THE BALANCE, GRADUATED CYLINDER, BURET, AND BUNSEN BURNER
DISCUSSION (Reference: Equipment Guide: Weighing and the Electronic Balance, Liquid
Volume Measuring Devices, and The Bunsen Burner)
The balance, graduated cylinder, buret and bunsen burner are four items that you will routinely use in lab this semester. This experiment is designed to give you direct experience using this equipment and to develop basic skills in making measurements with the balance, graduated cylinder, and buret.
However, our goal is to learn much more than the operational aspects of these devices. This is certainly not unimportant. Indeed, correct use of any measuring equipment is an integral part of the measuring process. But our interest is the whole process, not just part of it, since measurements are made for a purpose – a means to an end.
There are a number of measuring techniques each of which have their advantages and disadvantages. You will learn some measuring techniques in this experiment, and you will explore and evaluate them in the context of different measuring situations.
A measurement is a datum; multiple measurements are data. Measurements are the foundation of science. Data are recorded and subsequently analyzed, manipulated, or utilized. You are responsible for obtaining data and recording it, and you should strive to obtain data that is of the highest quality; data which you are confident of. The best way to reach this level is to develop good measuring habits by practice and more practice. Learn to exercise care in making measurements and be alert. If you suspect a problem with any measurement, re-check it.
To this end, it is also important to immediately record data as it is obtained and to do so accurately. Lastly, data needs to be organized in a manner that facilitates its use.
If you take a look at the next experiment, you will notice that the buret and balance are again featured. However, the emphasis in the next experiment is on the establishment of physical properties from measurement. This activity will also include an introduction to techniques that are used to assess uncertainty and error in measurement. Together the first two experiments are designed to introduce you to and acquaint you with the many sides of measurement.
Indeed, most of what you learn in this experiment and the next will be utilized throughout this and subsequent chemistry lab courses. In this course, the beginning experiments will provide some direction and guidance with respect to making and using measurements. However, as the semester progresses, you will be expected to become ‘expert’ in these areas. And little or no direction or guidance on measuring or the use of measurements will be given.
EXPERIMENTAL WORK INDIVIDUALLY NO PARTNERSHIPS
Goggles are required.
Follow the directions on the report sheet.
Record your data – include labels and units – and illustrate necessary calculations in the space provided below each directive.
Some beakers will be used more than once. If necessary, rinse and dry them between uses.
Answer the questions as you go along.
Record all displayed digits on the electronic balance.
Read and record the graduated cylinder scale to the tenths place and the buret scale to the hundredths place.
Use the same balance for all weighings.
Clean Up and Disposal
Clean all glassware that you used with soap and water; give it a final rinse with deionized water.
Empty your wash bottle, and return it and all other equipment to your drawer. Lock the drawer.
Clean your work area with a sponge; sponges are located in the common cabinet below each sink.
1 LABORATORY TOOLS NAME ___________________________
DATA AND RESULTS 1315 Section ______
1. Weigh a solid object – wooden block, flask, or test tube – on the balance. First zero the balance using the tare (zero) function, then place the object on the weighing pan, and record its mass. (Identify the object.)
2. Weigh out some salt (sodium chloride) following the two methods described below.
Method 1: zero the balance; weigh and record the mass of a 50-mL beaker. Remove the beaker from the balance and transfer two small scoops of salt to it. Re-zero the balance; weigh and record the mass of the beaker and salt. Determine the mass of salt in the beaker.
Method 2: place a 100-mL beaker on the balance and zero the balance using the tare (zero) function. Remove the beaker from the balance and transfer two small scoops of salt to it. Place the beaker back on the balance and record the mass of salt in the beaker.
Which weighing method do you prefer? Why?
Can you anticipate a situation where you might not be able to use your preferred method? Explain.
Why is a container used to weigh salt?
Suppose you re-zeroed the balance in method 2 after removing the empty beaker. When the beaker is returned to the pan after salt has been added to it, will the mass displayed by the balance correspond to the mass of salt? Explain. (Hint: if you are uncertain, try it!)
3. Stand a 15-cm test tube in a 150-mL beaker. Weigh the beaker and tube and bring them back to your bench. Grasp the tube near its mouth with a test tube clamp, and heat it in a burner flame along the bottom half of its length until a persistent yellow flare is observed when the flame touches the hot glass. Stand the hot tube in the 150-mL beaker and after thirty seconds have elapsed weigh the tube and beaker. Keep the tube and beaker on the balance pan for two additional minutes and record the combined mass at thirty-second intervals.
Weigh the beaker and tube again after fifteen minutes have elapsed.
According to your data, is it okay to weigh an object while it is hot or should you allow it to cool to room temperature before weighing it? Explain.
4. Use weighing method 1 to complete the following tasks:
Determine the mass of deionized water in a 50-mL graduated cylinder that is filled exactly to the 20-mL mark with deionized water.
Transfer the water in the cylinder to a 100-mL beaker and determine the mass of water transferred.
How do the two masses of water compare?
If the masses differ, why are they different? Explain, but make certain your explanation is consistent with your results.
Would it be feasible to use weighing method 2 to determine the mass of water in the graduated cylinder and the mass of water transferred to the beaker? Why or why not?
A student doing this part of the experiment forgot to weigh her graduated cylinder before adding water to it. Instead she weighed it after she transferred the water from it to the beaker. When she analyzed her data, she discovered that the mass of water in the graduated cylinder equalled the mass of water in the beaker. Do her results match your results? If yes, state so. If no, account for the discrepancy between her results and your results.
5. Obtain a buret and rinse it with deionized water. Then half-fill it with deionized water. Bring the buret to a nearby sink, and open the stopcock and drain about 2 mL or so of water from it. This operation will remove trapped air bubbles and ensure that the tip is filled with liquid. Check for leaks; the reformation of an air bubble in the tip is indicative of a leak. If you can’t fix the leak, consult you instructor.
Fill the buret with water until the liquid level is just above the zero mark. Clamp the buret to a ringstand. Carefully drain water from the buret into a 50-mL beaker until the liquid level is at the zero mark.
Weigh a 150-mL beaker. Transfer exactly 20 mL of water from the buret into the beaker. Weigh the water and beaker. Determine the mass of water in the beaker. (Record both mass and volume data.)
Refill the buret to the zero mark with deionized water. Weigh a 250-mL beaker. Transfer exactly 20 mL of water from the buret into the beaker. Weigh the water and beaker. Determine the mass of water in the beaker.
If two masses differ by more than 0.100 g, repeat one more time.
In the second determination, you were instructed to refill the buret to the zero mark. Could you have successfully made this determination without refilling the buret? If so, describe how you would do it and identify the measurements that you would make.
If you need to measure and transfer an accurate volume of liquid to a beaker which device would you choose the graduated cylinder or the buret? Explain.