Free of subscriptions • Self-paced • Text-based and keyboard accessible
Course homeModule I-03 · Recommended before: Before measurements, density and laboratory work
Measurements, units and significant figures
Measured results communicate both a numerical value and its reliability. Trailing zeros after a decimal can be significant; leading zeros are not.
Learn the method
density = mass / volume
- Record units with each measurement.
- Count significant figures correctly.
- For multiplication/division use the fewest significant figures.
- For addition/subtraction use the least precise decimal place.
- Round only the final answer.
Common mistake: Significant figures are not the same as the number of decimal places.
Worked example
Problem: How many significant figures are in 0.00450 g?
Step-by-step solution: There are 3: 4, 5 and the final zero. Leading zeros only locate the decimal place.
Check the units and whether the answer is reasonable before continuing.
More applications of this mathematics
Try each extension before expanding its solution.
Addition vs multiplication rounding
12.30 + 0.4 = 12.7 (one decimal place), whereas 2.0 × 3.14 = 6.3 (two significant figures).
Density with units
A 15.0 g solid occupies 5.00 mL. Density = 15.0/5.00 = 3.00 g/mL.
Temperature scales
Convert 25.0 °C to absolute temperature for a gas law: 25.0+273.15=298.15 K (report according to measurement precision).
Independent practice — five questions
Suggested mastery threshold: four out of five. Incorrect answers display a worked explanation after submission.
Supporting reference
OpenStax Chemistry 2e / NIST — reference for this subject. This lesson’s wording, examples and questions are original.
IntoChemistry original instructional content, 2026. Formative feedback is not an official course grade. Sources are linked for independent reading. Accessibility testing with your institution and assistive technology is still recommended.