Lab 1-Density of a Liquid Flashcards
What is the purpose of this lab?
Provide practice with measuring devices such as balances, graduated cylinders, and volumetric pipets, as well as to determine the density of a liquid substance.
What is the definition of density?
The ratio of the mass (m) of a substance to the volume (V) occupied by that mass; Density= mass/Volume; The units of density are g/cm^3 and g/mL.
What are some things that effect the volume of a substance?
Pressure and temperature changes. Though the mass of a substance remains constant, its density will change. Therefore, when stating the density of a substance, ONE SHOULD SPECIFY THE TEMPERATURE AT WHICH THE DENSITY WAS DETERMINED AT.
What will be done in part A of the lab?
Several objects will be weighed and you will become more familiar the use of substances. In the second part, you will measure the mass and volume of water and use this data to accurately determine the density at a specific temperature.
What will be done in part B of the lab?
You will be using graduated cylinders, pipets, and burets (along with balances) to determine the density of liquid water. A volumetric pipet can be used to measure the volume of a liquid.
What is a volumetric pipet?
A glass tube with an enlarged barrel and a calibration mark. When the pipet is filled and the liquid level is at the calibration mark, the pipet will deliver the indicated quantity of the liquid to two decimal places. (MAKE SURE TO READ THE LIQUID LEVEL AT THE BOTTOM OF THE MENISCUS)
What are some general guidelines to follow while using a pipet?
One should handle the pipet with care because the tips are very fragile. Also, never fill the pipet by mouth suction; always use a rubber suction bulb. Before each use, make sure the pipet is clean (in fact, never assume that any glassware is clean from before). No water droplets should be observed in the inner walls. The volumetric pipet is calibrated to deliver (TD) the indicated amount of liquid by gravity drain only. When the flow of liquid from the pipet is complete, a small amount of liquid will remain at the tip. This type of pipet was calibrated taking this into account. Never shake the pipet or add the amount of retained liquid to the liquid delivered by gravity.
What is am important thing to consider regarding pipets, volume, and temperature?
Note that the volume of a given liquid will increase as the temperature rises. For example, the volume occupied by 1000mL of water at 15 degrees Celsius will occupy 1002mL at 25 degrees Celsius. Pipets, therefore, are normally calibrated at a specific temperature (20 degrees Celsius). Using a pipet at room temperature (25 degrees Celsius) will produce an error in measured volume of about 1 part per thousand (0.1%). This error is negligible for our purposes.
List the experimental procedure for part A of the experiment.
After receiving instructions on the operation of lab balances from your TA, weigh a coin and measure its mass to 0.001g. Record the mass of the coin the lab notebook. With the coin on the balance, re-zero your balance by pressing the “TARE” bar. Place another coin on the balance (again, record its mass). Remove both coins from the balance, re-zero the balance, and weigh both coins together. (record this mass as “Mass of coins #1 and #2 weighed together”).
In addition to measuring the mass of the coins, weigh several other objects to become familiar with the use of balances. Some suggestions are a paperclip, a hair, a beaker, or a beaker with a fingerprint. It is also worthwhile to weigh an object on all of the balances in the lab to make sure hat they’re all accurate.
List the experimental procedure for part B of the experiment.
Add approximately 20mL of DISTILLED WATER to a beaker. While you’re weighing the graduated cylinder and the flask in the following steps, periodically measure the temperature of the water until it comes to a constant value. Record this temperature in the notebook. You will use this sample of water for the rest of the experiment. ALL OF THE TECHNIQUES IN THE FOLLOWING CARDS ARE A PRT OF THE EXPERIMENTAL PROCEDURE FOR PART B
What is the technique for using a graduated cylinder in this experiment?
Weigh a clean 50-mL graduated cylinder. Add distilled water from the BEAKER to about the 40-mL mark. Measure and record the actual volume to the nearest 0.1 mL (estimating between the smallest markings).
Empty and dry the graduated cylinder. Repeat the mass and volume measurement two more times using the same graduated cylinder. Calculate the density of water sample for each trial. If the densities of the 3 trials vary by more than 1%, perform a 4th trial.
What is the technique for using a pipet in this experiment?
Weigh a clean 25-mL Erlenmeyer flask (WITH GLASS STOPPER) to the nearest 0.001g. Carefully pipet 25.00mL of water into the flask. (note: rinse the interior of the pipet with a small amount of water first to remove contaminants from the pipet). Replace the stopper and weigh the flask and liquid. Repeat this density determination twice more by repeating the pipetting and the weighing steps. Calculate the density for each trial. If the densities of the three trials vary by more than 1%, do a 4th trial.
What is the technique for using a buret in this experiment?
Weigh the 25-mL flask and stopper to the nearest 0.01g. Fill a buret to just below the zero mark with distilled water from the beaker. Measure and record the initial volume to the nearest 0.01mL. Deliver approximately 25-mL of distilled water into the flask. NOTE: It is not necessary to deliver exactly 25.00mL but you must measure the actual volume accurately. Measure and record the final volume to the nearest 0.01mL. Weigh the flask with the stopper and the water. Calculate the density of the sample. Repeat this procedure 3 times to obtain the density calculations. Again, you may need to do this for the 4th time.
Calculate the average density and standard deviation for the three closest trials obtained by each of the two techniques.
Compare the average value of the density of the water you obtained to the true value shown in Appendix H. Be sure to compare the values in the table corresponding to the same temperature as your water sample.
Calculate the percent error of your determination by the following equation. PERCENT ERROR= Observed Value-True Value/True Value x 100. Do this for the density of each technique you obtained.
