1. Unit 1 – Making Measurements
Unit 1: Introduction to Chemistry and Measurements Credited to J. Wyatt GHS

2. Why Do We Measure?
Measurements allow us to keep track of things, look at how things change over time, make comparisons, establish relationships between things, make predictions, and draw conclusions.
They help us to understand the world around us!

3. What Do We Measure?
Qualitative Measurements – descriptive measurements, not absolute
Ex: Hot, cold, heavy, light, color, rough, smooth, smell, health
Quantitative Measurements - numerical, units, defined, standardized
Ex: time, length, density, speed, count – use SI Units!

4. Systeme Internationale (SI Units)
Based on the metric system
Helps for consistency among scientists
Quantity
Base Unit
Symbol
Time
second s
Length
meter m
Mass
kilogram kg
Temperature
kelvin K
Count/quantity
mole
mol
Electric Current
ampere A
Luminous Intensity
candela cd

5. Derived Units
Not everything that can be measured can be measured with the seven base SI units
Derived units to the rescue! Made of the seven base units
Ex: Force = mass x acceleration
Mass: measured in kg
Acceleration: measured in m/s2
kg * m/s2 = Newton (N), unit of force

6. Why Use the Metric System?
Makes conversions easy!
English/Imperial Units have no standard conversion factor to make units larger or smaller
Metric units are based on powers of 10
Fewer conversion factors to memorize!
Can apply the conversion factor to any base unit, no matter what you are measuring.

8. Metric Prefixes You must memorize the ones that are starred!
Remember…King Henry Died By Drinking Chocolate Milk!
Prefix
Symbol
Factor
Scientific Notation
giga G
1 billion =
109
mega M
1 million =
106
*kilo k
1 thousand = 1 000
103
*hecto h
1 hundred = 100
102
*deca da
ten = 10
101
*base unit
none
one = 1
100 = 1
*deci d
1 tenth = 0.1
10-1
*centi c
1 one hundredth = 0.01
10-2
*milli m
1 one thousandth = 0.001
10-3
micro
1 one millionth =
10-6
nano n
1 one billionth =
10-9
Make the base unit BIGGER
Make the base unit smaller

9. Accuracy and Precision
Is there a difference? YES!!!
Accuracy is getting close to your target; close to the accepted value of the measurement
Precision is getting the same answer over and over again, regardless of how correct (accurate) the measurement is
Good measurements are BOTH!

10. Accuracy and Precision Examples
If the true density of the substance is 1.59 g/cm3, which student collected the most accurate data?
Which student was the most precise?
Density Data Collected by Three Different Students
Student A
Student B
Student C
Trial 1
1.54 g/cm3
1.40 g/cm3
1.70 g/cm3
Trial 2
1.60 g/cm3
1.68 g/cm3
1.69 g/cm3
Trial 3
1.57 g/cm3
1.45 g/cm3
1.71 g/cm3
Average
1.51 g/cm3

11. Accuracy and Precision Examples
Four students each took 3 temperature readings of a sample of water. The actual temperature of the water was 80.0°C. Which student’s measurements were both accurate and precise?
Student Measurements of Temperature
Reading 1 (°C)
Reading 2 (°C)
Reading 3 (°C)
Student 1
78.6
78.5
78.7
Student 2
82.4
80.0
81.4
Student 3
80.8
78.9
81.8
Student 4
80.1
79.9

12. If you had to measure 15.0 mL of a liquid, which would you choose?
Making Measurements
If you had to measure 15.0 mL of a liquid, which would you choose?

13. Estimating Measurements
Must always read all of the certain numbers plus one estimated digit
The pencil is cm long
The decimal place of the estimated digit is determined by how the instrument is calibrated
The estimated digit is in the 0.01 (one hundredths) place
This instrument is precise to the place
Which of the rulers to the right would allow you to be the most accurate and precise in your measurement?

14. Significant Figures
All of the numbers that we are certain about + estimated digit are significant to the measurement
Called significant figures (sig figs) or significant digits (sig digs)
When numbers are converted into scientific notation, the number of sig figs must be the same as in the original measurement
Only measurements have sig figs!
Definitions and counting numbers have infinite sig figs
Ex: 12 cats
Ex: 12 inches = 1 foot

15. Determining Significant Figures
Any number that is not a zero is always significant
Zeros may or may not be!
Trailing zeros are only significant if a decimal point is present
A zero in between two non-zero numbers is significant
Zeros that act as place holders are not significant

16. Determining Significant Figures – Made Easy!
The rules for sig figs can be simplified by applying the Atlantic/Pacific rule
Ask yourself – Is there a decimal?
Yes! – Pacific (present) – start from the left and count all numbers from the first non zero number
NO! – Atlantic (absent) – start from the right and count all numbers from the first non zero number

17. Uncertainty in Measurements
Sig figs help you to determine where the uncertainty in your measurements lies
The last significant figure is the estimated one – this is the uncertain digit
All of these scales measure mass, but their uncertainty is different.
Uncertainty +/- 0.1
Uncertainty +/- 0.01
Uncertainty +/
Uncertainty +/
Think about how the mass of the same object would look on these different scales.

18. Uncertainty in Measurements - Range
Since the scales are rounding to different places, the real value lies within a range. The uncertainty defines that range.
If the object on the first scale reads 12.3 g, then the true value should be between 12.2g and 12.4 g (12.3 +/- 0.1 g)
On the second scale, the uncertainty is +/ g, so a possible measurement might be g, and the true value should be between 12.34g and g ( /- 0.01g)
On the third scale, the uncertainty is +/ g, so a possible measurement might be g, and the true value should be between and g ( / g)
What is a possible value for the same object if it were placed on the last scale?

19. Explain This!
Using what you know about measurements, how could this possibly be true?

20. Calculating With Measurements
Oftentimes we have to add, subtract, multiply, or divide with measurements.
We must take uncertainty into account when we do this! Your final answer cannot be more certain than your least certain measurement.
56.0 g / 13 cm3 = g/cm3 …..etc. Where do you cut off the calculator vomit?
Sig Figs to the rescue!

21. Adding and Subtracting Measurements
Your final answer must have the same number of decimal places as your measurement with the fewest decimal places when you add and subtract measurements
Add normally then round answer to the appropriate place

22. Multiplying and Dividing Measurements
Your final answer must have the same number of significant figures as your measurement with the least amount of significant figures
Calculate normally then round final answer to the proper number of sig figs

23. Rounding is Serious Stuff!
Twenty-eight Americans were killed on February 25, 1991 when an Iraqi Scud hit the Army barracks in Dhahran, Saudi Arabia. The Patriot defense system had failed to track and intercept the Scud