube;
- Calibration Checking an instrument against a known standard so that its readings can be trusted.
- Absolute uncertainty The possible error in a reading, quoted with the same unit as the reading.
- Parallax error A false reading caused by viewing a scale from an angle rather than at eye level.
- Precision How closely repeated measurements agree with one another.
Common Mistakes
- Quoting the uncertainty of a titre as ±0.05 cm³ when it comes from two readings and should be ±0.10 cm³.
- Dropping the final zero in a digital reading, which hides the true precision of the instrument.
- Using a measuring cylinder when a burette or pipette is needed for a quantitative result.
- Confusing accuracy with precision; the two describe different qualities of a measurement.
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