Lab Section. Experiment 2 Density. Introduction

Transcription

1 Experiment 2 Density Introduction Density (given the symbol d or ρ depending upon which book you read) is an intrinsic property of materials. The term intrinsic means that it is independent upon the amount of the substance. Thus, the density of anything remains the same, no matter the shape and size of the sample. The density of water at 4 C is regardless if the sample is 1 cup or 1 swimming pool. For regular solids, those that have a simple formula for calculating their volume, calculating the density is straightforward: simply weigh the object and measure its dimensions. The density is calculated by dividing the mass by the volume. Each regular solid has its own formula for calculating its volume. For this lab, the volume equals length times width times height will work for calculating the volume (. Note: 1 ml = 1cm 3. For irregular solids, those that do not have a formula for calculating their volume, the volume can be determined by measuring the volume of liquid that the solid displaces. To do this, the solid is submerged in a liquid and the volume displaced is measured. This is done by taking an initial reading and a final reading and determining the volume difference. The mass of the object is then divided by the volume, and the density is determined. Measuring the density of a liquid is very similar. Although the volume cannot be measured with a ruler, it can be determined using volumetric glassware, for instance, a pipet. This measured volume can be weighed in a container, and its mass determined. Typically this is done by taking the difference of the mass of the empty container before and after the liquid has been added. A new technique needed for this lab is called conditioning. This is a technical term for rinsing out something. To condition a pipet, you will suck up some of the liquid you are going to measure using a pipet wheel, and allow it to drain out. Be sure that you rinse out the entire working area of the pipet, not just the end, but be sure to not invert the pipet, as this will contaminate the pipet wheel. In this lab you will determine the densities of the following liquids and solids: a regular solid, an irregular solid, water at room temperature, an unknown liquid. In addition your instructor will demonstrate the effect that density has on different liquids and solids. Also, you will determine the thickness of aluminum foil using its known density. Equipment Needed: Erlenmeyer flask 100 ml beaker 250 ml beaker Aluminum Foil 10 ml pipet and wheel Rubber stopper Unknown Solid Unknown liquid

2 Prelaboratory Questions: Which is more dense, water or ice? How do you know? What does it mean to condition a pipet? What is meant by mass by difference? How do you do it? What is meant by volume by difference? How do you do it?

3 Examples: Density of a Regular Solid Consider the following regular solid. The dimensions are measured to be: 3.85 cm long, 1.20 cm wide, and 2.45 cm high. It weighs g cm 3.85 cm 1.20 cm The volume of the solid is calculated by: : The density is then calculated by dividing the mass by the volume: Density of an Irregular Solid Volume by Difference or in proper significant figures, ρ = 8.78 The volume of a g, unknown, irregular solid is determined by difference. Calculate its density g 43.5 ml 62.0 ml The initial volume of the graduated cylinder is 43.5 ml. Once the object is placed in the graduated cylinder the volume increases to a final volume of 62.0 ml. The difference in the volumes yields the volume of the object placed inside: 62.0 ml 43.5 ml = 18.5 ml. The density of the solid can now be calculated. Divide the mass of the solid by the volume it displaced: or in proper significant figures, ρ = 3.86

4 Density of a liquid Mass by Difference The mass of a 25.0 ml sample of an unknown liquid is determined by difference. Calculate its density ml liquid added g g Mass of empty beaker is g. The mass after 25.0 ml of liquid is added is g. Taking the difference of these masses yields the mass of the liquid: g g = g. The density of this liquid can now be calculated. Divide the mass of the liquid by the volume of the liquid: in proper significant figures ρ = 1.53 Thickness of a Material A 10.0 cm by 22.5 cm piece of tin weighs g. If the density of tin is is this foil?, how thick The volume of the object can be found by dividing the mass by the density, or multiplying the mass by the inverse of the density: (recall: ρ ) or Remembering that, or alternatively, In proper significant figures: cm or 126 µm

5 Procedure Relative Densities Instructor Demonstration Record each of the components that your instructor uses during this demonstration including: the different liquids and their relative positions, as well as the solids and their final resting places within the liquids Density of an Unknown Regular Solid Obtain an unknown solid from your instructor. Weigh the solid. Measure its length, width, and height using the centimeter ruler and calculate the volume of the solid. Calculate the density of the unknown solid. Repeat these steps using the millimeter ruler. Density of an Irregular Solid Volume by Difference Weigh the irregular solid provided by your instructor. Fill a graduated cylinder to between 50 and 70 ml with water. Record this initial volume. Gently place the irregular solid into the graduated cylinder. This is best accomplished by tilting the graduated cylinder and sliding the solid into the water. This will avoid splashing. Record the new, final volume of the graduated cylinder. displaced by difference. Determine the total volume Calculate the density of the irregular solid by dividing the mass of the solid by the volume of water it displaced. Repeat these steps for reproducibility.

6 Density of Water Mass by Difference Weigh the Erlenmeyer flask. Obtain approximately 100 ml of deionized water in a 250 ml beaker. Use this water to condition your pipet. Transfer 10.0 ml of the water from the beaker into the Erlenmeyer flask. Reweigh the Erlenmeyer flask containing the 10.0 ml of water. Determine the mass of the water by difference. Calculate the density of water for each trial by dividing the mass of the water by the volume of water added Repeat these steps for reproducibility. Note: it is unnecessary to dry the flask in between measurements as the mass of the added water is obtained by the differences in mass. However, it is necessary to reweigh the Erlenmeyer flask between runs. Density of the Unknown Liquid Mass by Difference Obtain approximately 50 ml of an unknown liquid sample from your instructor in a clean, dry 100 ml beaker. Record the unknown liquid number. Weigh your Erlenmeyer flask Condition your pipet with the unknown liquid. Put all waste into a waste beaker for disposal at the completion of the lab. Once the pipet has been conditioned, transfer 10.0 ml of the liquid into the Erlenmeyer flask and reweigh the flask. Determine the mass of the liquid by difference. Calculate the density of the unknown liquid by dividing the mass of the liquid by the volume of liquid added. Repeat these steps for reproducibility. Thickness of Aluminum Foil Measure the length and width of the aluminum foil using the millimeter ruler. Weigh the piece of aluminum foil. To minimize errors due to wind currents, fold the foil in fourths before weighing. Using the density of aluminum, ρ2.70, calculate the thickness of the aluminum foil.

7 Data Table Relative Densities Observations

8 Density of Unknown Solid Trial 1 (centimeter ruler) (millimeter ruler) Initial mass of Unknown Solid Volume of solid Length of solid Width of solid Height of solid Volume of solid show calculation: cm cm cm cm cm cm Density of Unknown Solid show calculation:

9 Density of Irregular Solid Trial 1 Mass of irregular solid Initial volume Final volume Total volume displaced Density of Irregular Solid show calculation: Trial 1 Average density of solid

10 Density of Water Trial 1 Initial mass of Erlenmeyer flask Final mass of Erlenmeyer flask Volume of water Density of water show calculation: Trial 1 Average density of water Density of Unknown Liquid Initial mass of Erlenmeyer flask Final mass of Erlenmeyer flask Volume of Unknown Liquid Density of Unknown Liquid show calculation: Trial 1 Trial 1 Average density of liquid

11 Thickness of Aluminum Foil Mass of aluminum foil Length of aluminum foil Width of aluminum foil g cm cm Volume of aluminum foil show calculation cm 3 Thickness of aluminum foil cm

12 Postlaboratory Questions: 1) Why is it unnecessary to dry the Erlenmeyer flask in between measurements for the water and the unknown liquid? 2) A 400 troy ounce gold bar is the standard in gold trading. The U.S. Mint reports that a standard gold bar weighs 27.5 pounds, or 12.5 kg. It measures 7 inches by 3 inches by 1 inches, which is cm by 9.21 cm by 4.45 cm. What is the density of gold in? Knowing that the density of gold is 19.3, is this a pure gold bar? 2) An g, uncut diamond was dropped inside of a graduated cylinder containing 45.5 ml of water. If the water level rose to 51.0 ml, what was the density of the diamond? 3) A 150 ml beaker weighing g had a 50.0 ml sample of an unknown liquid put inside of it. The beaker was then reweighed and found to be g. What was the density of the liquid?

13 4) A block of iron was measured to be 2.00 cm by 3.15 cm by 5.25 cm. If the density of iron is 7.87, how heavy was the block of iron? 5) A thin sheet of nickel measured 5.00 cm by 7.00 cm was found to weigh g. If the density of nickel is 8.90, what is the thickness of this sheet? 6) Cats love to play with balls of yarn. If a 4.00 inch diameter ball of yarn weighs 1.40 lbs, what is its density in? [volume of a sphere is π ]