1. Chapter 2A: Measurements & Calculations West Valley High School
General Chemistry
Mr. Mata
Cartoon courtesy of NearingZero.net

2. Standard 1B
Students will know how to use the Periodic Table to identify metals, nonmetals, & semimetals.

3. Essential Question
How are measurements and units used in chemistry?

4. Section 2-1 Steps in the Scientific Method
1. Observations
- quantitative (numbers) ex: 25 g
- qualitative (descriptive) ex: red
2. Formulating hypotheses
- possible explanation for observation.
3. Performing experiments
- gathering new information to decide whether the hypothesis is valid.

5. Outcomes Over Long-Term
Theory (Model)
- set of tested hypotheses that gives an overall explanation of observations.
Natural Law
- The same observation applies to many different systems.
- Ex: Law of Conservation of Mass

6. Law vs. Theory Law: summarizes what happens in nature. (math equation)
Theory: (model) attempt to explain why it happens(plausible explanation).

7. Units of Measurement Measurement - quantitative observation;
consists of 2 parts
Part 1 - number
Part 2 - unit
Ex: 20 grams or 6.63 x Joules
Quantity: value that has magnitude, size or amount.

8. The Fundamental SI Units (le Système International, SI)
Physical Quantity Name Abbreviation
Mass gram g
Length meter m
Time second s
Temperature Kelvin K
Amount of substance mole mol

9. Derived SI units
Derived unit - obtained from combinations of fundamental units. Ex: volume, density.
Volume - amount of space occupied by an object; units in mL or cm3
Density= mass/Volume
Density: quantity of matter per unit volume; g/mL or g/cm3

10. Conversion Factors
Conversion Factor: ratio from 2 different units; used to convert from one unit to another.
Ex: days or year
1 year days
Temperature conversions:
K=˚C + 273
˚ C= K- 273
1mL= 1 cm3

11. Precision and Accuracy
Accuracy: closeness to true value.
Precision: agreement in several trials.

12. Percent Error
Percent Error = /accepted value – experimental value/ X 100
accepted value

13. Uncertainty in Measurement
A digit that must be estimated is called uncertain.
A measurement always has some degree of uncertainty.

14. Why Is there Uncertainty?
Measurements are performed with instruments.
No instrument can read to an infinite number of decimal places.

15. Rules for Counting Significant Figures
Nonzero integers always count as significant figures (sig figs).
3456 has 4 sig figs.
Zeros -Leading zeros do not count as sig figs.
has only 3 sig figs.

16. Zeros –”captive” zeros always count as sig figs.
16.07 has 4 sig figs.
Zeros -Trailing zeros are sig only if the number contains a decimal point.
9.300 has 4 sig figs.
Exact numbers have an infinite number of sig figs.
1 inch = 2.54 cm, exactly

17. Sig Fig Practice #1
How many significant figures in each of the following?
m  5 sig figs
17.10 kg  4 sig figs
L  5 sig figs
3.29 x 103 s  3 sig figs
cm  2 sig figs
 2 sig figs

18. Scientific Notation
Scientific notation – shorthand for very small or very large numbers.
Ex:
is = 6.5 x 10 4
is = 1.2 x