CHM 101/102 Laboratory Manual Significant Figures and Density General Chemistry 101/102 Laboratory Manual University of North Carolina at Wilmington

CHM 101/102 Laboratory Manual Significant Figures and Density Purpose 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 Safety Considerations  There are no special safety considerations associated with this laboratory exercise other than washing your hands before leaving the lab.

CHM 101/102 Laboratory Manual Significant Figures and Density Accuracy vs. Precision 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.

CHM 101/102 Laboratory Manual Significant Figures and Density Significant Figures 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 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: => 4 sig. fig. 10,200 => 5 sig. fig.

CHM 101/102 Laboratory Manual Significant Figures in Calculations 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. Significant Figures and Density Example: g 3.72 g g g (4 decimal places) (2 decimal places) (3 decimal places) (2 decimal places)

CHM 101/102 Laboratory Manual Significant Figures in Calculations 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. Significant Figures and Density Example: mL x 1.7 g = 21 g mL 4 sig. fig.2 sig. fig.  Note: Definitions and counted numbers have an unlimited number of significant figures.

CHM 101/102 Laboratory Manual Procedure – Part 1 a & b 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. Significant Figures and Density read from the bottom of the meniscus

CHM 101/102 Laboratory Manual Procedure – Part 1 c & d 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. Significant Figures and Density

CHM 101/102 Laboratory Manual Procedure – Part 2 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”. Significant Figures and Density

CHM 101/102 Laboratory Manual Procedure – Part 2 Procedure – Part 2 Significant Figures and Density the difference between these two readings is the volume of the metal