Scientific Measurements Chemistry I Mr. Skeen Ch. 3

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

Percent Error Significant Figures Ratio of an error to an accepted value % error = error/accepted value X 100 Error = accepted value – measured value Significant Figures All known digits plus one estimated digit

Rules for recognizing significant figures (sig. fig.) Non-zero numbers are always significant Zeros between significant numbers are always significant All final zeros to the right of the decimal are significant Zeros that act as place holders are not significant Convert to scientific notation Counting #s and defined constants have an infinite # of significant figures

Rules for Rounding If the digit to the immediate right of the sig. fig. is < 5 don’t change the last sig. fig. > 5 round the last sig. fig. up = 5 and followed by a nonzero digit round the last sig. fig. up = 5 and not followed by a nonzero digit, look at the last sig. fig. If it’s odd round up, if it’s even do not change the last sig. fig.

Applying Rounding Rules Addition and Subtraction Answer must have the same # of sig. fig. to the right of the decimal as the value with the fewest digits to the right of the decimal Multiplication and Division Answer must have the same # of sig. fig. as the measurement with the fewest sig. fig.

Units of Measurement SI Units Base Unit Derived unit Systeme International d’Unites 1960 International Committee Base Unit Based on an object or event in the physical world Derived unit A unit defined by a combination of SI base units

Quantity Base Unit Abbreviation Time second s Length meter m Mass kilogram kg Temperature kelvin K Amt. of a Substance mole mol Electric current ampere A Luminous Intensity candela cd

Prefix Symbol Factor giga- G 109 mega- M 106 kilo- k 103 deci- d 1/10, 10-1 centi- c 1/100, 10-2 milli- m 1/1000, 10-3 micro 10-6 nano- n 10-9 pico- p 10-12

Dimensional Analysis Conversion Factor Dimensional Analysis Ratio of equivalent values used to express the same quantity in different units Dimensional Analysis Method of problem-solving that focuses on units

Density Ratio comparing the mass of an object to the volume of that same object. Relationship with temperature and pressure