• Use the SI system.
  • Know the SI base units.
  • State rough equivalents for the SI base units in the English system.
  • Read and write the symbols for SI units.
  • Recognize unit prefixes and their abbreviations.
  • Build derived units from the basic units for mass, length, temperature, and time.
  • Convert measurements from SI units to English, and from one prefixed unit to another.
  • Use derived units like density and speed as conversion factors.
  • Use percentages, parts per thousand, and parts per million as conversion factors.
• Use and report measurements carefully.
  • Consider the reliability of a measurement in decisions based on measurements.
  • Clearly distinguish between
    • precision and accuracy
    • exact numbers and measurements
    • systematic error and random error
  • Count the number of significant figures in a recorded measurement. Record measurements to the correct number of digits.
  • Estimate the number of significant digits in a calculated result.
  • Estimate the precision of a measurement by computing a standard deviation.

Measurement is the collection of quantitative data. The proper handling and interpretation of measurements are essential in chemistry - and in any scientific endeavour. To use measurements correctly, you must recognize that measurements are not numbers. They always contain a unit and some inherent error. The second lecture focuses on an international system of units (the SI system) and introduces unit conversion. In the third lecture, we'll discuss ways to recognize, estimate and report the errors that are always present in measurements.

• quantitative observations
• include 3 pieces of information
  • magnitude
  • unit
  • uncertainty
• measurements are not numbers
  • numbers are obtained by counting or by definition; measurements are obtained by comparing an object with a standard "unit"
  • numbers are exact; measurements are inexact
  • mathematics is based on numbers; science is based on measurement

• Le Systéme Internationale (SI) is a set of units and notations that are standard in science.

  Four important SI base units (there are others)

  Quantity	SI Base Unit	English Equivalent
  length	meter (m)	1 m = 39.36 in
  mass	kilogram (kg)	1 kg = 2.2 lbs
  time	second (s)
  temperature	kelvin (K)	°F = 1.8(oC)+32 K = °C + 273.15

• derived units are built from base units

  Some SI derived units

  Quantity	Dimensions	SI units	Common name
  area	length × length	m2	square meter
  velocity	length/time	m/s
  density	mass/volume	kg/m3
  frequency	cycles/time	s-1	hertz (Hz)
  acceleration	velocity/time	m/s2
  force	mass × acceleration	kg m/s2	Newton (N)
  work, energy, heat	force × distance	kg m2/s2	Joule (J)

• Prefixes are used to adjust the size of base units

  Commonly used SI prefixes (there are others).

  Prefix	Meaning	Abbreviation	Exponential Notation
  Giga-	billion	G	109
  Mega-	million	M	106
  kilo-	thousand	k	103
  centi-	hundredths of	c	10-2
  milli-	thousandths of	m	10-3
  micro-	millionths of	µ	10-6
  nano-	billionths of	n	10-9
  pico-	trillionths of	p	10-12

• several non-SI units are encountered in chemistry

  Non SI unit	Unit type	SI conversion	Notes
  liter (L)	volume	1 L = 1000 cm3	1 quart = 0.946 L
  Angstrom (Å)	length	1 Å = 10-10 m	typical radius of an atom
  atomic mass unit (u)	mass	1 u = 1.66054×10-27 kg	about the mass of a proton or neutron; also known as a 'dalton' or 'amu'

• addition and subtraction: units don't change
  2 kg + 3 kg = 5 kg
  412 m - 12 m = 400 m
  • consequence: units must be the same before adding or subtracting!
    3.001 kg + 112 g = 3.001 kg + 0.112 kg = 3.113 kg
    4.314 Gm - 2 Mm = 4.314 Gm - 0.002 Gm = 4.312 Gm
• multiplication and division: units multiply & divide too
  3 m × 3 m = 9 m²
  10 kg × 9.8 m/s² = 98 kg m/s²
  
  • consequence: units may cancel
    5 g / 10 g = 0.5 (no units!)
    10.00 m/s × 39.37 in/m = 393.7 in/s

• 5 step plan for converting units
  1. identify the unknown, including units
  2. choose a starting point
  3. list the connecting conversion factors
  4. multiply starting measurement by conversion factors
  5. check the result: does the answer make sense?
• Common variations
  • series of conversions
    • example: Americium (Am) is extremely toxic; 0.02 micrograms is the allowable body burden in bone. How many ounces of Am is this?
  • converting powers of units
  • converting compound units
  • starting point must be constructed
• using derived units as conversion factors
  • mass fractions (percent, ppt, ppm) convert mass of sample into mass of component
  • density converts mass of a substance to volume
  • velocity converts distance traveled to time required
  • concentration converts volume of solution to mass of solute

• making a measurement usually involves comparison with a unit or a scale of units
  • always read between the lines!
  • the digit read between the lines is always uncertain
  • convention: read to 1/10 of the distance between the smallest scale divisions
• significant digits
  • definition: all digits up to and including the first uncertain digit.
  • the more significant digits, the more reproducible the measurement is.
  • counts and defined numbers are exact- they have no uncertain digits!

• counting significant digits in a series of measurements
  • compute the average
  • identify the first uncertain digit
  • round the average so the last digit is the first uncertain digit
• counting significant digits in a single measurement
  • convert to exponential notation
  • disappearing zeros just hold the decimal point- they aren't significant.
  • exception: zeros at the end of a whole number might be significant
• Precision of Calculated Results
  • calculated results are never more reliable than the measurements they are built from
  • multistep calculations: never round intermediate results!
  • sums and differences: round result to the same number of fraction digits as the poorest measurement
  • products and quotients: round result to the same number of significant digits as the poorest measurement.

• Precision vs. Accuracy

  good precision & good accuracy poor accuracy but good precision

  good accuracy but poor precision poor precision & poor accuracy

  
  

  Precision	Accuracy
  reproducibility	correctness
  check by repeating measurements	check by using a different method
  poor precision results from poor technique	poor accuracy results from procedural or equipment flaws
  poor precision is associated with 'random errors' - error has random sign and varying magnitude. Small errors more likely than large errors.	poor accuracy is associated with 'systematic errors' - error has a reproducible sign and magnitude.

• Estimating Precision
  • Consider these two methods for computing scores in archery competitions. Which is fairer?

    	Score by distance from bullseye
    	Score by area or target

  • The standard deviation, s, is a precision estimate based on the area score: where
    x_i is the i-th measurement
    is the average measurement
    N is the number of measurements.
    

Copyright © 1997-2005 by Fred Senese
Comments & questions to fsenese@frostburg.edu
Last Revised 02/23/18.URL: http://antoine.frostburg.edu/chem/senese/101/measurement/index.shtml