Chemistry Chapter 2 MeasurementsandCalculations

Steps in the Scientific Method 1.Observations - quantitative - qualitative 2.Formulating hypotheses - possible explanation for the observation 3.Performing experiments - gathering new information to decide whether the hypothesis is valid whether the hypothesis is valid

Outcomes Over the Long-Term Theory (Model) - A set of tested hypotheses that give an overall explanation of some natural phenomenon. overall explanation of some natural phenomenon. Natural Law - The same observation applies to many different systems different systems - Example - Law of Conservation of Mass

Law vs. Theory A law summarizes what happens  A law summarizes what happens  A theory (model) is an attempt to explain why it happens.

Nature of Measurement Part 1 - number Part 2 - scale (unit) Examples: 20 grams 6.63 x Joule seconds Measurement - quantitative observation consisting of 2 parts consisting of 2 parts

The Fundamental SI Units (le Système International, SI)

SI Units

SI Prefixes Common to Chemistry PrefixUnit Abbr.Exponent Kilok10 3 Decid10 -1 Centic10 -2 Millim10 -3 Micro  10 -6

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 Which of these balances has the greatest uncertainty in measurement?

Precision and Accuracy Accuracy refers to the agreement of a particular value with the true value. Precision refers to the degree of agreement among several measurements made in the same manner. Neither accurate nor precise Precise but not accurate Precise AND accurate

Types of Error Random Error (Indeterminate Error) - measurement has an equal probability of being high or low. Systematic Error (Determinate Error) - Occurs in the same direction each time (high or low), often resulting from poor technique or incorrect calibration.

Rules for Counting Significant Figures - Details Nonzero integers always count as significant figures has 4 sig figs.

Rules for Counting Significant Figures - Details Zeros - Leading zeros do not count as significant figures has 3 sig figs.

Rules for Counting Significant Figures - Details Zeros - Captive zeros always count as significant figures has 4 sig figs.

Rules for Counting Significant Figures - Details Zeros Trailing zeros are significant only if the number contains a decimal point has 4 sig figs.

Rules for Counting Significant Figures - Details Exact numbers have an infinite number of significant figures. 1 inch = 2.54 cm, exactly

Sig Fig Practice #1 How many significant figures in each of the following? m  5 sig figs kg  4 sig figs 100,890 L  5 sig figs 3.29 x 10 3 s  3 sig figs cm  2 sig figs 3,200,000  2 sig figs

Rules for Rounding Off 1.If the digit to be removed a.is less than 5, the preceding digit stays the same for example, 1.33 rounds to 1.3. b.is equal to or greater than 5, the preceding digit is increases by 1. For example, 1.36 rounds to 1.4 and 3.15 rounds to 3.2.

Rules for rounding off 2. In a series of calculations, carry the extra digits through t the final results and then round off. This means that you should carry all of the digits that show on your calculator until you arrive at the final number ( the answer) and then you round off, using the procedure in rule 1.

Rules for Significant Figures in Mathematical Operations Multiplication and Division: # sig figs in the result equals the number in the least precise measurement used in the calculation x 2.0 =  13 (2 sig figs)

Sig Fig Practice # m x 7.0 m CalculationCalculator says:Answer m 2 23 m g ÷ 23.7 cm g/cm g/cm cm x cm cm cm m ÷ 3.0 s m/s240 m/s lb x 3.23 ft lb·ft 5870 lb·ft g ÷ 2.87 mL g/mL2.96 g/mL

Rules for Significant Figures in Mathematical Operations Addition and Subtraction: The number of decimal places in the result equals the number of decimal places in the least precise measurement =  18.7 (3 sig figs)

Sig Fig Practice # m m CalculationCalculator says:Answer m 10.2 m g g g 76.3 g 0.02 cm cm cm 2.39 cm L L L709.2 L lb lb lb lb mL mL 0.16 mL mL

Direct Proportions  The quotient of two variables is a constant  As the value of one variable increases, the other must also increase  As the value of one variable decreases, the other must also decrease  The graph of a direct proportion is a straight line

Inverse Proportions  The product of two variables is a constant  As the value of one variable increases, the other must decrease  As the value of one variable decreases, the other must increase  The graph of an inverse proportion is a hyperbola

Converting from one Unit to Another Step one To convert from one unit to another, use the conversion that relates to that statement. 1m=100cm Step twp Choose the appropriate conversion factor by looking at the direction of the required change. Make sure the unwanted units are on the bottom, cancel out. 12in x 2.54cm/1in

Converting from One Unit to Another Step 3 Multiply the quantity to be converted by the conversion factor to give the quantity with the desired units. 12in x 2.54cm/1in= cm Step 4 Check that you have the correct number if significant figures. 30 cm

Converting from One Unit to Another Step 5 Ask whether your answer makes since.