The use of the analytical balance, and the buret.

Transcription

1 1211L Experiment 1. Density 2015 by H. Patterson Instructor Notes: Students make measurements individually then share data to make the graph. There are four volumetric measurements to be studied; 3.00 ml, 6.00 ml, 9.00 ml and ml. Assign one volume to each student at a bench. This unit introduces the student to basic laboratory procedures, physical properties of matter, and units, Students work independently. Educational Objectives The student will become familiar with: The intensive property density and its units. The use of the analytical balance, and the buret. The techniques of accurate measurement and the determination of experimental error and standard deviation. Pre-Lab: In icollege Safety Goggles and proper shoes must be worn. Lab Notebook Content Your instructor may provide specific instruction for the preparation of your lab notebook. Prior to lab, your notebook should include the title, date, the purpose of the lab and the required data tables. This will provide you with experience in record keeping and proper notebook set up. Sample data tables are in the icollege course in the DENSITY content section. Your instructor may require that you write an experimental procedure and provide other information. Data and observations are to be recorded in the lab notebook as you perform the experiment. Calculations must be workout in detail in the lab notebook at the end of the experiment.

2 Introduction The density of matter is the mass per unit volume in the sample. The units of density are grams per cubic centimeter (g/cm 3 ), used to describe the density of solids and grams per milliliter (g/ml), for the density of liquids. Density = Mass Volume Densities vary with temperature and atmospheric pressure. Most materials expand when heated and thus their densities decrease with increasing temperature. Literature values for density are determined and reported at 20 C. The temperature is always specified when a density is reported. The density of an element or compound is a characteristic physical property and, therefore, is useful in identifying the substance. Unlike liquids and, especially, gases, the densities of most solids are not greatly influenced by small changes in the temperature or pressure. However, the density of a solid may depend somewhat on the method of sample preparation (i.e., whether it has been rolled, cast, or powdered). Volume is the amount of space occupied by matter. Liquid volumes are measured using volumetric glassware (volumetric pipets, burets or graduated cylinders).

3 A solid that has a regular shape may be measured and dimensional analysis used to determine its volume as follows: Cubic solid (cm 3 ) Volume = (edge) 3 Rectangular solid (cm 3 ) Volume = Length x width x height Cylindrical solid (cm 3 ) Volume = π x( radius) 2 x height Spherical solid (cm 3 ) Volume = π (radius) Alternately, the volume of any solid may be determined by measuring the volume of water it displaces. This method requires that a known volume of water, at a known temperature be added to a graduated cylinder. The solid is placed in the graduated cylinder and the apparent volume of water in the cylinder is read at the bottom of the meniscus in the graduated cylinder. The difference in the two volumes is the volume of the solid. Purpose: This laboratory exercise will improve your ability to read a buret to two decimal places, to use the analytical balance, to prepare graphs using experimental data and to recognize linear relationships of the form y=mx + b. Procedure: Clean and dry a small beaker. In this beaker, place no more than ~ 45 ml of the unknown liquid that is to be studied. This liquid is to be kept free of water and other contaminates at all times. At the end of the lab period, it is returned to the stock bottle to be used by other lab sections. Use a clean and dry thermometer to measure the temperature of the liquid and record this value, in degrees Celsius, in your lab notebook. Rinse a buret with no more than 3 ml of the stock liquid. This rinse liquid is discarded from the buret, directly down the sink drain. Close the stop cock on the buret and add the remaining liquid. Adjust the liquid level so that the meniscus is within the calibrated region of the buret. Record the initial buret

4 reading to two decimal places. Good Lab Practice: Always fill a buret by holding it in the sink and use a funnel. Never fill the buret when it is clamped above your eye level; chemicals could splash down in your eyes. After you have dispensed an aliquot don t fill the buret up again unless less than 15 ml of reagent remains. Obtain three empty screw cap test tubes from the supply bench. Dry these tubes with lab tissues. Number the tubes and the caps with a pencil. Weigh each of three tubes with the caps on and record the mass of each in your lab notebook. A tarred 1 weigh dish may be used to contain the tube while it is on the balance pan. Good Lab Practice: You should always use the same lab balance for measurements during a particular experiment. Add your assigned volume of liquid to the test tube by dispensing liquid from the buret. Record the final volume reading on the buret after you make this addition. Fill each of the three tubes, carefully reading and recording the initial and final buret volumes for each addition. 2 Return the cap to its matching tube, making sure that tubes do not leak when tipped on their sides. Weigh each filled tube and record the mass. 1 To tare the plastic weighing dish: Place the dish on the balance pan. Use the zero button on the balance to reset the weight of the dish to zero before adding a test tube. 2 Do not use the volume markings on the test tube to determine the volume of solution added to the tube; they are inaccurate and will introduce errors in your measurements.

5 Data Analysis: All calculations are to be hand written in your lab notebook. For each trial: 1. Calculate the volume of liquid added to each tube: Final Volume(buret) Initial Volume(buret) 2. Calculate the mass of liquid in each tube: Mass of the tube with liquid mass of the empty tube 3. Calculate the average temperature of the liquid: Sum of each temperature reading / number of trials 4. Collate data for the four volumes (3mL, 6mL, 9mL and 12mL) from other students at your bench. Summarize this data in a table. 5. Prepare a graph of mass of the liquid as a function of volume of the liquid using MS EXCEL. The graph must have a title, date, your name, data and all axes labeled to receive full credit. Please see the accompanying online document How to make a graph using MS EXCEL. This document is located at: 6. Use the linear regression function of MS EXCEL to obtain the best straight line through the data. Display the equation of the line and the R 2 value on the graph. Print two copies of the graph on the lab printer. Attach one copy to your lab notebook (staples or glue ok). The other copy is turned in with your lab report. Discussion The relationship between the mass of the liquid and its volume is linear at constant temperature. The graph of the group data should show a straight line, with little scatter and a R 2 value above If the R 2 value is below 0.90 or the data is not linear, discuss the data with the instructor and repeat measurements as needed. The slope of the line drawn through the data is the experimental density of the liquid. Liquids expand in volume when heated, and densities change as the temperature is increased. When reporting a density, include the temperature at which the measurements were made.

6 When your measurements are complete the instructor will give you the true density of the liquid at 20⁰ C. Calculate the relative percent error in your experimentally determined density. Error is the difference between the measured value of the quantity and its true value. Error = True value- measured value Absolute Error is the absolute value of the Error. Absolute Error = True value- measured value Relative Percent Error is the quotient of the Absolute Error divided by the true value. Relative Percent Error = True value- measured value x 100 True value Precision The most common way to specify the precision of a set of measurements is the standard deviation, s, which for a small number of measurements is: 1/2 n S = Σ (xi - xavg ) 2 i=1 (n-1) Where: xi is the individual result xavg n is the average is the total number of measurements

7 Clean Up: Return the liquid to the stock bottle. Check with your instructor if you are uncertain whether your liquid is still usable. Please remember that other classes will also use them. Contaminated liquids may be flushed down the drain with water. Dry the test tubes and return them to the lab bench. Open the buret stop cock and rinse the buret with a small volume (~2 ml) of water. Store it in the lab draw. Return thermometers and buret stands to their storage areas. Wash and dry the beakers and any other glassware that you used. Wipe down the bench countertops. The instructor may designate a bench captain to inspect the lab draws for order and cleanliness. Clean the balances and balance counter tops. Wash your hands. The instructor will excuse you when the cleanup and check out process is complete. Turned in one week after the lab: (except in summer) 1. Yellow lab notebook pages with purpose, procedures, data and calculations completed. 2. One copy of the graph. 3. Summary Reporting Sheet. 4. Other material as required by the instructor.

8 Name: Date: Day: M T W R F S Time: Summary Reporting Sheet: Density of a Liquid Attach this sheet to the top of the yellow lab sheets and other reports your instructor requires Suggested Points Experimental density of the liquid. 15 (report value here ->) Relative percent error in the density of the liquid. (report value here ->) 15 Graph; proper titles, etc. 10 Experimental set up, following procedures efficiently, clean-up of glassware, balances and lab drawers. Notebook preparation as assigned by instructor. Data collection-(recording data directly in the lab notebook in neat and orderly tables with correct units.the instructor will check for: mass of empty tubes (g) mass of tubes with liquids (g) initial and final buret readings (ml) Accurate data reporting; summary data table. --- Calculations (are all calculations written neatly in the lab notebook) 5 10 Total 100 End of Experiment