1. CSSI PGCC Barbara A. Gage
Measurement
CSSI PGCC Barbara A. Gage

2. CSSI PGCC Barbara A. Gage
Measurement
Determined magnitude of a property
Based on a standard (string…)
Must have a unit (which is based on the standard)
Number of digits in a measurement depends on the device used
CSSI PGCC Barbara A. Gage

3. CSSI PGCC Barbara A. Gage
Height, Width, Depth…?
CSSI PGCC Barbara A. Gage

4. Significant Figures
When making a measurement you must record all digits you are sure of and one that is a reasonable estimate (regardless of where the decimal place falls)
The object below is 4.86 cm. You can be certain of the 4 and 8. The 6 is an estimate and can vary +/- 1 between measurements.
CHM PGCC Barbara A. Gage

5. CSSI PGCC Barbara A. Gage
Significant figures
The graduated cylinder at the
right contains mL of liquid.
It is assumed that you can visually
divide the space between lines
into ten parts.
CSSI PGCC Barbara A. Gage

6. CSSI PGCC Barbara A. Gage
Significant Figures
When you encounter a measurement assume that all non-zero digits or zeros between other digits are significant.
dm sig figs
2.04 gal sig figs
52 kg sig figs
CSSI PGCC Barbara A. Gage

7. CSSI PGCC Barbara A. Gage
Significant Figures
Zeros may serve as actual measured values or place holders in estimated measurements. Place holder zeros are NOT significant because they are really not measured values.
Place holder zeros can be removed by expressing the number in scientific notation.
1,000,000 = 1 x = 4.5 x 10-5
CSSI PGCC Barbara A. Gage

8. CSSI PGCC Barbara A. Gage
Significant Figures
120,000 2 significant figures This is an estimate that can be written as 1.2 x significant figures The zeros before and immediately after the decimal point can be eliminated using scientific notation. The last zeros remain because they are actual measurements x 10-3
CSSI PGCC Barbara A. Gage

9. Significant Figures –Addition and Subtraction
When you are adding and subtracting numbers you only count the columns where you are sure of all the values in that column.
CSSI PGCC Barbara A. Gage

10. Significant Figures –Multiplication and Division
When multiplying or dividing two or more values, the answer should contain the number of digits in the value with the least number of significant figures.
sig figs
X sig figs
sig figs (calculator answer)
You can only trust the answer to 3 sig figs so the correct answer is 8.35 x 10-2.
CSSI PGCC Barbara A. Gage

11. CSSI PGCC Barbara A. Gage
Measurement Systems
Everyday measurements in the USA are generally made using the English system.
The scientific community and most other countries use the metric system.
CSSI PGCC Barbara A. Gage

12. CSSI PGCC Barbara A. Gage
Units
English System
Metric System
Length
inch, foot, yard, mile
meter (or metre) (m)
Volume
teaspoon, cup, gallon
liter (or litre) (L)
Mass
ounce, pound, ton
gram (g)
The units in the English system do not have a common
conversion factor.
12 in = 1 ft 3 ft = 1 yd yd = 1 mi
The units in the metric system have a common
factor which is 10. The metric system uses a common
base unit and prefixes to change the size of the unit.
CSSI PGCC Barbara A. Gage

13. CSSI PGCC Barbara A. Gage
Metric Prefixes
CSSI PGCC Barbara A. Gage

14. Converting Measurements
Often it is necessary to convert a measurement made in one unit to another unit.
ex cm = ? m
In the metric system you can just shift the decimal point or set up a conversion factor.
2.45 cm = m
CSSI PGCC Barbara A. Gage

15. Converting Measurements
Using a conversion factor:
100 cm = 1 m
100 cm or m both ratios = 1
1 m cm
CSSI PGCC Barbara A. Gage

16. Converting Measurements
If Baltimore is 35 miles away, how far is it in km?
Using a conversion factor:
1.61 km = 1 mi
CSSI PGCC Barbara A. Gage

17. Converting Measurements
How many mL are in gal?
1.06 qt = 1 L 4 qt = 1 gal
CSSI PGCC Barbara A. Gage

18. CSSI PGCC Barbara A. Gage
Problem…
A gas particle has a velocity of 752 m/s. What is its velocity in mi/hr?
1.61 km = 1 mi
CSSI PGCC Barbara A. Gage

19. CSSI PGCC Barbara A. Gage
Problem…
A synthesis process requires 6.2 fl. oz. of activator for every 2.5 tons of starting material. What is the concentration of activator in the final product in mL/kg?
1 fl oz = 29.6 mL lb = 1 ton lb = 1 kg
CSSI PGCC Barbara A. Gage

20. Accuracy and Precision
Accuracy = how close a result comes to the true value
Precision = reproducibility of a measurement
CSSI PGCC Barbara A. Gage

21. Precision and Accuracy
Consider 3 persons shooting darts at a target…
“a” is precise but
not accurate
“b” is accurate
and precise
“c” is not precise
or accurate
CSSI PGCC Barbara A. Gage

22. Accuracy and Precision
Measurements of the same object made by three students; actual value = cm
Student 1 Student 2 Student 3
14.72 cm cm cm
Precise not accurate Not accurate or precise Accurate and precise
CSSI PGCC Barbara A. Gage

23. CSSI PGCC Barbara A. Gage
Density
Property derived from two measurements, mass and volume
Density = Mass/Volume
Will have a unit that contains both mass and volume such as g/cm3, lb/gal, kg/L
Does not depend on the size of the sample
CSSI PGCC Barbara A. Gage

24. CSSI PGCC Barbara A. Gage
Density
What is the density of a sample of metal that has a mass of g and when placed in 15.0 mL of water causes the level to rise to 22.4 mL?
CSSI PGCC Barbara A. Gage

25. CHM PGCC Barbara A. Gage