Trends in the Mass of a Penny

Transcription

1 Trends in the Mass of a Penny Formal Lab Report Course Name Student Name-Class Period Abstract: Before the year 1982, pennies were made up of a majority of Cu. However, after that year, the composition of pennies chaned to a majority of Zn. Our raph of the masses of ten pennies and the year they were minted in showed a trend in the masses of pennies before the year 1982 and after In order to find out what caused this chane, we used the density of the pennies. Density is an intensive property of a system which means it is a physical property of the system that does not depend on the system size or the amount of material in the system. So we took the laboratory further and found the masses and volumes of roups of pennies separated by whether they were minted before or after By raphin the mass vs. volume of these samples, and findin the densities of each sample, we found that it was a chane in penny composition which supports the fact that after 1982, pennies were made with a zinc core and copper coat instead of mostly copper. Also, we proved the chaned in composition by puttin a nicked old penny and a nicked new penny both into a solution of HCl (hydrochloric acid) and the new penny was the only one to react. Introduction: Pennies minted before 1982 were made up of a mixture of 95% copper and around 5% zinc. However, after awhile, makin pennies with such a hih content of copper became too expensive. Instead, pennies after the year 1982 were minted with a zinc core and a copper coatin, around 97.5% zinc and 2.5% copper. The differences in the pennies compositions chaned many of their characteristics. Chanin the composition of an object and keepin the volumes constant means some properties of the object such as the mass, density and some chemical properties of the object must have chaned. So the penny densities were chaned and their reaction to HCl chaned as well. In this lab, we analyzed penny masses to determine trends between the masses, the years there were minted in, and the causes of these trends. We calculated the densities of the older and newer pennies by raphin our obtained values on a mass () vs. volume (ml) raph, ivin us the density. The results we obtained explained why the masses before 1982 followed a certain trend in their masses and why after 1982 they followed a different trend. These results supported the historical fact that the composition of pennies was chaned that year. Experimental: For the first part of the laboratory, we used a 50-mL raduated cylinder, a 50-mL buret and some tap water to find the averaed calibrated volume (V) of the raduated

2 cylinder. First, we filled the buret with water and recorded the initial volume readin to the nearest hundredth of a ml. Then, we filled the raduated cylinder to the 45.0 ml mark without lettin the buret volume fall lower than the 50 ml mark and took the final buret volume readin (aain, to the nearest hundredth). We repeated the procedure two more times, calculated the calibrated volume for each of the three trials by subtractin the initial V from the final V, and then calculated the averaed calibrated volume of the 3 trials, recordin it all in a table. Then, for the second part of the procedure, we used 10 pennies (a mixture of old and new), weihed each one and recorded each mass, alon with the year they were minted, in a table. We entered the values into a computer alon with the rest of the class s and used Graphical Analysis to raph a Mass vs. Year dot raph. For the third part of the experiment, we had three samples of pennies, each with a different number of pennies. There was a roup of 8 new pennies (pennies minted after 1982) and two roups of old pennies (pennies made before 1982), one with 4 pennies and one with 10 pennies. We first found the mass of each of the samples and recorded the masses to the nearest thousandth of a ram. Then we filled the buret with water, recorded the initial buret readin, added a bit of water (around 5 10 ml) to a dry raduated cylinder and slid the roup of the 4 old pennies into the cylinder, tappin at the cylinder to make sure there were no air bubbles in between the pennies. Then we finished fillin the raduated cylinder to the 45 ml mark and took the final buret readin, recordin it to the nearest hundredth of a ml. We redid this final and initial readin for both of the other roups of pennies, makin sure the cylinder was dry, and recorded it all in a table, and then calculated the densities (D = m/v)) and the averae volume of each sample, as well as the volume of a sinle penny for each roup. Alon with the rest of the class, we entered the volumes and masses of our pennies into the master calculator under their correspondin old/new list and we used Graphical Analysis to raph a linear reression of the new and old penny raphs, makin sure the startin point was at (0, 0). For the last part of the experiment, we used copper and zinc samples and recorded their masses to the nearest thousandth of a ram. We filled the buret with water, recorded the initial buret readin, added a bit of water (around 5 10 ml) to a dry raduated cylinder and added the metal sample, tappin at the cylinder to et rid of the air bubbles. Then we finished fillin the raduated cylinder to the 45 ml mark and took the final buret readin to the nearest hundredth of a ml. We did the same for both the zinc and the copper samples, and calculated the volumes and densities for each metal. Usin Graphical Analysis, we inserted the mass and the volumes for zinc in one raph, and preformed a linear reression of the data. We did the same for copper, makin sure to start the raph at (0, 0). This ave us the density of zinc and the density of copper values, allowin us to find the percent error of our new and old penny densities (since we had the percentaes of Zn and Cu each contained). As a class, we placed a nicked old and new penny in a 25 ml sample of hydrochloric acid (HCl) in a 100-mL beaker to see the effect the acid would have on them. Analysis:

3 Our first data recorded was the mass of ten pennies and the years there were minted. With our ten values alone, my lab partner and I did not see a trend in the masses. However, in the raph of all the class s combined data, we saw a drop in the masses at round On the raph Mass vs. Year-Old and New Pennies, we saw that pennies minted before 1982 had a mass around rams and those minted after 1982 had masses around 2.4 to 2.5 rams. However, there were sliht differences in the masses within the two major mint year roups we saw. When we found the calibrated volumes of the 50 ml cylinder, we used the final and initial buret volumes. *final buret volume = ml *initial buret volume = 0.00 ml Calibrated volume = final buret volume initial buret volume Calibrated Volume = ml 0.00 ml Calibrated Volume = ml For the averae calibrated volume, we took the averae of our three calibrated volume trials: Av. Calibrated Volume = Trial 1CalibratedV Av. Calibrated Volume = 3 Av. Calibrated Volume = ml Trial 2 CalibratedV 3 Trial 3 CalibratedV For Data Table 3, we had to calculate the sample volume (V), the sample density (D) and the averae volume of a penny. We also had to do the same for the zinc and copper samples in Data Table 4. In the Old 4 penny sample: *Av. Calibrated V = ml *Volume of H 2 O Added = ml Sample Volume = Av. Calibrated V V of H 2 O added Sample Volume = ml ml Sample Volume = 2.12 ml *Total mass of sample = *# pennies in sample = 4 Total massof sample mass (m) = # of penniesinsample = =

4 *Sample V = 2.12 ml Sample Density = V m D = ml D = ml *Sample Volume = 2.12 ml *# of pennies in sample = 4 SampleVolume Av. Volume of a penny = # of penniesinsample 2.12 ml Av. Volume of a penny = 4 Av. Volume of a penny = 0.53 ml We also calculated the exact density of the pennies usin the density values of pure copper (Cu) and zinc (Zn) and the percentaes of those elements in pennies. Since we had the percent of each element in a penny we multiplied the density of each element by its correspondin percentae in the penny and added the density of Cu and Zn to et the total density of the penny. For an old penny: *pennies made before 1982 = 95% copper and 5% Zn *Density of pure Cu (D Cu ) = 8.92 /ml *Density of pure Zn (D Zn ) = 7.14 /ml D Cu = (0.95 Cu)(8.92 /ml) = /ml of Cu per old penny D Zn = (0.05 Zn)(7.14 /ml) = /ml of Zn per old penny Density of old pennies = D Cu per old penny + D Zn per old penny Density of old pennies = /ml /ml Density of old pennies = /ml Usin our experimental density value obtained from the slope of the fit Mass vs. Volume-Old Pennies raph (the averaed density of all the data we collected) and the correct density value (the density we calculated from iven percentaes and densities of pure Cu and Zn), we found our percent error for the density of each penny class (old

5 and new). We also did the same for the zin and copper samples. In the case of a new penny: *correct density value of new penny = /ml *experimental value of new penny = 6.56 /ml (correct value - experimental value) % error = 100 correct value ( ) ml % error = ml ml % error = 8.6% Results/Conclusion: After examinin the Mass vs. Mint Year raph, my partner and I hypothesized that the drop in the mass after the year 1982 sinified either a chane in the composition of pennies after 1982 or a chane in the amount of material used (chane in mass). The sliht ranes of the masses each year could just be attributed to discrepancies in the different locations in which the pennies were minted. We used the property of density which is independent of the amount or size of the material used to find out what actually caused the chane in mass after The masses and volumes we athered for the old and new pennies were raphed on two separate raphs; one for each the old and new pennies. The slope of the linear fits for the Mass vs. Volume New Pennies and Old Pennies raphs ave us the averae densities for an old and new penny. The New Penny raph shows the averae density as 6.56 /ml and the Old Penny density as 7.79 /ml. This chane in density shows that in fact, it was the composition of pennies that chaned after the year 1982, explainin the chane in masses in the first raph. By raphin the mass vs. volume for zinc and copper, we found the densities of pure Cu and Zn in the same way we did for old and new pennies. This way, we could see that density is independent of the amount of material in a sample(here a penny) and the size of the sample. The percent error we found for the densities of the old and new pennies and the copper and zinc samples are due to the buret volume measurements. Another possible source of error was accidentally not dryin the raduated cylinder enouh trial after trial. For the final class section of the lab, when we placed the two nicked pennies in the HCl acid, we saw that the old penny did not react at all while bubbles bean to form around the new penny. When we removed the two pennies from the HCl, we saw that the new penny was now a mere shell. This is because copper does not react in HCl while zinc does and new pennies are made with a zinc core and a copper coatin. By usin an intensive property (here, density), which does not depend on the amount of material in an object or the size of the object, we found out that the chane in

6 mass was caused by a chane in composition of pennies. The history our results back up is that in 1982 zinc replaced copper as the major component in pennies. A possible way to better the experiment desin is by doin more trials because that would improve our averae densities. An experiment in the future could be findin out why exactly zinc reacts with HCl and copper does not. Since HCl is also known as stomach acid because it is the acid in our stomach, another possible future experiment could be findin out what happens when HCl reacts with zinc and if the reaction would be harmful if it happened in a human stomach.