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Chemistry Chapter 2 Units of Measurement SI Units Base Units The updated metric system Based on factors of ten Time - second Length - meter Mass – kilogram Derived Units – combination of base units Volume – space occupied by an object Density – mass per unit volume Density= mass/volume Temperature Scientific Notation and Dimensional Analysis Scientific Notation Expresses numbers as multiple of two factors Example: 6.02x1023 Adding/subtracting – FIRST thing you must do is make sure exponents are the same #1 – number between 1 and 10; #2 – ten raised to a power Example: 15.6x106 + 0.165x108 = ? 1.56x107 + 1.65x107 = 3.21x107 Multiplying/dividing – also a two-step process Multiplying - #1 – multiply first factors; #2 add exponents Example: (2x103) x (3x102) = 6x105 Dividing - #1 – divide first factors; #2 subtract divisor exponent from dividend exponent Example: (9x108) / (3x10-4) = 3x1012 Dimensional analysis – problem-solving method that focuses on units Conversion factors Ratio of equivalent values Used to express different units Used when you need to change units How reliable are measurements? Accuracy – how close a measured value is to an accepted value Precision – how close a series of measurements are to one another How repeatable it is Percent error – ratio of error to accepted value How “right” it is %error = error/accepted value x 100 Significant figures – include all known digits plus one estimated digit Non-zero numbers are ALWAYS significant Example: 72.3 has three Zeros between non-zero numbers are ALWAYS significant All final zeros to the right of the decimal place are significant 6.20 has three Zeros that act as placeholders are NOT significant; convert to scientific notation to remove the placeholder zeros 60.5 has three 0.0253 and 4320 each have three Counting numbers and defined constants have an infinite number of significant figures 6 desks 60 s = 1 min Rounding Addition/subtraction Answer should have no more sig figs than the data with the fewest sig figs Rules Answer must have same number of digits to right of decimal as value with fewest digits to right of decimal Line up decimals, then perform the math Multiplication/division Answer must have same number of sig figs as measurement with fewest sig figs Representing Data Graphing Circle (pie) graph – shows parts of a whole Bar graph – shows how a quantity varies with factors such as time, etc. Line graph – points represent intersection of data for 2 variables Independent variable on x-axis; dependent variable on y-axis If best fit line is straight (linear) – directly related Line rises to right – positive slope Line sinks to right – negative slope Slope = (y2 – y1)/(x2 – x1) Interpreting graphs Interpolation – educated guess of data that falls between measured points on a line graph Extrapolation – extending the line beyond the plotted points and estimating values for variables