Measurement and Laboratory Practice
What Measurement and Laboratory Practice tests
This domain tests instrument selection, calibration, reading scales, significant figures, uncertainty calculations, data recording, and safe laboratory practice. It rewards attention to the quantity being measured and to the limits of the apparatus.
Core rules
- The least count is the smallest scale division; read to one estimated digit beyond it when the instrument allows.
- Calibrate or check for zero error before use. View a meniscus or pointer perpendicularly to avoid parallax.
- For a product or quotient, relative uncertainties combine approximately as %total = √[(%a)^2 + (%b)^2] for independent random errors.
- Percent error compares an experimental value with an accepted value: |exp − accepted|/|accepted| × 100%. Percent difference compares two experimental values.
- Choose an instrument whose resolution is appropriate: a micrometer for a wire diameter, a graduated cylinder for rough volumes, a balance for mass. Follow electrical, laser, and radiation safety rules.
Worked example
A sample has mass 25.0 g ± 0.1 g and volume 10.0 mL ± 0.2 mL. Find the density and its percent uncertainty.
ρ = m/V = 25.0/10.0 = 2.50 g/mL. Mass percent uncertainty is 0.1/25.0 × 100 = 0.40%; volume is 0.2/10.0 × 100 = 2.0%. Combined: √(0.40^2 + 2.0^2) ≈ 2.0%. The density is 2.50 g/mL ± 2.0%, or about ± 0.05 g/mL. The volume measurement dominates the uncertainty.
Traps
Recording more digits than the instrument supports; treating zero error as random; adding absolute uncertainties when multiplying; reading a buret or graduated cylinder from the top of the meniscus; using a ruler with 1 mm marks to claim 0.01 mm precision; ignoring safety constraints when choosing apparatus.