PART I: MEASURING MASS

Transcription

1 Chemistry I Name Dr. Saulmon School Year Laboratory 1 Measuring Mass, Volume, and Temperature Monday, August 25, 2014 This laboratory is broken into three parts, each with its own introduction, procedure, data table, and conclusions. PART I: MEASURING MASS OBJECTIVE The measurement of mass is fundamentally important in any chemistry laboratory. In this laboratory, you will learn how to use the balances that we use in this classroom. INTRODUCTION The accurate determination of mass is one of the most fundamental techniques for students of experimental chemistry. Mass is a direct measure of the amount of matter in a sample of substance; that is, the mass of a sample is a direct indication of the number of atoms or molecules the sample contains. Since chemical reactions occur in proportion to the number of atoms or molecules of reactant present, it is essential that the mass of reactant used in a process be accurately known. There are various types of balances available in the typical college general chemistry laboratory (balances measure mass, scales measure weight). Such balances differ in their construction, appearance, operation, and in the level of precision they permit in mass determinations. Our laboratory uses the triple-beam balance. We ll begin the laboratory with a demonstration of its use. There are some general points to keep in mind when using any laboratory balance: 1. Always make sure that the balance gives a reading of grams when nothing is present on the balance pan. Adjust the zero knob if necessary. If the balance cannot be set to read zero, ask Dr. Saulmon for help. 2. All balances are damaged by moisture. Do not pour liquids in the immediate vicinity of the balance. Clean up any spills immediately from the balance area. 3. No reagent chemical substance should ever be weighed directly on the pan of the balance. Ideally, reagents should be weighed directly into the beaker or flask in which they are to be used. Plastic weighing boats may also be used if several reagents are required for an experiment.

2 4. Procedures are often written in such terms as weigh 0.5 grams of substance (to the nearest milligram). This does not mean that exactly grams of substance is needed. Rather, the statement means to obtain an amount of substance between and grams, but to record the actual amount of substance taken (e.g., grams). Unless a procedure states explicitly to weigh out an exact amount (e.g., weigh out exactly 5.00 grams of NaCl ), you should not waste time trying to obtain an exact amount. However, always record the amount actually taken to the precision of the balance used. 5. For many types of balances, there are likely to be small errors in the absolute masses of objects determined with the balance, particularly if the balance has not been properly calibrated or has been abused. For this reason, most mass determinations in the laboratory should be performed by a difference method: an empty container is weighed on the balance, and then the reagent or object whose mass is to be determined is added to the container. The resulting difference in mass is the mass of the reagent or object. Because of possible calibration errors, the same balance should be used throughout a procedure. PROCEDURE 1. Record all data and observations directly on the result form in ink. 2. Listen to the demonstration of use of the balance by Dr. Saulmon. 3. You will be provided with a small object whose mass you will determine. The objects are coded with an identifying number. Record the identification code on the result form. 4. Label the two small beakers A and B. Write directly on the glass do not write on the white space if there is one. 5. Determine and record the masses of beakers A and B. The determination of the beakers masses should be to the level of precision permitted by the particular balance you are using. 6. Transfer the unknown object to Beaker A, and determine the combined mass of Beaker A and the object. Record. Determine the mass of the unknown object by subtraction. Record. 7. Transfer the unknown object to Beaker B, and determine the combined mass of Beaker B and the object. Record. Determine the mass of the unknown object by subtraction. Record. 8. Although the two beakers used for the determination undoubtedly had different masses when empty, you should have discovered that the mass of the unknown object was the same, regardless of which beaker it was weighed in. 9. When the group across the bench from you has gotten to this step, switch places with them and determine the mass your unknown object on their balance using the procedure already described. Only use Beaker A this time. 10. Compare the masses of the object as determined on the two different balances. Is there a difference in the masses determined for each object? In future experiments, always use the same balance for all mass determinations in a given experiment. 11. Compare the results of your mass determinations of the unknown objects with Dr. Saulmon s results. If there are any major discrepancies, take some time to discuss it and make sure you are using the balances correctly.

3 PART II: MEASURING VOLUME OBJECTIVE Much of what we know about the physical world has been obtained from measurements made in the laboratory. Skill is required to design experiments so that careful measurements can be made. Skill is also needed to use lab equipment correctly so that errors can be minimized. At the same time, it is important to understand the limitations of scientific measurements. The purpose of this laboratory is to introduce you to reading volumes of liquids and to give you practice in recording of values that have the correct number of significant figures. INTRODUCTION Experimental observations often include measurements of mass, length, volume, temperature, and time. There are three parts to any measurement: Its numerical value The unit of measurement that denotes the scale An estimate of the uncertainty of the measurement The numerical value of a laboratory measurement should always be recorded with the proper number of significant figures. The number of significant figures depends on the instrument or measuring device used and is equal to the digits definitely known from the scale divisions marked on the instrument plus one estimated or doubtful digit. The last, estimated, digit represents the uncertainty in the measurement and indicates the precision of the instrument. Measurements made with rulers and graduated cylinders should always be estimated to one place beyond the smallest scale division that is marked. If the smallest scale division on a ruler is centimeters, measurements of length should be estimated to the nearest 0.1 cm. If a ruler is marked in millimeters, readings are usually estimated to the nearest 0.2 or 0.5 mm, depending on the observer. The same reasoning applies to volume measurements made using a graduated cylinder. A 10-mL graduated cylinder has major scale divisions every 1 ml and minor scale divisions every 0.1 ml. It is therefore possible to read the volume of a liquid in a 10-mL graduated cylinder to the nearest 0.02 or 0.05 ml. Three observers might estimate the volume of liquid in the 10-mL graduated cylinder shown below as 8.32, 8.30, or 8.33 ml. These are all valid readings. It would NOT be correct to record this volume of liquid as simply 8.3 ml. Likewise, a reading of ml would be too precise. Accuracy and precision are two different ways to describe the error associated with measurement. Accuracy describes how correct a measured or calculated value is, that is, how close the measured

4 value is to an actual or accepted value. The only way to determine the accuracy of an experimental measurement is to compare it to a true value if one is known! Precision describes the closeness with which several measurements of the same quantity agree. The precision of a measurement is limited by the uncertainty of the measuring device. Variations among measured results that do not result from carelessness, mistakes, or incorrect procedure are called experimental errors. Experimental error is unavoidable. The magnitude and sources of experimental error should always be considered when evaluating the results of an experiment. PROCEDURE There are six graduated cylinders, each labeled and each containing a specific quantity of liquid to which some food coloring has been added to make the volume easier to read. 1. Record the capacity and the major and minor scale divisions of each graduated cylinder in the Data Table. 2. Measure the volume of liquid in each cylinder and record the results in the Data Table. Remember to include the units and the correct number of significant figures. 3. Estimate the uncertainty involved in each volume measurement and enter the value in the Data Table.

5 PART III: MEASURING TEMPERATURE OBJECTIVE In this experiment, you will check your thermometer for errors by determining the temperatures of two stable reference equilibrium systems. INTRODUCTION The most common laboratory device for the measurement of temperature is, of course, the thermometer. The typical thermometer used in the general chemistry laboratory permits the determination of temperatures from -20 C to 120 C. Most laboratory thermometers are constructed of glass and so they are very fragile. Mercury was traditionally used as the temperature sensing fluid in thermometers. However, if the thermometer was broken, poisonous mercury may be released, which is a problem. If you ever do use mercury thermometers, any mercury spills must be reported immediately to the lab instructor. Because of the danger of mercury, other liquids (such as colored alcohol) are commonly used in student-grade laboratory thermometers. The typical laboratory thermometer contains a bulb (reservoir) of mercury or other liquid at the bottom; it is this portion of the thermometer that actually senses the temperature. The glass barrel of the thermometer above the liquid bulb contains a fine capillary opening in its center, into which the liquid rises as it expands in volume when heated. The capillary tube in the barrel of the thermometer has been manufactured to very strict tolerances, and it is very regular in cross-section along its length. This ensures that the rise in the level of liquid in the capillary tube as the thermometer is heated will be directly related to the temperature of the thermometer s surroundings. Although the laboratory thermometer may appear similar to the sort of clinical thermometer used for determination of body temperature, the laboratory thermometer does not have to be shaken before use. Medical thermometers are manufactured with a constriction in the capillary tube that is intended to prevent the liquid level from changing once it has risen. The liquid level of a laboratory thermometer, however, changes immediately when removed from the substance whose temperature is being measured. For this reason, temperature readings with the laboratory thermometer must be made while the bulb of the thermometer is actually present in the material whose temperature is being determined. To check whether or not your thermometer is reading temperatures correctly, you will calibrate the thermometer. To do this, you will determine the reading given by your thermometer in two systems whose temperature is known with certainty. If the readings given by your thermometer differ by more than one degree from the true temperatures of the systems measured, you should exchange your thermometer and then calibrate the new thermometer. A mixture of ice and water has an equilibrium temperature of exactly 0 C and will be used as the first calibration system. A boiling-water bath, whose

6 exact temperature can be determined from the day s barometric pressure, will be used as the second calibration system in this experiment. SAFETY PRECAUTIONS This is our first laboratory with serious safety considerations. Please be completely aware of all requirements below. Safety eyewear must be worn at all times while you are in the laboratory, whether or not you are working on an experiment. Know fully how to use a Bunsen burner and how to act around the burner. Dr. Saulmon will demonstrate the procedure for lighting and adjusting the burner. Regardless of your past experience with burners, you must do exactly as he instructs. o SHOULD THE BUNSEN BURNER GO OUT, IMMEDIATELY TURN OFF THE GAS AT THE GAS OUTLET VALVE. If you wish to turn off the burner, do so by turning off the gas at the gas outlet valve first, then close the needle valve and barrel. Never reach over an exposed flame. o Never leave a lit burner unattended. Never leave anything that is being heated or is visibly reacting unattended. Always turn the burner or hot plate off when not in use. o Heated metals, glass, and ceramics remain very hot for a long time. They should be set aside to cool on a trivet and then picked up with caution. Use tongs or heat-protective gloves if necessary. Determine if an object is hot by bringing the back of your hand close to it prior to grasping it. If a thermometer or other piece of glassware becomes broken, immediately notify Dr. Saulmon. Do not try to clean up the shards yourself. PROCEDURE 1. You will be provided with a thermometer inserted into a rubber stopper. A viscous, slick liquid known as glycerin was used as a lubricant to put the thermometer into the stopper you may notice some on the surface of the stopper. Make sure the rubber stopper is completely above the 100 C mark on the thermometer. 2. Fill a 400-mL beaker with ice, and add tap water to the beaker until the ice is covered with water. Stir the mixture with a stirring rod for 30 seconds. 3. Clamp the rubber stopper to a ring stand so that the bottom 2-3 inches of the thermometer is dipping into the ice bath. Make sure that the thermometer is suspended freely in the ice bath and is not touching either the walls or the bottom of the beaker. 4. Allow the thermometer to stand in the ice bath for 2 minutes, and then read the temperature indicated by the thermometer to the nearest 0.2 degree. Remember that the thermometer must be read while still in the ice bath.

7 5. Allow the thermometer to warm to room temperature by resting it in a safe place on the laboratory bench. 6. Set up an apparatus for boiling as indicated in the Figure on the next page, using a 100-mL beaker containing approximately 75 ml of water. Add 2-3 boiling chips to the water. Do not yet put the thermometer in the water. 7. Light the burner and adjust the flame as instructed by Dr. Saulmon. 8. Heat the water to boiling. 9. Once the water is boiling, suspend the thermometer so that it is dipping halfway into the water. Make certain the thermometer is not touching the walls or bottom of the beaker. Allow the thermometer to stand in the boiling water for 2 minutes; then record the thermometer reading to the nearest 0.2 C. 10. A boiling-water bath has a temperature near 100 C, but the actual temperature of boiling water is dependent on the barometric pressure, which in turn is dependent on altitude and changes in the weather. The current barometric pressure will be written on the board write it in your data table. After the lab, we will use a handbook of chemical data to determine that actual boiling point of water at that pressure. 11. Cleanup consists of: a. Turning the gas valve off and disconnecting the burner hose from the nozzle. b. Removing the beaker from the ring stand and allowing it to cool. REMEMBER, A HOT BEAKER DOES NOT LOOK ANY DIFFERENT FROM A COOL BEAKER. Pour the water out and set the beaker in a safe place to cool. c. Make sure all water is dried from the bench surface and all the equipment is neatly organized on the bench.

8 Laboratory I: Measuring Mass Name RESULTS/OBSERVATIONS Your balance: Beaker A Beaker B Mass of empty beaker ID number of first object Mass of beaker plus object Mass of object itself Other group s balance: Beaker A Mass of empty beaker A ID number of object Mass of beaker plus object Mass of object itself Difference in mass between the two balances (Beaker A only) ID number of object Difference in mass

9 POST-LAB QUESTIONS 1. Why is it important always to use the same balance during the course of an experiment? Explain using examples from your own data. 2. Why should reagent chemicals never be weighed directly on the pan of the balance? 3. How might you account for any differences detected when measuring the mass of the same object on different balances?

10 Laboratory II: Measuring Volume Name RESULTS/OBSERVATIONS DATA TABLE: VOLUME MEASUREMENTS Graduated Cylinder Capacity Major Scale Divisions Minor Scale Divisions Volume of Liquid Estimated Uncertainty A B C D E F POST-LAB QUESTIONS 1. What is the relationship between the scale divisions marked on the graduated cylinders and the estimated uncertainty in volume measurements? 2. Which graduated cylinder(s) gave the most precise volume measurement? Does the number of significant figures allowed for each volume measurement A reflect the precision of the graduated cylinders?

11 Laboratory III: Measuring Temperature Name RESULTS Reading of thermometer in ice bath, C: Reading of thermometer in boiling water, C: Barometric pressure, mm Hg: True boiling point of water, C: POST-LAB QUESTIONS 1. Why was the barometric pressure needed in the calibration of your thermometer? If the barometric pressure is higher than normal, would the boiling point of a liquid be higher or lower? Explain. 2. Why were the temperatures of an ice bath and a boiling-water bath chosen for the calibration of the thermometer?

12 3. Calculate the percent error of each temperature measurement. In contrast to the previous lab, we are calculating the error due to accuracy, because we are comparing our measured value to the defined melting and boiling point of water. The equation for percent error is: measured value accepted value percent error = % accepted value 4. Evaluate your percent errors. Even if you don t have any frame of reference, I want to know if you think your errors are reasonable and acceptable. Explain the reasons behind your evaluation.

13 Laboratory I: Measuring Mass Name PRE-LAB QUESTIONS 1. What are the differences in meaning between mass and weight? In the laboratory, do we determine the mass or the weight of objects? 2. Explain the following: Weigh approximately 5 grams of NaCl to the nearest milligram. 3. Why are masses in the chemistry laboratory usually determined by a difference method (using a beaker to contain the object to be weighed rather than just placing the object directly on the pan of the balance)? 4. Why is it important to use the same balance when making several mass determinations of a given object?

14 Laboratory II: Measuring Volume Name PRE-LAB QUESTIONS 1. How does the concept of significant figures relate to uncertainty in measurement? 2. A pipet is a type of specialized lab glassware that is used to deliver a specified volume of liquid. A 5-mL pipet has major scale divisions marked for every milliliter and minor scale divisions marked for every 0.1 ml. How would you estimate the uncertainty in volume measurements made using this pipet? Would it be proper to report that the pipet was used to deliver 3.2 ml of liquid? Explain.

15 Laboratory III: Measuring Temperature Name PRE-LAB QUESTIONS 1. Explain the difference between a medical thermometer and a laboratory thermometer. 2. In the laboratory, why must you always record the temperature while the thermometer is submerged in the substance? 3. If the thermometer has marking for each 1 C, to how many decimal places, if any, will you record your temperatures?