1. Measurement in Chemistry

2. SI Units
All systems of weights and measures, metric and non-metric, are linked through a network of international agreements supporting the International System of Units.
The International System is called the SI, using the first two initials of its French name Système International d'Unités.

3. SI Units Base SI Units: meter - distance kilogram - mass second - time
kelvin - temperature
mole -amount of substance

4. All other SI Units are derived from the seven standard units.
For example:
Units of volume -derived from units of length.
1 mL = (1 cm)(1 cm)(1 cm) = 1 cm3
1 L = 1 dm3
Units of energy -derived from a combination of units.
1 J = 1

5. SI Units
SI units have universally accepted symbols. Some common features of these symbols include:
no periods after SI units
Symbols for SI units never pluralized
SI units are case-sensitive (IF I WRITE SOMETHING UPPER CASE, YOU NEED TO WRITE IT IN UPPER CASE!)

6. Significant Figures What are they?
All “certain” digits and ONE “uncertain” digit in a scientific measurement or calculation.
What do we mean by “certain” and “uncertain”?

7. What is the measurement of this paper clip?

8. Significant Figures Why do we use them?
To account for the limits of ourselves.
To account for the limits of our instruments.
Basically, we use sig. figs. to keep from lying about the precision of our instruments.

9. Significant Figures How do I use sig. figs. when measuring?
If the instrument is digital, record every digit (even zeroes).
If the instrument is lined (rulers, graduated cylinders, etc.), estimate one digit beyond the markings.

10. Reporting Measurements
Report what is known with certainty
Add ONE digit of uncertainty (estimation)
By adding additional numbers to a measurement – you do not make it more precise. The instrument determines how precise it can make a measurement. Remember, you can only add ONE digit of uncertainty to a measurement.

11. Practice Measuring cm 1 2 3 4 5 4.5 cm cm 1 2 3 4 5 4.54 cm cm 1 2 3 41 2 3 4 5
4.5 cm cm 1 2 3 4 5
4.54 cm
PRACTICE MEASURING
Estimate one digit of uncertainty.
a) 4.5 cm
b) * 4.55 cm
c) 3.0 cm
*4.550 cm is INCORRECT while 4.52 cm or 4.58 cm are CORRECT (although the estimate is poor)
By adding additional numbers to a measurement – you do not make it more precise. The instrument determines how precise it can make a measurement. Remember, you can only add ONE digit of uncertainty to a measurement.
In applying the rules for significant figures, many students lose sight of the fact that the concept of significant figures comes from estimations in measurement. The last digit in a measurement is an estimation.
How could the measurement be affected by the use of several different rulers to measure the red wire?
(Different rulers could yield different readings depending on their precision.)
Why is it important to use the same measuring instrument throughout an experiment?
(Using the same instrument reduces the discrepancies due to manufacturing defects.) cm 1 2 3 4 5
3.0 cm

12. Accuracy and Precision
In chemistry, the meanings of accuracy and precision are quite different.
Accuracy is a measure of how close a measurement comes to the actual or true value of whatever is measured.
Precision is a measure of how close a series of measurements are to one another, irrespective of the actual value.

13. Accuracy and Precision
To evaluate the accuracy of a measurement, the measured value must be compared to the correct value.
To evaluate the precision of a measurement, you must compare the values of two or more repeated measurements.

14. Darts on a dartboard illustrate the difference between accuracy and precision.
Good Accuracy, Good Precision
Poor Accuracy, Good Precision
Poor Accuracy, Poor Precision
The closeness of a dart to the bull’s-eye corresponds to the degree of accuracy. The closeness of several darts to one another corresponds to the degree of precision.

15. Accurate? Precise?

16. Accurate? Precise?

17. Accuracy, Precision, and Error
Determining Error
There is a difference between the accepted value, which is the correct value for the measurement, and the measured (experimental) value, the value measured in the lab.
The difference between the experimental value and the accepted value is called the error.
Error = measured value – accepted value

18. Accuracy, Precision, and Error
Determining Error
Suppose you use a thermometer to measure the boiling point of pure water at standard pressure.
The thermometer reads 99.1°C.
You probably know that the accepted value of the boiling point of pure water at these conditions is actually
100.0°C
The error = measured - accepted
= 99.1°C – 100°C
= – 0.9°C.

19. Accuracy, Precision, and Error
Determining Error
For the boiling-point measurement, the error is 99.1°C – 100°C, or –0.9°C.
The percent error of a measurement is the absolute value of the measured experimental value minus the accepted value divided by the accepted value, multiplied by 100%.
Percent error =
measured - accepted
accepted value
100% x

20. Significant Figures
How do I tell if a digit in a number is significant?
All nonzero digits are significant. (any number 1-9)
All “captured” zeroes are significant. (301)
“Leading” zeroes are never significant. (.005)
“Trailing” zeroes are significant only if the number has a decimal in it.

21. Significant Figures
What is the difference between a measurement of 20 mL and 20. mL? 20.0 mL? mL?

22. Significant Figures How many sig. figs. do these numbers contain?
3.582
0.503
4600
520.
.1000
3250
.0036 60