Introducing Measurements in the Laboratory Objectives The objectives of this laboratory are a To use a metric ruler to measure the dimensions of regular geometric shapes and to use these measurements to determine the areas of the shapes b To measure the volume of a sample of water using a graduated cylinder and a beaker in order to compare their precision c To measure the mass of an item using a triple beam balance and an analytical electronic balance in order to compare their precision also to determine the mass of a powder by weighing by difference d To measure the melting point of an unknown solid and identify it using this measured value Background Our knowledge of chemistry and chemical processes largely depends on our ability to obtain correct information about matter Often this information is quantitative in the form of measurements In this lab students will be introduced to some common measuring instruments so that they can practice making measurements and to learn about instrument precision In Part A of this lab a metric ruler will be used to measure length in centimeters cm In Part B a beaker and a graduated cylinder will be used to measure liquid volume in milliliters mL In Part C an electronic balance and a triple beam balance will be to measure mass in grams g In Part D a thermometer will be used to measure temperature in degrees Celsius C Since all measuring devices are subject to some error it is impossible to make exact measurements Scientists record all the digits of a measurement that are known exactly plus the first one that is uncertain These digits are collectively referred to as significant digits Digital instruments such as an electronic balance are designed to limit themselves to the correct number of significant digits and their readings are properly recorded as given However when using analog instruments such as rulers and thermometers the experimentalist is responsible for determining the correct number of significant figures These instruments are properly read to one place beyond the graduations of the scale Example 1 Measuring Length The ruler markings are every 0 1 centimeter The correct reading is 1 67 cm The first 2 digits 1 67 are known exactly The last digit 1 67 is uncertain You may have instead estimated it as 1 68 cm Example 2 Measuring the Volume of a Liquid When measuring liquid volumes the graduated scale must be read from the lowest point of the curved surface of the liquid the liquid meniscus The graduated cylinder markings are every 1 milliliter The correct reading is 30 0 mL The first 2 digits 30 0 are known exactly The last digit 30 0 is uncertain Even though it is a zero it is significant and must be recorded Page 1 of 5 Example 3 Measuring Temperature Here the thermometer markings are every 1 degree The correct reading is 33 6 C The first 2 digits 33 6 are known exactly The last digit 33 6 is uncertain You may have instead estimated it as 33 5 C Note that the measuring devices used in this lab may have different scale graduations than the ones shown in these examples Thus be sure to make it a regular habit to check the scales on all equipment When making measurements it is important to be as accurate and precise as possible Accuracy is a measure of how close an experimental measurement is to the true accepted value Precision refers to the degree of uncertainty in a measurement For example a mass measurement of 48 26 g has an uncertainty of 0 01 g while a measurement of 48 3 g has an uncertainty of 0 1 g Since the measurement of 48 26 g has less uncertainty it is the more precise measurement In general the more decimal places provided by a device the more precise the measurement will be Since measurements are often used in calculations to obtain other values of interest it is important to consider the number of significant figures that should be recorded for the results of such calculations If multiplying or dividing measured values the result should be reported with the lowest number of significant figures used in the calculation If adding or subtracting measured values the result should be reported with the lowest number of decimal places used in the calculation Example 4 Significant Figures in Calculated Values a A student runs 18 752 meters in 54 2 seconds Calculate his average velocity or speed velocity distance time 18 752 m 54 2 s 0 345978 m s from calculator 0 346 m s to 3 significant figures b The mass of a glass is measured to be 12 466 grams If 10 33 grams of water are added to this glass what is the total combined mass total mass 12 466 g 10 33 g 22 796 g from calculator 22 80 g to 2 decimal places Page 2 of 5 The temperature that will be measured in this lab is the melting point of an unknown solid Melting point is a physical property When a solid is heated continuously a point will eventually be reached where it undergoes a physical change and becomes a liquid The temperature at which liquid first appears is defined as the melting point of that substance Since all pure substances have unique melting points a measured melting point can be used to identify an unknown substance by comparing it with a list of known substances and their accepted true melting points The accuracy of a measured value such as a melting point may be evaluated by a calculation of percent error Percent error is a common way of reporting how close a measured experimental value EV is to the true value TV Percent Error EV TV 100 TV Accurate measurements will typically have low percent errors of 5 Procedure Safety In Part D you will be heating a solid powder and several pieces of equipment with an open Bunsen burner flame Exercise extra caution while using the Bunsen burner and please remember that the heated items will be very hot to the touch Materials and Equipment Metric ruler shape sheet electronic balance large test tube 100 mL beaker 100 mL graduated cylinder triple beam balance 250 mL Erlenmeyer flask electronic balance sugar Bunsen burner thermometer 400 mL beaker stand and ring clamp small watch glass wire gauze capillary tube latex tubing scoopula and unknown solids Part A Measuring the Dimensions of Regular Geometric Shapes 1 Check out a ruler from the stockroom 2 Obtain a shape sheet from your instructor and then use the ruler to measure the dimensions of the two geometric shapes on it Measure the length and width of the rectangle and the diameter of the circle Record these measurements on your report form 3 When finished return the ruler