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Chemical Foundations. Scientific method Units of measurement Significant figures Dimensional analysis Temperature Density Classifications of matter. Some Definitions. Science What is demonstrably true about nature Technology
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Chemical Foundations • Scientific method • Units of measurement • Significant figures • Dimensional analysis • Temperature • Density • Classifications of matter
Some Definitions • Science • What is demonstrably true about nature • Technology • The application of science and natural resources to improve human life. (see post-it-note pg.7) • Chemistry • The study of the interaction of matter and energy
Parts of Scientific Method A Theory attempts to explain why it happens A Law summarizes what happens
Dimensional Analysis • For any number (N) N x 1 = N • Identical units in numerator and denominator cancel each other. • 5.00cm x inch/2.54 cm = 1.97 inch • Remember 2.54 cm = inch so we’re essentially multiplying 5.00cm by 1 thus changing only the units and not the magnitude.
Converting prefixes • Multiply by units you wish to convert to divided by units you wish to go from and multiply numerator and denominator by inverse prefix: • Given 2.56 cm Want m • 2.56 cm x 106um/102cm = 104 um • How many um in a mile?
Dimensions, Two Cubes m3 = 103 dm3 = 103 L = 106 cm3 = 106 mL
Accuracy vs. Precision Accuracy is closeness to true value Precision is reproducibility of results Precise but not accurate Not accurate or precise Accurate and precise
Measurement of Volume 20.15 mL Estimate measurements to 1/10 the smallest measuring division Here the smallest division is 0.1 mL
Measurement • Components of Measurement • Numerical quantity • Unit • Name of substance • For example, 20.15 mL water Numerical quantity unit Name of substance
Significant Figures • Equivalent to number of integers in number • Zeros between integers are significant • Leading zeros are never significant • Trailing zeros are significant if there is a decimal point.
Exact Numbers • These numbers have unlimited number of significant figures. • They are a result of countable or defined quantities. • 5 dollars or 12 eggs • Some are exact by definition, ie. 12in/ft or 2.54cm/inch or 5280 ft/mi • Measured quantities are never exact.
Rounding Off • Last step in calculation • Round off to least number of sig. figs. in multiplication or division problems. • Round off to least # digits right of decimal in addition or subtraction • If number beyond significant figure is ≥ 5 round up. If ≤ 5 leave unchanged.
Addition & Subtraction • Round off to least number of digits beyond the decimal point. • 12.0g + 11.93g + 12g + 18.2g = 54.13 ≈ 54g • Notice 12g has no digits beyond the decimal point therefore the result must not also.
Multiplication or division • These calculations have all to do with the least # significant figures in the calculation. • 2.563g/1.5 mL = 1.7086 g/mL ≈ 1.7 g/mL
Addition/Subtraction and Multiplication or Division • If possible do addition/subtraction first to determine significant figures. • (2.2 -2.015)/2.015 x 100% = 0.185/2.015 x 100% = 9.181% ≈ 9% (only one sig. fig.) • If possible round off only at end of calculation. • (40. oF - 32.00 oF)/1.8 oC/ oF = 8/1.8 = 4.444 oC ≈ 4 oC
Careful! • Your calculator may not report the correct significant figures. • 8.00 g ÷ 2.00 mL =4.00 mL • 2.560 x 8.8/275.15 = .082
Density • Mass/volume = kg/L = g/mL = g/cm3 • Constant value for all pure substances at constant temperature. • Water has a density of 1.00 g/mL at 4oC. • Insoluble substances which have density >1g/mL will sink in water while substances <1g/mL will float.
