Measurements & Calculations

Chapter 2 Chemistry Review

Scientific Method• Logical approach• Observe & collect data (research topic)• Form hypothesis (a proposed explanation)• Test hypothesis (set up & do experiment)• Record & analyze data from experiment• Summarize & make conclusions• Modify, support, or reject hypothesis• Communicate methods & results

Formulating Hypothesis

• Statement must be testable• Can use “If…, then….” statement– If you use Dan’s SureFire Bait, then you’ll always

catch premium bass.• Can be worded as if it were a fact– Fisherman using Dan’s SureFire Bait catch more

premium bass.– More premium bass are reeled in on Dan’s

SureFire Bait.

Planning the Experiment• Independent Variable (I.V.)– What is being tested; what’s different from control

• Dependent Variable (D.V.)– What is being measured as a result of I.V.

• Control group– Those not receiving/ treated with the I.V.– That to which the experimental group is compared

• Experimental group– Those receiving/ treated with the I.V.

• Constants– What is the same in both groups

Experimentation

Control group

• Bait of same appearance

• but not Dan’s

Constants

• Same climate, place, time of day, boat/bank,

• user, rod & reel

Experimentalgroup

• Dan’s SureFire Bait (I.V.)

• average size

Experimental vs. Control

Results [# fish] (D.V)

From Hypothesis to Theory

• Hypothesis is supported by data• Methods & Results are published• Others test hypothesis & verify results• Others relate their hypotheses to yours• Others can build models & predict outcomes

based on the collective hypotheses• The scientific community adopts a new theory

(or expands/ modifies existing one)

International System of Units

• Adopted in 1960 by the General Conference on Weights and Measures

• Has 7 base units– Length (m, meter); Mass (kg, kilogram); Time (s,

second); Temperature (K, kelvin); Amount of substance (mol, mole); Electric current (A, ampere); Luminous intensity (cd, candela)

• All scientists understand & use same system

SI Prefixes

• tera- T 1012 1 000 000 000 000• giga- G 109 1 000 000 000• mega- M 106 1 000 000• kilo- k 103 1 000• hecto- h 102

100• deka da 101 10• Base unit 100 1

SI Prefixes, negative exponents

• Base unit 10 1.000000• deci- d 10-1 0.1• centi- c 10-2 0.01• milli- m 10-3 0.001• micro- m 10-6 0.000 001• nano- n 10-9 0.000 000 001• pico- p 10-12 0.000 000 000 001• femto- f 10-15 0.000 000 000 000 001

Derived Units

• Combinations of base units– Area m2

– Density kg/ m3 ; g/cm3

– Molar mass kg/ mol; g/ mol– Energy kg m2 /sec2 (J, joule)– Volume m3

• Liter = (10 cm)3 = 1000 cm3 = 1000 cc = 1000 mL

Conversion Factors

• Ratio derived from equality of 2 different units– 100 cm/ m; 1000 mm/ m; 1 000 000 mm/ m– 102 x 10-2 = 100 = 1 Just change sign of exponent– 1000 m/ km; 1 000 000 m/ Mm– 103 m/ 103 m = 1; kilo = 103, M = 106; 106/ 106 = 1

• Used in dimensional analysis to solve problem– How many mm are there in 2.3 km?– 2.3 km(1000 m/ km)(1000 mm/ m) = 2.3 x 106 mm

Accuracy vs. Precision

• Accuracy = close to accepted value• Hits the bull’s-eye

• Percentage error% Error= (valueexperimental – valueaccepted)/ valueaccepted

Some error always exists with measurements!!!• Precision = close to one another’s values• All hit same area but not necessarily the bull’s-eye

Significant Figures (S.F.)

• = All digits known + 1 final digit (estimated)• Rules for S.F. – All digits 1 through 9 are S.F. (327)– All zeros between non-zero digits (30207) are S.F.– All zeros at the end of non-zero digits & right of

the decimal point (327.00 or 3.2700) are S.F.– All zeros after non-zero digits & before a decimal

point (327000.) are S.F.

Rounding Significant Figures

• Products or quotients can have no more S.F. than # of S.F. of what’s multiplied or divided

• Sums or differences can have no more decimal places than the least # of decimal places in the problem

• Rounding– Must round answer to correct # of S.F.– If the last digit is > 5, increase by 1 UNLESS THE 5

FOLLOWS AN EVEN NUMBER

Scientific Notation

• Written in the form of M x 10n

• M = all significant figures• n = can be a positive or negative # or zero• (+n) = # is > 10; (-n) = # is between 0 & 1• When n = 0, # is between 1 & 10, not including 10

• Rule of exponents• 10A x 10B = 10A+B

• 10A / 10B = 10A-B

Proportionality

• Directly proportional• x k = y, where k is a constant• As x increases, y increases & vice versa• (Pressure) (constant volume) = Temperature

• Indirectly proportional• x y = k, where k is a constant• As x increases, y decreases & vice versa• (Pressure)(volume) = Constant Temperature