Chem Report - Be nice but be honest!

Density

Linda Southard

Nora Mena

CHM151L-005

Pete Golden

October 1, 2012

OBJECTIVE:

            The purpose of this lab was to determine the density of a Light Pink Plastic Rock by way of both Buoyancy/Flotation and Displacement methods.  Density is the ratio of the mass (m) of an object to its volume (v).  Therefore, they should be able to determine what the density is of the rock when utilizing two different methods.  In doing so, they will take from one single light pink rock to seven light pink rocks to help us in determining the density.  The light pink rocks that are being used are of all different sizes and shapes with different cuts throughout.  (Fig. 1) 

Fig. 1

            The first method that will be utilized is that of displacement.  In the displacement method, they will weigh their rocks and work with a baseline.  Everyone will utilize a graduated cylinder and add different amounts of liquid as well as different amounts of rocks to ensure that there is a minimum of 10 mL change.  Displacement occurs after an object, in this case the pink rock, is submerged into the graduated cylinder pushing the water out of its way and taking up the space.  The volume of the water that is “displaced” is then measureable which will then give the mass of the pink rock that was put in to the water.  When working with this method, everyone will want to do a minimum of four tests.  Within these four tests, they are looking for four “good” results.  (By good it is meant that the results are within range of each other, not results that go from 0-50). They will change the factors of this in four different ways.  They will change either the liquid, the rocks or both.  After they have gotten all four of their good readings, everyone will begin to calculate their density.

            The second method that will be utilized is that of Buoyancy/Flotation.  Here everyone will again take their graduated cylinder however, this time it will be weighed independently at first.  Next, they will add exactly 10 mL of liquid (Water or Ethanol, Ethyl Alcohol) Fig. 2.  They should then take two pink rocks however; they shouldn’t be using them at the same time. They will take the first rock, setting the second rock off to the side to use later.  At this point, add it to the cylinder and liquid.  Get a good measurement on the scale and begin to add some glycerin to the mixture.  Stirring throughout the procedure, be sure to continue to add and stir until the rock is seen beginning to float upwards to which it will stay just below the bauble of the liquid line.  Next, return the cylinder with all liquid and the rock to the scale for a final weight.  At this point, write down all information and then repeat all steps using the second rock which we set to the side earlier.  After completing this the second time, begin to calculate the density.  The density should again be within range of each other.  If numbers happen to come up with one number that is extremely off from the other, a repeat should be done after all of the math is checked.

   

(Fig. 2)            Water                                      Glycerin                                  Ethanol

            One of the important things that needs to be kept in mind while working on this experiment is that of Intensive Properties.  Intensive Property is defined as the physical quantity or property that is independent of the amount of substance present nor does the value depend on the amount of the substance for which it is measured.  Therefore, it is best to keep in mind that just because the volume of the liquid is getting greater, the rock will remain with the same mass regardless.  The rocks mass won’t be changed by the volume of the liquid.

            Another thing that is needed to be kept in mind, is that of the difference between precision and accuracy.  Although they have their difference, they are both used to help to determine how well a measured quantity is or if the value from a measured quantity is known.  Or in other words, are they close enough or do we have the correct answer?  If we are going to go by precise measurements, then we will be working with those types of measurements that have repeated variable that are in agreeance with each other.  If we are going to go by accurate measurements, then we will have both our experimental and true values in agreeance.  In the case of this lab, we are going to base our results on the precise measurements.  The results of our experiment should result in measurements that are within a close enough range that they will be precise yet not accurate due to we do not know the true and exact measurement. 

DATA:

Displacement Method

Linda

# Rocks	Vi (mL)	Vf (mL)	V Rocks	Mass of Rocks	Density
4	35 mL	48.5 mL	13.5 mL	16.1 g	1.19 g/mL
7	35 mL	58.5 mL	23.5 mL	28.0 g	1.19 g/mL
7	48 mL	74.0 mL	23.0 mL	28.0 g	1.22 g/mL
4	48 mL	61.5 mL	13.5 mL	16.1 g	1.19 g/mL

16.1 g     = 1.1925 = 1.19 g/mL

13.5 mL 

28.0 g     = 1.1914 = 1.19 g/mL

23.5 mL

28.0 g       = 1.2173 = 1.22 g/mL

23.0 mL

16.1 g       = 1.1925 = 1.19 g/mL

13.5 mL

Linda’s average density was   1.20 g/mL

Linda’s range of density was 1.22 – 1.19 = 0.03 g/mL

Nora’s

The density that was calculated by Nora was:

1.20 g/mL

1.10 g/mL

1.10 g/mL

1.20 g/mL

Nora’s average density was    1.15 g/mL

Nora’s range of density was 1.20 – 1.10 = 0.10 g/mL

Group Information:

The eight density calculations that the group received were the following:

1.19 g/mL

1.19 g/mL

1.22 g/mL

1.19 g/mL

1.20 g/mL

1.10 g/mL

1.10 g/mL

1.20 g/mL

Group’s average density was 1.17375 = 1.17 g/mL

Group’s range of density was 1.22 – 1.10 = 0.12 g/mL

Buoyancy Method

Linda:

*Cylinder = Graduated Cylinder       

Graduated Cylinder Weight	Cylinder* weight with Pink Rock and 10 mL Water	Vf	Cylinder ending weight	Mass	Density
139.56 g	153.28 g	42 mL	189.24 g	35.96 g	0.86 g
139.90 g	152.90 g	40 mL	185.96 g	33.06 g	0.83 g

189.24 g – 153.28 g = 35.96 g

35.96 g     = 0.8561 = 0.86 g

42.00 mL

185.96 g – 152.90 g = 33.06 g

33.06 g     = 0.8265 = 0.83 g

40.00 mL

Linda’s average density was 0.845 = 0.85 g

Linda’s range of density was 0.86 – 0.83 = 0.03 g

Nora’s

The two densities’ that were calculated by Nora were:

0.77 g/mL

0.52 g/mL

Nora’s average density was 0.645 = 0.65 g/mL

Nora’s range of density was 0.77 – 0.52 = 0.25 g/mL

Side Note:  Nora felt like she either missed a step or miscalculated. 

Group Information:

The four density calculations that the group received were the following:

0.86 g/mL

0.83 g/mL

0.77 g/mL

0.52 g/mL

Group’s average density was 0.745 = 0.75 g/mL

Group’s range of density was 0.86 – 0.52 = 0.34 g/mL

Solvent Table

Water	Glycerin	Ethanol
1.00 g/mL	1.26 g/mL	0.789 g/mL

Discussion:

            Of the two methods that the group has worked with, it is in the opinion believed that the displacement method is the better of the two methods.  It seems to have worked best because when working with this method, the group was changing in an intentional manner different variables.  This was allowing the group to get the mass of the pink rocks.  They were still able to end up with mostly the same result even though the group had changed things up.  For example, when Linda had used four rocks and 35 mL of water, she had gotten a density of 1.19 g/mL, when she used seven rocks and 35 mL of water, again the end result was 1.19 g/mL.  On a third attempt, she changed up again and used seven rocks with 48 mL of water and end result was 1.22 g/mL.  The final test was again four rocks yet with 48 mL of water which again gave her a density of 1.19 g/mL.  Linda was intentionally changing water volume and number of rocks yet she was still getting the same result.  The Density didn’t change because she changed the volume of the water nor did it change because she used more or less of the pink rocks.  Finally, it is also felt that this was a better method due to the fact that the group did more testing of the rocks to get the same result versus the flotation method to where they only used two testing attempts and were really limited to the variable that could change. 

            When the group compared our results with each other, it was noticed that they didn’t have the same results.  Linda’s average density was 1.20 g/mL while Nora’s averages was 1.15 g/mL.  Even though there is only a 0.05 g/mL difference in their average, it is significant enough in the world of science.  The group could have come up with this difference due to the pink rocks that were used; they aren’t of the exact same cut for each stone.  Each pink rock is cut in different sizes and shapes with an average general appearance that they all appear to be the same.  Another difference could be that they used different amounts of each and one of them  could have unknowingly picked up all smaller rocks than that of the other.  Finally, the group could have come up with different results due to the difference in their testing methods.  The group only used that of Glycerin and Water, they didn’t use any of the ethanol to get any floatation.  They limited extremely their variables. 

            While working with both of these methods, the group is not free from making errors or mistakes.  Although there is always the risk of doing incorrect math or looking at something incorrectly, that isn’t what should be looked at.  While working with the Displacement method, one thing that causes an error in this form of measurement is by using two different rocks.  While your group is changing up the variables, you’ll want to make sure that they only change those that can still remain similar.  Don’t change from pink rocks to that of the small beads because most certainly your group won’t come up with a density within the same range.  You will notice that your group is changing too many of the wrong variables. By doing so, your group will come up with the density of one rock versus the density of the multi beads and neither of the averages nor ranges will be correct.  The group will have incorrect data in the equation which throws the theory out the window.  While working with the Buoyancy/Flotation method, your group may run the risk of not paying attention and adding too much or too little of the second liquid.  Linda had added too much of the glycerin and after stirring and mixing it with the water, she realized that the pink rock was above the liquid level.  Linda had to correct it by adding water, re mixing the mixture and then waiting for the pink rock to settle just below the surface of the liquid mixture.  If you aren’t paying attention to your experiment and rushing through it then you will again come up with incorrect data.  Had Linda not noticed that one of the pink rocks was above the liquid line, then she would have come up with a density that would be less than that of the liquid.  Having a Density that is less than the density of the liquid and having a density that is equal to or greater than that of the liquid will not give you a correct density. 

            When you have density which is equal to that of the liquid then you will get a 0.xXX density.  When you have a density which is greater than that of the liquid, you will have a density that is 1.XX .  When you try to average them out, you will still not have a valid density due to basically, two rights don’t make a wrong.  Two inconsistencies don’t make a consistency. 

            If Linda were to do this experiment again, she would probably change the Buoyancy method.  Linda would do the two pink rocks (separately) with water and Glycerin and then again do two pink rocks (separately) with Glycerin and Ethanol.  She would think and, most likely hope, that by doing this four times and by using all three variables at her disposal that she would be able to come up with more of a precise and accurate density. 

Conclusion:

            In conclusion, it is still felt that the displacement method was the better of the two methods.  Linda was able to do more testing and get more of an average and possible smaller range.  She got four different tests which she was able to intentionally change the variables, rocks and liquid.  Yet, while this was done, she still only got two numbers out of four.  Three of the results were 1.19 g/mL while only one was 1.22 g/mL.  While working with the Buoyancy method, she was only doing one rock at a time so she would have to do every single rock in the package to get an average.

           

CHM 151L
Cooperative Project #1: Density
Maximum Points Possible: 100

Title Page (4 points)

Title of Lab	1 pt.
Your name and name of your partner	1 pt.
Lab and section number	1 pt.
Instructor’s name and Date Submitted	1 pt.
	TOTAL:

Introduction (19 points) (2-3 paragraphs)

Identify the objective of this lab; define and explain density.	3 pts.
Identify quantities that must be measured and how, and describe the physical object being studied.	3 pts.
Clearly name and DESCRIBE the 2 methods used, what the parameters are, what materials are needed, how they are used, and tell how many times each method is carried out.	8 pts.
Discuss the relevance of the intensive property and explain the significance of varying the sample size.	2 pts.
Explain the difference between precision and accuracy.  Which is relevant in this experiment?  Why?	3 pts.
Points deducted if outcome, results, and/or conclusions are included in this section.	-3 pts.
	TOTAL:

Results (24 points) (should ONLY be data, no explanations)

Data table for displacement, including averages and ranges for each individual and the group	2 pts.
Proper labels and units on data and table	6 pts.
Calculation for density using the displacement method (use actual data and remember units)	2 pts.
Data table for floatation, including averages and ranges for each individual and the group	2 pts.
Proper labels and units on data and table	6 pts.
Calculation for density using the floatation method (again, use actual data and remember units)	2 pts.
Solvent Table	1 pt.
Proper labels and units on data and table	3 pts.
	TOTAL:

Discussion (22 points) (should be at least 3 in depth paragraphs)

Of the two methods used, which is the better method?	2 pts.
Primary reason for choosing this method. (NOTE: MUST use data from tables to receive full credit) Need to show an understanding of the concept of density and the experimental design (science, data, collection, precision, etc).	4 pts.
List at least one other reason this method is better.	2 pts.
Give ideas as to why the density results might vary between: a) the trials of an individual, b) the two partners, and c) the two methods (NOTE: full points will not be awarded unless real trials are used to support observations).	6 pts.
Identify at least one source of error per method; this is to be about experimental technique (NOTE: “misreading measurements,” and/or “incorrect calculations” are not acceptable answers and will not be awarded any points). These sources should be ACTUAL error, not hypothetical situations.	2 pts.
Explain how these sources of error would affect the results (BE SPECIFIC! Tell whether the error would cause the density to be too high or too low, and explain why).	4 pts.
If this experiment was to be done over, what would be changed or done differently? How would this change positively impact the experiment?	2 pts.
	TOTAL:

Conclusion (6 points) (at least 3-5 sentences)

Briefly restate which method was chosen as the best.	2 pts.
Briefly explain why this method was chosen (include data)	4 pts.
Points deducted if new information appears here that has not been seen in the paper previously	-5 pts.
	TOTAL:

Overall Quality (15 points)

Sections are in order and labeled properly	2 pts.
Paper was written in third person point of view	3 pts.
Shows depth and understanding of experiment	10 pts.
	TOTAL:

Grammar and Spelling (10 points maximum)

No grammatical/spelling errors; easy to read and follow	10 pts.
1-3 grammatical/spelling errors; easy to read and follow	8 pts.
4-6 grammatical/spelling errors; easy to read and follow	6 pts
1-3 grammatical/spelling errors; not easy to read or follow	4 pts.
4-6 grammatical/spelling errors; not easy to read or follow	2 pts.
7 or more grammatical/spelling errors	0 pts.