Page 1 of 5Introducing 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 2 of 5Example 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 placesPage 3 of 5The 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 = 100||×−TVTVEV 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,