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Chapter 2A : Measurements & Calculations. West Valley High School General Chemistry Mr. Mata. Cartoon courtesy of NearingZero.net. Standard 1B. Students will know how to use the Periodic Table to identify metals, nonmetals, & semimetals. Essential Question.
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Chapter 2A:Measurements & Calculations West Valley High School General Chemistry Mr. Mata Cartoon courtesy of NearingZero.net
Standard 1B • Students will know how to use the Periodic Table to identify metals, nonmetals, & semimetals.
Essential Question • How are measurements and units used in chemistry?
Section 2-1Steps 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.
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
Law vs. Theory • Law: summarizes what happens in nature. (math equation) • Theory: (model) attempt to explain why it happens(plausible explanation).
Units of Measurement • Measurement - quantitative observation; consists of 2 parts • Part 1 - number • Part 2 - unit • Ex: 20 grams or 6.63 x 10-34 Joules • Quantity: value that has magnitude, size or amount.
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
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
Conversion Factors • Conversion Factor: ratio from 2 different units; used to convert from one unit to another. • Ex: 365 days or 1 year 1 year 365 days • Temperature conversions: K=˚C + 273 ˚ C= K- 273 1mL= 1 cm3
Precision and Accuracy • Accuracy: closeness to true value. • Precision: agreement in several trials.
Percent Error Percent Error = /accepted value – experimental value/ X 100 accepted value
Uncertainty in Measurement • A digit that must be estimated is called uncertain. • A measurement always has some degree of uncertainty.
Why Is there Uncertainty? • Measurements are performed with instruments. • No instrument can read to an infinite number of decimal places.
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. • 0.0486 has only 3 sig figs.
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
Sig Fig Practice #1 • How many significant figures in each of the following? 1.0070 m 5 sig figs 17.10kg 4 sig figs 100 890 L 5 sig figs 3.29 x 103 s 3 sig figs 0.0054 cm 2 sig figs 3 200 000 2 sig figs
Scientific Notation • Scientific notation – shorthand for very small or very large numbers. Ex: 65 000 is = 6.5 x 10 4 0.00012 is = 1.2 x 10 - 4
