DETERMINATION OF DENSITY

Transcription

1 DETERMINATION OF DENSITY Dr. Barbara B. Bunn. Adapted from Experiments in General Chemistry ; Wiley 1996J.G. Wardeska, T.T-S. Huang, R.W. Kopp; used by permission. Tested by Ms. Janice Orr s Chemistry classes, Chilhowie High School, Chilhowie, VA and Brandon Inge and Barry Williams in Mrs. Mary Lou Hearn s Chemistry class, Nottoway High School, Nottoway, VA, Spring, 2002 Introduction Chemists use many kinds of measurements in their study of chemical phenomena. In this experiment we will illustrate a few of the more common measurements that are made in chemical laboratories. We will also use these measurements to determine a physical property of a substance: its density (d)., as all physical properties is characteristic of a substance under a given set of conditions. is defined as the mass to volume ratio and its units are mass/volume. In the metric system these units are typically g/cm 3 or g/ml. In table 1 you will find densities of some common substances at 25 o C. A simple density measurement can be performed as follows: 1. Weigh a solid object. 2. Measure the volume of water in a graduated cylinder. 3. Add the object to the cylinder. 4. Note the new level of water. The new level of water minus the old level equals volume of water displaced, which equals the volume of the object. 5. The density of the object = mass /volume. Table 1. Densities at 25 o C of some common substances. SUBSTANCE at 25 o C (g/ml) Water Copper metal 8.92 Benzene, C 6 H Ethanol Sodium Chloride 2.16 Iron metal 7.9 Nickel metal 8.90 Magnesium metal 1.74 Zinc metal 7.14 Aluminum metal Lead metal 11.3 Mercury metal Sodium metal 0.97 Gold metal Osmium metal 22.5 Titanium metal 4.51 Antimony metal 6.70 Vanadium metal 6.11 Silver metal 10.5 One of the most common measurements a chemist performs is determination of the mass (in grams) of a substance. We refer to this operation as weighing and it is very easily done using a balance. Your instructor will demonstrate the proper use of the balance. Page 1 of 5

2 Volume in liters (L) or milliliters (ml) can be determined by several techniques depending upon the desired precision of the measurement. One of the most precise methods for determining the volume of a container such as a flask is by weighing the water, which exactly fills the flask at a given temperature. Since the mass and volume are related by density, (d = m/v) the volume may be obtained if the density is known. The density of water is known quite precisely for a wide range of temperatures (Table 2). The device used for measuring density is called a pycnometer. A simple pycnometer can be made with a 25 ml erlenmeyer flask and a one-hole stopper. By allowing excess liquid to escape through the hole when the stopper is fitted snugly on the liquid-filled flask, we are assured of having the same amount of liquid for each measurement. Commercial pycnometers have a ground glass stopper with a hole rather than a one-hole rubber stopper. With an etched marker on stopper and flask, the stopper always fits in exactly the same way and to the same depth maintaining the constant volume. In your rubber stopper pycnometer, you will have to be careful to put the stopper in to exactly the same depth in each of the measurements where you fill the flask with a liquid! Otherwise you will make an error. Table 2. of water at selected temperatures. T, o C, g/ml T, o C, g/ml Goals: To make a pycnometer, a device used for measuring density, using a 25 ml flask and a one-hole stopper. To determine the volume of a flask by weighing the water that is required to fill the flask completely at a particular temperature, and using the known density of water at that temperature, calculate the volume of the water. To determine the density of an unknown solid by accurately weighing the solid, and then measuring the volume of the solid by water displacement. To determine the density of an unknown liquid. Prelab Questions: 1. A dry flask weighing g is completely filled with water and reweighed. The filled flask was found to weigh g; the temperature of the water was measured at 24.0 o C. a. What is the weight of the water in the flask? b. What is the volume of the water (hence the volume of the flask)? Page 2 of 5

3 2. The density of copper is 8.92 g/ml. What is the mass of a sample of copper occupying a volume of 2.52 ml? 3. The mass of ml of a certain liquid was found to be 7.89 g at 25 0 C. What is the density of the liquid? From table 1, what is the identity of the liquid? Safety: Eye protection should be worn. Procedure and Observations: I. Determination of the Volume of a Pycnometer: 1. Weigh a clean, DRY 25 ml flask tightly fitted with a one-hole stopper (called a pycnometer) and record the value in the data table below. This will determine the mass of the flask and stopper. 2. Completely fill the flask with water and place the stopper snugly so that excess water squirts out of the hole in the stopper. 3. Dry the outside of the flask and weigh the filled flask and stopper, recording the weight in the data table. Subtract the mass of the flask from the mass of the flask plus the water (# 2 #1). This gives you the mass of the water (#3) 4. Measure and record the temperature of the water in the flask. 5. Determine the volume of the flask (hence the volume of water it can contain), using the density of water at the observed temperature. Recall: Mass = Volume II. Determination of the density of an unknown metal 1. Obtain a dry unknown metal. Fill the thoroughly dry, weighed pycnometer about ½ full of metal and replace the stopper. Weigh the pycnometer and solid together. Record your results in the data table. 2. You can obtain the mass of the metal by subtracting #7 from #8. Part II #7 is the same as Part I #1. 3. Fill the pycnometer still containing the metal with water and replace the stopper snugly, allowing excess water to escape through the hole. Be sure that there are no bubbles in the flask. 4. Weigh the flask, metal and water and record it. 5. Subtract (#10 - #8) to obtain the mass of water in the pycnometer. 6. Determine the volume of water using the density. 7. By subtracting (#6 - # 11) you obtain the volume of metal. 8. From these data, calculate the density, and using table 1 determine the identity of the unknown solid. Page 3 of 5

4 DATA TABLE Part I. 1. Mass of dry pycnometer g 2. Mass of pycnometer plus water g 3. Mass of water g 4. Temperature of water o C 5. of water at recorded temperature g/ml 6. Volume of pycnometer (and water). ml Part II. 7. Mass of pycnometer (from #1) g 8. Mass of pycnometer and metal g 9. Mass of metal g 10. Mass of pycnometer, metal and water g 11. Mass of water (#10 - #8) g 12. Volume of water in pycnometer ml 13. Volume of metal (#6 - #12) ml 14. of metal g/ml 15. Identity of metal Page 4 of 5

5 Part III. Determination of the of an Unknown Liquid 1. Weigh a dry pycnometer. 2. Using a pipette and pipette pump, place EXACTLY ml of the liquid in the flask and stopper it. 3. Weigh the pycnometer with the liquid. 4. Subtract the pycnometer from pycnometer + liquid to obtain the mass of the liquid. 5. Determine the density by d = mass/volume DATA TABLE Method 1 1. Weight of pycnometer g 2. Weight of pycnometer and liquid g 3. Weight of liquid g 4. Volume of liquid ml 5. of liquid g/ml Postlab questions 1. How would you determine the density of a solid that is water-soluble (such as sodium chloride)? 2. The balance on which the weights were determined was inaccurately calibrated and read 0.05 g low on all readings. Would this affect the density determination? How? 3. Determine the percent error in your result. Example: if the true density = 2.78g/mL and your result is 2.94 g/ml then: X100 = 5.76% 2.78 error Page 5 of 5