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discuss_12.pptxCHM117 Lab 1: Scientific Measurements 2018
Overview
Introduction to:
Uncertainty
Lab glassware/instruments
Five Sections
(1) Measuring Solids
(2) Measuring Liquids
(3) Determining Density
(4) Using Density
(5) Measuring with Different Techniques
Evaluating Data
Accuracy – “closeness” to accepted value
Precision – “closeness” of repeated measurements
Indicated by uncertainty (for glassware, etc.) and standard deviation (for experimental averages)
http://www.esri.com/news/arcuser/0807/gdb_precision.html
Uncertainty
Measure of precision
expressed with for each number
Two types:
Absolute Uncertainty - expressed in same units
(ex) 20.0 0.2 cm
Relative Uncertainty - expressed in percentage
(ex) 0.2/20.0 *100 = 1% 20.0 cm 1%
Uncertainty of Measurements
Each instrument has inherent level of absolute uncertainty
Last digit is an estimate
0 cm
1
2
For example:
1.21 0.01 cm
Significant Figures
Shorthand for uncertainty
Written digits include all known plus one estimate
Determined by measuring device
Sig. figures indicate # decimal places for uncertainty
Rules for significant figures:
Leading zeros don’t count
(e.g.) 0.0075 in = 2 significant figures
Trailing zeroes do count
(e.g.) 0.00750 in = 3 significant figures
Addition/Subtraction of Measurements
For significant figures:
Answer has lowest # decimal places
For uncertainty:
Answer has the sum of absolute uncertainties
Example:
16.01 ± 0.01 cm
33.47 ± 0.01 cm
49.48 ± 0.02 cm
+
Multiplication/Division of Measurements
For significant figures:
Answer has lowest # significant figures
For uncertainty:
Answer has the sum of relative uncertainties
Example: 16.01 ± 0.01 cm (± 0.06%)
33.47 ± 0.01 cm (± 0.03%)
535.9 ± 0.09%
Convert back to absolute uncertainty: (0.09/100)*535.9
535.9 ± 0.5 cm2
x
Mass Measurements
Mass: measurement of resistance to acceleration
Unit: kilogram
Measured with analytical balance
Contains standard masses
Same principle as beam balance
Balance point detected with light beam
Weighing Notes
Tare the balance before use
Can be done empty or with empty container
Don’t touch glassware being weighed
Add chemicals outside balance
Keep doors closed while measuring
Write down units AND uncertainty
This balance is ± 0.0002 g.
Clean up the balance after use
Volume Measurements
Volume: quantity of three-dimensional space
Unit: Liter
Precision determined by type of glassware
More tick marks give lower uncertainty
(e.g.) beaker vs. cylinder
± 5 mL
± 0.5 mL
Volume Notes
Place glassware on flat surface
Look at liquid at eye-level
Read volume from bottom of meniscus
Concave (e.g. water)
Convex (e.g. mercury)
Density
Measurement of mass per unit volume
d = m/V
Changes with temperature
Unit: kilogram/cubic meter
Common form: g/mL, g/cm3
For aluminum:
d = 2.70 g/cm3
(CRC Handbook, 51st Edition)
