Lacey_Che131_F2011_Lect-2s

Lacey_Che131_F2011_Lect-2s - Lec2: Measurements and units L...

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Lec 2: Measurements and units 1 Roy A. Lacey, Stony Brook University; Che 131, Fall 2011 Measurements ` Accurate measurements are essential for characterizing UNITS MATTER!! the physical and chemical properties of matter. ` Standardization of the units of measurements is essential. ` Quantitative easy to check ` Easy to agree upon, no personal bias ` The measuring instrument limits how good th t i 2 Roy A. Lacey, Stony Brook University; Che 131, Fall 2011 the measurement is.
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Scientific Method A hypothesis is a tentative and Why are measurements important? testable explanation for an observation or a series of observations. A scientific theory is a general explanation of widely observed phenomena that have been extensively tested extensively tested. Measurements are used to test th i d tb l ih l 3 Roy A. Lacey, Stony Brook University; Che 131, Fall 2011 theories and establish laws SI Units Quantity Base Unit Abbrev. Symbol Length meter m l Mass Tim kilogram secon kg m Time Temp second Kelvin s K t T Amount mole mol n 4 Roy A. Lacey, Stony Brook University; Che 131, Fall 2011
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Prefixes for SI units 5 Roy A. Lacey, Stony Brook University; Che 131, Fall 2011 Scientific Notation 9 A simple way of writing numbers that accommodates values too large or small to be conveniently written in standard decimal notation Sign Exponential 10 b a × μ 2 321 3 21 10 →× 0 1 0 1 10 10 b b xa ya = × ii f i d 3 4000 4 10 3.21 10 6 5720000 5 72 10 01 10 bb xy a a a x + significand 5.72 10 6 0.0000075 7.5 10 0 1 10 6 Roy A. Lacey, Stony Brook University; Che 131, Fall 2011 For addition and subtraction make all of the exponents the same
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Derived units Quantity Name Symbol Area square meter m 2 Volume cubic meter m 3 Force newton N =kg.m/s 2 Pressure pascal Pa = 1N/m 2 Energy joule J = 1 Kg.m 2 /s 2 Power watt W = J/s Electric charge coulomb C Current ampere A = C/s Voltage volt V = W/A = J/C Frequency hert H hertz Hz 7 Roy A. Lacey, Stony Brook University; Che 131, Fall 2011 How good are the measurements? We use two words to describe how good measurements are ` Accuracy how close the measurement is to the actual value ` Precision how well can the measurement be repeated All scientific measurements are subject to error. 9 These errors are reflected in the number of figures reported for the measurement. 9 These errors are also reflected in the observation that two successive measures of the same quantity can be different. 8 Roy A. Lacey, Stony Brook University; Che 131, Fall 2011
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Accuracy vs. Precision ` Accuracy can be true of an individual measurement or the average of several ` Precision requires several measurements before anything can be said about it Good accuracy Poor accurac Poor accurac Random errors: reduce precision Good precision Poor accuracy Good precision Poor accuracy Poor precision Systematic errors: reduce
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This note was uploaded on 03/12/2012 for the course CHEMISTRY 131 taught by Professor Lacey during the Fall '11 term at SUNY Stony Brook.

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Lacey_Che131_F2011_Lect-2s - Lec2: Measurements and units L...

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