Lab 8 Counting by Weighing Online PDF

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This assignment showed us the proper counting for not only measurements but how to properly do the equations involved to get a more exact number....

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LAB 8: COUNTING BY WEIGHING -- ONLINE After the completion of this lab, the student will 1) Explain what an isotope is 2) Be able to determine the average mass of a sample INTRODUCTION The elements on the periodic table are listed with masses that are decimal numbers. The atomic weights on the periodic table take into account the mass and abundance of each isotope of an element. An isotope is an atom that differs in the number of neutrons from atoms of the same type. A differing number of neutrons will cause a different atomic mass. For example, carbon-12 has six neutrons while carbon-13 has seven neutrons. They are both atoms of carbon. A differing number of neutrons will affect the stability of an atom. Carbon-12 is very stable; carbon-13 is not as stable. Carbon-14 is even less stable. When looking at a periodic table, you will see that the mass listed for carbon is 12.01. This number indicates that the most abundant isotope of carbon is carbon-12. Because it is hard to work with single atoms in the general chemistry lab, we will be working with pennies. Prior to 1982, pennies were composed of solid copper. Because of the rising price of copper, the US government began to mint pennies with a zinc core and a copper foil exterior after 1982. We are going to use the mass of pre-1982 pennies and post-1982 pennies to determine the number of each type of penny in a sample. PRE-LAB QUESTIONS 1.

What year did the US government change the composition of the penny from 100% copper to a zinc core with a copper coating?

The year 1982 is the year that the US government changed the composition of the penny from 100% copper to a zinc core with a copper coating. 2. How many neutrons does a carbon-14 atom contain? There are 8 Neutrons in a Carbon-14 atom. 3. Why did the US government continue to use copper in the penny? Why could they not make a penny out of 100% zinc? The zinc core is naturally a silvery color, so it was easy to confuse the penny for a dime. The copper coating differentiates the two coins as to lessen the confusion. PROCEDURE 1. Obtain 25 pennies. Separate into two piles – 1) 1981 and older; and 2) 1983 newer. Set 1982 pennies aside.

and

2. Use a paper towel to ‘label’ the pennies. See image to the right. 3. Mass all of your pre-1982 pennies and record the masses in the table below. Calculate your average pre-1982 mass and record in the blank. Perform the calculations in the Results/Observations section using the your average. 4. Mass all of your post-1982 pennies and record the masses in the Lab Results/Observations section. Calculate your average post-1982 mass and record in the blank. Perform the calculations in the Results/Observations section using the your average. 5. Your instructor will give you the mass of a 25-penny sample. Use the your average of each type of penny to determine how many pre-1982 and post-1982 pennies are in the sample. Show your work in the Results/Observations section.

LAB RESULTS/OBSERVATIONS Pre-1982 Pennies – Mass (g) 3.0913

3.1440

3.1099

3.0667

3.1743

3.1092

3.1284

3.0748

3.0855

3.0832

3.0993

3.0822

Average mass of pre-1982 pennies _____3.1041 G_______________________ Mass of 55 pre-1982 pennies ___________170.724G_______________ Number of pre-1982 pennies present in total mass of 310. G _______100 pennies______________________ Post-1982 Pennies – Mass (g) 2.4948

2.5125

2.4669

2.5075

2.4811

2.5452

2.5235

2.4884

2.4971

2.5024

2.5499

2.4800

2.5241

2.4952

2.4854

2.5011

2.5012

2.4786

Average mass of post-1982 pennies ____2.5019 G________________________ Mass of 75 post-1982 pennies _____187.6425_____________________ Number of post-1982 pennies present in total mass of 250. G __________100 pennies___________________

Instructor 25-penny mass ____64.8900______________________ Calculations for determining how many pre-1982 and how many post-1982 pennies are in the Instructor sample: X represents the pre 1982- pennies, y will be the post 1982 X+Y = 25- x=25 – y X(average mass of pre-1982 pennies) + y(average mass of the post 1982 pennies) = 64.8900 (25-y)(average mass of pre-1982 pennies) + Y(average mass of the post-1982 pennies)= 64.8900 25(average mass of pre-1982 pennies)- Y(average mas of pre-1982 pennies) + y(average mass of post-1982 pennies) = 64.8900 77.6025 – Y(3.1042) + Y(2.5019) = 64.8900

77.6025- -3.5195+ -2.8367=64.8900

-y(3.1042)+Y(2.5019)= -12.7125 2Y(5.6061)= -12.7125 2Y=-2.2676 Y=-1.1338 POST-LAB ANALYSIS 0) Suppose several groups did this lab in a laboratory setting. How do you think your calculated averages would compare to the averages of the other groups? If differences did occur, what factor(s) might account for those differences? I don’t think the averages would be that different as no one can control how many of post or pre 1982 pennies you get, the varying averages would be due to that factor other than the environment that each group would be in.

2) The element krypton exists in numerous isotopic forms. Below are the six most abundant isotopes of krypton, along with their exact masses and relative abundances. Based on this data, calculate the average atomic mass of a krypton atom. Isotope

Mass

Abundance

Kr-78

77.920

0.35%

Kr-80

79.916

2.28%

Kr-82

81.913

11.58%

Kr-83

82.914

11.49%

Kr-84

83.911

57.00%

Kr-86

85.911

17.30%

The average mass of a krypton atom would be 0.838....