Density Determination 9/10/11
Introduction In the density lab we had four main objectives to complete, they were: 1.
Learn
about intensive and extensive properties
2. Determine the density of regularly-shaped objects 3. Determine the density of irregularly-shaped objects 4. Determine the density of distilled or deionized water In order to find the density of an object whether it is regular or irregularly shaped you must first know its mass and volume. ( Density= Mass of object/ volume of object) Mass and volume of an object are considered extensive properties because they depend on the quantity of an object. The density however is considered an intensive property because unlike mass and volume, it does not depend on the quantity. The basic units that density it reported in are: g/ml org/cm^3 at 20°C. In density the temperature must be recorded because volume changes in different temperatures, so the density calculation is effected as well. Density of a substance remains the same at constant temperatures; because of this we can identify different substances on their density. The table below shows some density values for various substances we used in this ex periment.
Densities of various substances at 20°C Substance
Density (g/cm³)
Steel
7.9
Aluminum
2.7
Brass
8.4-8.8
PVC
1.39-1.42
Copper
8.96
Oak
0.60-0.90
Pine
0.35-0.50
Acrylic
1.1-1.2
Polypropylene
0.91-0.94
Water
1.0
In this lab the density for regularly-shaped, irregularly-shaped objects and ionized and deionized water will be calculated by measuring both volume and mass for each object and sample.
Procedure In determining the density for both the objects and the water we have a certain procedure to follow: A. Determine the density of the regularly-shaped object.
1. Obtain one of the solid blocks that were provided in the lab and record its code number onto our data sheets. 2. Using a metric ruler (Figure 5), we were to measure the length, height and width of the block and record our measurements. By multiplying the length, width and the height of the block together we can calculate the volume of the cube.
olume=L x W x H) (V olume=L 3. Determine the mass of the block to the nearest 0.01g by using the top loading balance (Figure 2). 4. Calculate the density of the block by using the equation: (Density=Mass/Volume). Once you have calculated the density record your answer on your data sheet. 5. Once you have your density calculated you may do the experiment twice to check your answer and determine your average density value. Our professor said to only do it once. 6. Once you know the density, compare your findings to the density table to determine the substance of your cube and record your answer on your data sheet.
B. Determine the density of an irregularly-shaped object.
When determining the density of an irregularly-shaped object, you cannot use the volume equation because it is not easily measured. For this part of the experiment we used water displacement method to determine the volume of the irregularlyshaped object.
s 1. Obtain a sample of an unknown metal found in the Styrofoam cups in your lab, and record its number onto your data sheet. 2. Obtain approximately approximately 5 grams of the unknown metal; you can use the top-loading balance balance (Figure 2) to approximate your mass. Once you have gotten as close to 5 grams as you could record the value to the nearest 0.01 g on your data sheet. 3. Obtain a 10-ml graduated cylinder (Figure 4) and fill it about halfway with water. Once properly measured record its initial volume to the nearest 0.01 ml onto your data sheet. 4. Place the metal sample that you retrieved from the Styrofoam cup into the graduated cylinder that contains water. To remove air bubbles that will affect your calculations calculations just simply tap on the sides of o f the graduated gra duated cylinder cylinder lightly. 5. Measure the final volume of the water to the nearest 0.01 ml and record it to the nearest 0.01 ml onto the data sheet. 6. To determine the volume of the metal you must figure out the difference between the initial and final volumes of water, assuming that water does not react with metal. To determine the difference in water volumes you must subtract the initial volume from the final volume and record the metal sample volume onto the data sheet. 7. Once you have determined the density of the metal sample, compare it to the densities of various substance table to determine the identity of your substance.
C. Determine the Density of Water
1. Obtain and dry 50 ml or 100 ml beaker (Figure 6) and label it beaker 1. Weigh the beaker and record its mass to the nearest 0.01 g onto the data sheet. 2. Then transfer 30 ml of some distilled or deionized water into a clean beaker and label it beaker 2. Using a thermometer (Figure 3), measure the temperature of the water and record your findings on your data sheet.
3. Transfer 10.00 ml of the water to the pre-weighed beaker you labeled beaker 1. For accurate measuring measuring you should be using a 10-ml volumetric pipet (Figure 1), making sure there are no air bubbles. If you notice air bubbles in the pipet just gently tap on the side of it and refill the volumetric pipet to the line that corresponds to the 10.00 ml mark. 4. Weight beaker beaker 1 with the 10 ml of distilled or deionized water and record its mass to the nearest 0.01 g onto your data sheet. 5. Calculate Calculate the mass of the water by subtracting the mass of the empty beaker from the mass of the beaker containing the water. Record the calculated mass of the distilled water onto your data sheet. 6. Calculate the density of the water by dividing the mass of the water by its volume and record your calculations onto the datasheet. 7. Once you have the density of the water at a temperature you previously recorded. Compare your measurement of water density to 1.0 g/ml which is the value of water according to your lab book.
Equipment used in experiment Here
are some pictures of the equipment needed for this experiment:
Figure 3
Figure 1
Fi ure 2
Figure 4
Figure 5
Figure 6
Equations used in this experiment Volume
=
Length x Width x Height
Density =
Data and Calculations This is an accumulation of data and calculations that were recorded on the data sheet during the experiment. A.
Density of a regular-shaped object
Length Width Height Volume Mass Density Identity of unknown regularly shaped object
Determination 2.5 cm 2.5 cm 2.5cm 15.6 cm³ 10.19 g 0.65 g/cm³
Oak
Unknown Code
Number:
2__
Calculations for regular-shaped object: V=LxWxH 2.5cm x 2.5cm x 2.5cm= 15.625cm³ D=M/V 10.19 g/15.6cm³=0.65 g/cm³
Unknown code
B.
Density of an Irregularly-Shaped Object Mass of Metal Sample Initial volume of water Final volume of water Volume of metal Density of metal Identity of unknown irregularly-shaped object
Calculations for irregular-shaped object: V of metal= final volume of water initial volume of water 8.7ml -8ml= 0.7ml D=M/V 4.98 g/0.7ml=7.114 g/ml= 7.1 g/ml
4.98 g 8 ml 8.7 ml .7 ml 7.1 g/ml STEEL
number:
1
C.
Density of Distilled Water Mass of 50-ml beaker 1 Temperature of water Mass of beaker 1 + water Mass of water Volume of water Density of water Density of water reported in lab book
48.77g 21°C 58.8g 10.3 g 10.00 ml 1.003 g/ml 1.0 g/ml
Calculations for density of distilled water: Mass= 58.8g- 48.77g= 10.3 g D=M/V 10.3g/10.00ml= 10.3g/10.00ml= 1.003 g/ml
Results and Discussion
I determined the density of the block to be .65 g/cm³, based on this information the identity identity of the unknown regularly-shaped regularly-shaped object is Oak.
I determined the density of the irregularly-shaped object to be 7.1 g/ml, based on this
information the identity of this object is Steel. I determined the density of distilled water to be 1.003 g/ml; I find my calculations true based on the density of the water reported in the literature of being 1.0 g/ml.
Based on the density of substances at 20°C, I find my calculations to be true. I believe that the identities identities of my objects are valid because they correspond cor respond with the density constants. Possible errors in this experiment could include temperature. We measured the temperature of the room at 21°C, but the density table shows constants at 20°C.
Other errors that could affect the density include: y
y
Measuring with the pipet if there were air bubbles it would lower the density because the mass of the measured volume would be lower. Using a dirty pipet in measuring, the result would have a higher density because the volume would be higher than it should be.
y
And rushing through the experiment and not measuring everything exact. I observed in this experiment that if not all measurements of volume and mass are not valid than it effects the density, making it harder to identify the unknown substance.
Post Lab Questions 1. A lab technician has a volumetric pipet calibrated to deliver exactly 25.0 ml of liquid.
He
then measures the mass of the liquid to determine the density of the liquid. How will the density determination be affected by the following situations described below? Would the calculated density be higher, lower , or unaffected ? Explain your answers. a. A lab technician did not remove air bubbles from the pipet before delivering the liquid.
The calculated density would be lower than if he didnt have bubbles. If the measurement measurement of volume was less than it should have been because of the bubbles, than consequently the mass would be less. The volume would not be affected because because of the 10 1 0 ml constant in our experiment. If that constant was not in affect than volume being lower with a lower mass would result in a higher density. b. A student used a dirty pipet that had water droplets adhered to the inner walls of the pipet.
The result would have a higher density because the volume is higher than it should have been. Messing up calculations later in the experiment.
c. The student did not wait long enough to allow the li quid to empty from the pipet.
The calculated density would be lower because the volume is less than he expected it to be. Taking measurements of the mass from the volume amount would make the mass lower as well. Causing the lower density.
2. Do you expect that two objects with the same amount of mass but different density values would have the same volume? Explain your answer. No, they would not have the
same volumes. If two objects have the same mass but
different volumes than their density values would be different because, Density depends on mass/volume. mass/volume. If two objects had the same mass and volume, v olume, than their density would be the same.
3.
Determine the density of a regularly shaped object that has a mass 56.88g and the following dimensions: length = 3.65cm, width= 8.97cm and height= 6. 35cm.
Using the equation D=M/V
and V=L*w*h V=3.65x8.97x6.35 V=3.65x8.97x6.35 = 207.9cm³
D= 56.88g/207.9cm = 0.2736g/cm³
Density= 0.274g/cm³
