CHM 101/102 Significant Figures and Density General Chemistry 101/102 Laboratory Manual University of North Carolina at Wilmington Laboratory Manual CHM 101/102 Significant Figures and Density • Purpose To gain an understanding of how to interpret uncertainties in laboratory data. To become familiar with the use of common laboratory equipment and glassware. • Safety Considerations There are no special safety considerations associated with this laboratory exercise other than washing your hands before leaving the lab. Laboratory Manual CHM 101/102 Significant Figures and Density • Accuracy vs. Precision Accuracy – a statement of how close a measured value is to the true or accepted value. Precision – a statement of how reproducible the measured results are. Laboratory Manual CHM 101/102 Significant Figures and Density • Significant Figures The value reported for a measurement is rounded off so that it contains only the digits known with certainty plus one uncertain one which is the last digit. • Significant Figure Rules To determine the number of significant figures in a reported value, start at the left and count all digits beginning with the first non-zero digit. Examples: 0.003040 => 4 sig. fig. 10,200 => 5 sig. fig. Laboratory Manual CHM 101/102 Significant Figures and Density • Significant Figures in Calculations Addition & Subtraction: The number of decimal places in the answer should be equal to the number of decimal places in the value with the fewest decimal places. Example: Laboratory Manual 12.0035 g 3.72 g 0.015 g 15.74 g (4 decimal places) (2 decimal places) (3 decimal places) (2 decimal places) CHM 101/102 Significant Figures and Density • Significant Figures in Calculations Multiplication & Division: The number of significant figures in the answer should be the same as the value with the fewest significant figures. Example: 12.36 mL 4 sig. fig. x 2 sig. fig. 1.7 g mL = 21 g 2 sig. fig. Note: Definitions and counted numbers have an unlimited number of significant figures. Laboratory Manual CHM 101/102 Significant Figures and Density • Procedure – Part 1 a & b Pour approximately 8 mL of water into a 50 mL graduated cylinder. Record the measured volume to the appropriate number of significant figures. Repeat the procedure using a 10 mL graduate cylinder. read from the bottom of the meniscus Laboratory Manual CHM 101/102 Significant Figures and Density • Procedure – Part 1 c & d Use a 10 mL graduated pipet to transfer 8 mL of water to a 50 mL preweighed beaker. Reweigh the beaker and use the mass of water and the density of water to calculate the volume of water transferred. Laboratory Manual CHM 101/102 Significant Figures and Density • Procedure – Part 2 Determine the identity of an unknown metal by measuring its density (mass/volume). The volume of the metal is determined by “displacement”. Laboratory Manual CHM 101/102 Significant Figures and Density • Procedure – Part 2 the difference between these two readings is the volume of the metal Laboratory Manual