To become acquainted with simple laboratory measurements and calculations using common laboratory equipment.

Transcription

1 PURPOSE To become acquainted with simple laboratory measurements and calculations using common laboratory equipment. MATERIALS 250 beaker Piper and piper pump Hot plates Meter stick or ruler Balance Ice 100 graduated cylinder 50 erlenmeyer flask. Thermometer Wooden and metallic blocks, marbles Pennies CHEMICALS Liquids: isopropyl alcohol INTRODUCTION It is essential for the beginning chemistry students to learn how scientific measurements are carried out properly using common equipment. In chemistry as well as in all sciences, measurements are essential. The basic properties that can be measured are length, mass, time, and temperature. Other properties of matter such as volume, area, and density are called derived properties and are expressed as ratios or products of the basic properties. CHEM 1411 Experiment 1 Measuring Techniques and Calculations

2 Derived property Basic properties area length x length volume length x length x length density mass I (length x length x length) = mass I volume Any measured quantity should have a unit. The metric system and the Systeme International (SI) are both used in chemistry. Some of the basic SI units are the meter, the kilogram, and the second. Because the meter and the kilogram are generally too large for convenient use in the chemistry laboratory, we use related units such as the centimeter and the gram. Units in the metric system and SI are related to each other by powers of ten and are associated with prefixes such as, pico ), nano (l0-9 ), micro (l0-6 ), milli (l0-3 ), centi 00-2 ), deci (10-1 ), kilo (10 3 ), and mega (10 6 ). In the following experiment, you will become familiar with how length, mass, volume, and temperature measurements are carried out, and how the uncertainty in measured quantities are given through the use of significant figures. All measurements have an inherent uncertainty, which will depend on the instrument used to make the measurement. If a 5 gram object is weighed on a balance which has an uncertainty in measurement of ±0.01 g, we would report the weight as 5.00 g. The uncertainty in the measurement is assumed to be ± 1 of the last digit in the recorded value. If the mass is reported as 5.0 g, it is assumed that the uncertainty is ±0.1 g. This experiment has been designed to acquaint you with several types of measurements and measuring devices: The length of an object is one of its basic physical properties. In this experiment you will learn about the metric system of measurements using a ruler or meter stick. The mass of a liquid sample is measured by taking the difference between the mass of the container plus the liquid and the mass of the empty container. The mass of a chemical solid can be obtained similarly by placing the sample in a pre-weighed container. In this experiment you will practice using a triple beam balance to measure the mass. The volume of a liquid can easily be determined using graduated cylinders or pipers. The volume of regularly shaped solids (cube, rectangle, cylinder etc.) can be determined by measurement of the dimensions of the solid using the appropriate formula for the volume. However, if the shape is irregular, a convenient estimate of the volume is obtained from the volume of liquid displaced by the solid. In this experiment you will learn how to measure volume using a graduated cylinder and pipet and also learn how to calibrate the pipet. Thermometers are devices to measure temperature. In this experiment a thermometer will be calibrated comparing the measured boiling point and freezing point of water with literature values. 2 CHEM 1411 Experiment 1 Measuring Techniques and Calculations

3 w4.;;/ii ;;ii ;;;; ;:;,, ::J ', \,-;;,,-;, \,;;, :J ::J ' : One of the fundamental properties of matter is its density. As an intensive property, density is independent of the mass and the volume of matter. The relationship between density, mass, and volume is as follows: Density = mass/volume or D = M IV Thus for any density determination, the mass and the volume of the matter must be measured. The density of a solid or liquid is usually reported in units of g/ or g/cm 3 Because 1 is equivalent to 1 cm ', these units are interchangeable. The density of a gas is usually reported in units of grams per liter (g/l). Since volume changes with temperature, so does density. For example, the density of water is exactly g/cm 3 at 4 C and is slightly less at room temperature ( g/cm 3 at 25 C). Specific gravity is defined as the ratio of the density of a substance to that of another substance (usually water at 4 C for solids and liquids). Unlike density, specific gravity has no units. Your measurements and calculations in this experiment must follow the rules of precision, accuracy, rounding, and significant figures. Precision, which indicates the agreement between repeated measurements, is often reported as average deviation from the mean. EXPERIMENTAL PROCEDURE A. 1he Measurements of Length - Use of ruler or meter stick I. Use a ruler or meter stick and measure the length, width, and height of a wooden block in centimeters to the nearest millimeter and in inches to the nearest 1Il6 inch. II. For comparison, use dimensional analysis to convert the length in inches to centimeters. Record your measurements and calculations in the report form. III. Calculate the area of the largest face (in cm 2 ) and the volume (in cm 3 ) of the wooden block and record them in the space provided in the report form. You will need the same block for part B. B. 1he Measurement of Mass - Use of balances (scales) I. Measure the mass of three different pennies, and then calculate the average mass of a penny in grams and in milligrams. II. Weigh the wooden block and record your result on the report sheet.. C. 1he Measurement of Volume - Calibration of _pipet I. Obtain about 100 of water in a 250 beaker (about half filled with deionized water). Measure and record the temperature of the water in C. Weigh a clean, dry 25c or 50 Erlenmeyer flask and record the weight. Next use a 10 pipet to transfer 10 of water to the flask. Note: Some pi pets are marked from to 0 to 10 while others are only marked from 0 to 9. Be sure to use the pi pet correctly to measure 10 of liquid. CHEM 1411 Experiment 1 Measuring Techniques and Calculations 3

4 II. Weigh the flask after adding the water and calculate the mass of the water. III. Next, accurately calculate the volume of the water delivered by the pipet from the mass of the water and the density of water at the temperature the measurement was made (see Table 1). IY. Repeat this process three times and calculate: a. The mean (average) volume delivered, Vav = (V1 + V2 + V3) I 3 b. The deviation from the mean, /:),. v = IV measured -Vavl c. The average deviation from the mean, /:),. Vav= (b,. V1 + /:),. V2 +I::!,. V3) I 3 d. Report the volume delivered by pipette, V = Vav +/- /:),. Vav Table 1: Density of pure Water at Various Temperature c c: c; c; (;; ' c. ' c:, c. c;, c:;, ' c;,, D. The Measurement of Temperature - Calibration of Thermometer You will measure the temperature of ice water and boiling water in C. I. Make a slurry of ice and water in a 250 beaker. Be sure to use plenty of ice! Place the thermometer in the slurry and record the temperature after the thermometer reading is constant. II. Heat about I 00 of water on an electric hot plate until it is boiling, and measure the temperature after the thermometer reading is constant. This is your measured boiling point of water. Record this temperature on the report form. III. Since boiling points (but not freezing points) vary with atmospheric pressure, the measured boiling point can be compared to the true or calculated point based on the atmospheric pressure. Record the room pressure in units of mm Hg. Use the boiling point correction formula below to determine the boiling point of water at room pressure: 4 CHEM 1411 Experiment 1 Measuring Techniques and Calculations c c;, c- c c- c c c - c c

5 ,.., Boiling point correction = (760 mmhg- atmospheric pressure) (0.037 C/mmHg) True Boiling point = I 00 C - boiling point correction Example. The normal boiling point of water at 760 mm Hg (I atm) is I00 C. What is the calculated boiling point of water when the atmospheric pressure is mm Hg? ;;;, Boiling point correction = (760 mm Hg mm Hg)(0.037 C/mm Hg) True boiling point = I00.00 C - (+3.87 C) = C E. The Calculation of Density Solid: C Method I. Weigh a given solid (rock, metallic cylinder, or a piece of metal) on a balance to the nearest O.Olg. Fill a I 00- graduated cylinder about half full with tap water. Record the initial volume to the nearest 0.1. Place the object in the cylinder and record the volume after the solid is immersed. Discard the water and repeat this measurement. Calculate the average density of the solid. Method II. Weigh a wooden block on a balance to the nearest 0.0lg. Make sure you zero the balance before proceeding with each measurement. Measure the dimensions of the solid in centimeters (use a ruler or meter stick). Repeat these measurements twice. Determine the volume of the solid from these dimensions. Calculate the average density of the solid. Note: Use your data from Parts A.I and B.III for this calculation. Liquid: Weigh a dry, empty 50- Erlenmeyer flask. Using the provided pi pet, transfer I 0 isopropyl alcohol (rubbing alcohol) into the flask and reweigh. Calculate the density of the alcohol in grams per. CHEM 1411 Experiment I Measuring Techniques and Calculations 5

6 EXPERIMENT - I REPORT FORM INSTRUCTOR A. The Measurement of Length I. Length Width Height cm cm cm in in in mm mm mm m m m II. Conversion Calculation: length in inches to centimeters III. Area (L x W) = cm2 (show calculations) Volume (L x W x H) = cm3 (show calculations) B. The Measurement of Mass I. Mass of pennies Average mass of one penny (show calculation) mg IL Mass of wooden block Average mass of wooden block (show calculation) CHEM 1411Experiment1 Measuring Techniques and Calculations 7

7 C. The Measurement of Volume I. Temperature of water C Mass of empty 50- Erlenmeyer flask m = II. Mass of flask + 10 water Mass of 10 water ill= m1=-_g m= m= = m3=-_g III. Calculated volume of water (show calculation) V1= V 2 = V 3 = IV. a) Average volume of water (show calculation) Vav = b) Deviations (11 V) L'.1V1= L'.1V 2 = L'.1V 3 = (show calculation) c) Average deviations (show calculation) d) Volume of water delivered by piper +!- D. The Measure.m.ent of Te.m.perature c; c; G I"""-' ]1 't.:ft F' ;.<'.' G G...!"1 Convert room temperature (from Part C.I) to 0 P and K: (Show calculation for conversion of C to F and K.) oc op K I. Temperature of ice water II. Temperature of boiling water oc oc III. Atnospheric pressure: mm Hg 8 CHEM 1411 Experiment 1 Measuring Techniques and Calculations

8 Corrected boiling point temperature (show calculation) ac E. The Calculation of Density Type of Solid: Method I a) Mass of solid g g b) Initial volume of water c) Final volume of water d) Volume of solid e) Density of solid (Show calculation) g/ g/ f) Average density of solid (Show calculation) / Method II Wooden Block Average mass of solid (Part B.II) Volume of solid (Part A.III) cm3 Average density of solid (Show calculation) /cm 3 CHEM 1411 Experiment 1 Measuring Techniques and Calculations 9

9 Liquid : Identification Mass of empty 50 flask Mass of flask alcohol c;,. (;,. c;;.. c;,. Mass of 10.0 alcohol Volume of alcohol Density of alcohol (Show calculation) / irr irvr r ' r' 0 10 CHEM 1411 Experiment 1 Measuring Techniques and Calculations

10 EXPERIMENT - 1 Pre-Laboratory Review Questions and Exercises Due before lab begins 1. What precautions do we have to follow in this experiment? 2. Define the terms precision and accuracy. 3. Why does one calibrate a thermometer? 4. Name four intensive properties. 5. What is the distinction between density and specific gravity? What is the specific gravity of alcohol having a density of 0.79 g/? 6. Do the following conversions and round your answers to the proper number of significant figures: a) 5 ft, 11 in to cm b) 95.6 kg to lb 7. Calculate the boiling point of water if the atmospheric pressure is 630 mm Hg. CHEM 1411 Experiment 1 Measuring Techniques and Calculations 11

11 8. Calculate the density of a rectangular solid, which has a mass of g. It is 2.30 cm long, 4.01 cm wide, and 1.82 cm high. 9. A 10- pipette delivers 9.99, 10.01, 9.97, and in an experiment. Calculate the mean value and the average deviation from the mean. 12 CHEM 1411 Experiment 1 Measuring Techniques and Calculations

12 r r -4 ";).;I,. ::l ::J " - ".. EXPERIMENT - 1 Post-Laboratory Questions and Exercises Due after completing lab. 1. Complete the following table. 2. 'What is the purpose of repetition in measurements? 3. Calculate the atmospheric pressure if the boiling point of water is 81 C. 4. A perfect cube of aluminum metal was found to weigh g. The density of aluminum is 2.70 g/. 'What are the dimensions of the cube? 5. Using the appropriate table in the Handbook of Chemistry and Physics, report the density of each of the following substances: 0 rjl I 0 a) Copper metal b) 2-Propanol (Isopropyl alcohol) c) Acetic acid (from vinegar) CHEM 1411 Experiment 1 Measuring Techniques and Calculations 13