Experiment 1 Introduction to Some Laboratory Measurements

Transcription

1 Experiment 1 Introduction to Some Laboratory Measurements Introduction In this experiment you will familiarize yourself with the English & metric systems of measurement, weigh with a centigram balance, and study the operation of a laboratory burner. Volume will be measured directly with volumetric devices such as a graduated cylinder and a buret or calculated from dimensions if measuring a regular geometric shape such as a cylinder or rectangular bar. Density is found by dividing mass by volume. In this experiment, the density of water is determined. Procedure Part 1: Metric to English system of measurement using a ruler. Using either a meter stick or a centimeter ruler, measure the length and width of this page to the nearest 0.1 mm. After taking the measurements, convert all lengths to the specified units on the report sheet. Be sure to show all your work for the conversions. Next determine the volumes of test tubes by two different methods. In method one, use the cm ruler to measure inside diameters and lengths of two different test tubes to the appropriate degree of precision. (When measuring the height (h) of the inside of the tube, measure to where you think the curvature begins near the bottom of the tube.) If all your test tubes are the same, borrow one of a different size., Do not include the thickness of the glass in the reading. Now calculate the volume of each test tube using the formula: V = r 2 h keeping in mind = 3.14; r (the radius) = ½ the diameter. Round off your final calculation to the appropriate number of significant figures. Method two, measure the volumes of the same two test tubes, by filling them with water and pouring the water into a 25 or 50 ml graduated cylinder. Read the volume on the cylinder by estimating to the nearest 0.1 ml. What did you observe about the volume determined by the two different methods? Part II: Weighing objects using the balance. Your instructor will explain the operation of the balance you will use in your laboratory work. Weigh a clean dry 100 or 150-mL beaker to 2 decimal places using the centigram balance. Fill a 50-mL buret with tap water and mount it in a holder on a ring stand. Measure out about 20 ml of tap water into the weighed beaker. Read the initial and final volumes of the buret to the nearest 0.01 ml. Now re-weigh the beaker containing the water. Do not spill water on the pan! Record your measurements, both volume and associated weight, in your lab note book with the correct number of significant figures and correct units. Repeat the same steps for a sample of about 25 ml of tap water. It does not matter how much water you use, it ONLY matters that you record the actual volume. Technique Alert! Use the same balance for all your weighings to reduce the chance of systematic error. There is a direct relationship between an object s mass and its volume, called density. Using the formula d=m/v, where m stands for mass measured in grams and V stands for volume measured in milliliters, calculate the density of water for each of your trials. What did you discover? What do you

2 believe would happen if you measured out 15 ml of water, weighed it, and repeated the same calculations? Be sure to show all work for the conversions. Look up the accepted value for the density of water at a room temperature of 20 o C. (You can check the CRC Handbook for the value at 20 o C or use the internet). Report the value to three significant figures. Calculate the % error in your value for the density of water. The general formula for calculating a percent error is: % error = [ experimental value accepted value ] x 100 accepted value Keep the + or sign in the answer. It indicates whether your result is high or low. Part III. Temperature: The Thermometer & the Bunsen Burner. The "Bunsen" burner is commonly used for laboratory heating operations. Now you will learn to use the Bunsen burner. There are several types of burners, differing from each other in details of construction, but each has a gas inlet, gas control knob, a tube for mixing air and gas, and an opening near the base of the tube for introduction of air. Find these parts on your burner and make a neat labeled sketch of the burner. Light the burner by partially closing the air vent and the gas control knob, turning on the gas valve on the bench, and bringing a match in toward the top of the burner from the side, touching the top of the tube. Close the air hole at the base of the tube almost completely. This produces a luminous flame. Take a long copper wire and place the tip of the wire in the luminous flame, take note of your observations in your lab notebook. Next, open the air hole about half way and adjust the flame by adding more gas until the flame is about 8 to 10 cm high and the flame is blue. Take the same copper wire and place it in the blue flame and once again, record your observations in your lab notebook. Lastly, adjust the air intake such that the flame exhibits two blue tones, an inner light blue flame and an outer darker blue flame. Place your copper wire in several areas of the flame and record your observations. Can you confirm that a blue flame is hotter than an orange flame? Now it is time to use the Bunsen burner. Set up a ring stand with a ring clamp & wire gauze. Fill a 250 ml beaker half way with tap water and add two boiling stones. Place the beaker of water on the wire gauze and set up the Bunsen burner next to the ring stand. Measure the initial temperature of the room temperature water to the nearest 0.1 o C. Relight the Bunsen burner. If you are to boil water, which type of flame should you use? Place the lit Bunsen burner under the wire gauze holding the beaker of water. Heat the water to boiling. After 5 minutes of full boiling, measure the temperature of the water. Record that measurement in your lab notebook. Remove the thermometer and wait another 5 minute before remeasuring the temperature of the boiling water. Record that value. Repeat after another 5 minutes have past. Look up the boiling point of water and compare to your own reading. This technique is referred to as calibration of the thermometer. Is your thermometer accurate? The boiling point of any pure substance is characteristic of that material and can be used to identify that substance.

3 Experiment 1 Introduction to Some Laboratory Manipulations Data and Report Show all calculations on this report sheet. Name Part I. A. Length of page: cm mm km in (show calculations in space below) Width of page: cm mm km in (show calculations in space below) Data using the ruler: (show units) Test tube # diameter (d) height (h) volume (cm 3 ) Volume (L) Show the set-up of one volume calculation. Data using the graduated cylinder (show units) test tube #1 : volume test tube # 2: volume: What can you conclude about the two different methods for measuring volume? Which method, the measurement of dimensions, or the measurements of volume with a graduated cylinder, do you think gives a better result? Explain why. (recall the assumption about measuring the test tube as a cylindrical object).

4 Part II. Initial buret volume Final buret volume Volume of water Weight of beaker and water Weight of beaker Weight of water Room Temperature of water Density of water (calculated) Density of water from resource book % error in density Show setup of calculations of density and % error for one trial: o C g/ml o C Part III Sketch of burner (Label parts): What color is the flame when the oxygen intake has been minimized? Describe below what you observe when you placed the copper wire in the three different flames. What color is the hottest flame? Use the CRC handbook or other resource material to look up the melting point of copper and record that value here:. What can you conclude about the actual temperature of the hottest flame?

5 Temperature Manipulations Room Temperature of water o C Temperature of boiling water o C o C o C Theoretical temperature of boiling water from handbook Is your thermometer accurate? o C Questions 1. An empty graduated cylinder weighs g. 35 It was filled to the extent shown in this partial segment of a graduated cylinder with a liquid called carbon tetrachloride (CCl 4). The density of CCl 4 is 1.61 g/ml How many grams does the cylinder weigh together with the CCl 4? a) Look at the densities of the elements on the periodic chart given in the Introduction to Measurements resource in the Chem 65 lab materials, (online). What general trend, an increase or decrease in density, do you see as you look from the top to bottom in a given group of elements (that is, along a vertical column, for example Li to Fr)? b) What general trend do you see in the densities of elements on the left half of the periodic table, as you read from left to right in the same row, (or in other words, across a period, for example from K to Cr)? 3. a) What is the densest element? (refer to the density table in the Introduction to Measurements) b) If you have a coin made of this element that weighs g, and has a thickness of 4.00 mm, what would the diameter of the coin be, in cm? (A coin is shaped like a cylinder, so V = r 2 h where h = 4.00 mm) karat gold contains 50% gold and 50% copper. What do you think the density of 12 K gold should be? (Hint, look up the densities of pure copper and pure gold.)