University:
University of California, Berkeley
Course:
CHEM 1A | General Chemistry
Academic year:

2024
Views:

260

Pages:

15
Author:

africadas23u

Average Atomic Mass What is a weighted average? Isotopes determine the average mass of a sample A sample of matter consists of all naturally occurring isotopes in various percentages. Can we have a moment of silence? Since that time, average family size has declined, driven largely by declines in families with four or more children. Now, moms have 2.4 children on average – a number that has been fairly stable for two decades. What does “weighted average” mean? You don’t simply take the mass of each isotope and divide by the total number. This would imply they are found in equal amounts. (12+13+14)/3 = 13 This would imply that 12, 13, and 14 are found at 33% each. Carbon is not found in equal amount There is another way! You will get the same answer by multiplying each value by 33%, and then adding them up. (without sig figs of course) 12*.33=4.0 13*.33=4.3 14*.33=4.6 4.0 + 4.3 + 4.6 = 12.9 (or 13 if staying consistent) To calculate average atomic mass... You need to know two things: 1. The mass of each isotope present in the sample. 2. The percent makeup of the isotope in the sample, or the relative abundance (simply put, the percent already converted to a decimal.) Important sig fig issues to note Sometimes, the mass of the isotope given to you looks funny. 1. Sometimes the mass is given as a mass number, this is an exact number because it is based on a count of nucleons. ● i.e.-The mass number of a carbon-12 isotope is 12 amu Important sig fig issues to note 2. Sometimes the number is actually measured or calculated. Now it will be slightly less than the near whole number. ● i.e.-The mass of the carbon-12 isotope is 11.967 amu. Go based on the data given in the problem. Why is it less? I will explain, but don’t worry. Now for some examples! Go to the chapter 3 packet, page 11 for practice. Worksheet problem #1 (sig figs issued) .05 x 176 .19 x 177 .27 x 178 .14 x 179 .35 x 180 = 8.80 = 33.63 = 48.06 = 25.06 = + 63.00 178.55 Worksheet problem #2 .800 x 127 = 102 .170 x 126 = 21.4 .0300 x 128 = 3.84 102 + 21.4 + 3.84 127 amu Worksheet problem #3 .5000 x 197 = 98.50 .5000 x 198 = 99.00 98.50 +99.00 197.50 amu Worksheet problem #4 .0730 x 6.017 = .439 .9270 x 7.018 = 6.506 + .439 6.506 6.945 Worksheet problem #5 .3758 x 191.0 = 71.78 .6242 x 193.0 = 120.5 71.78 + 120.5 192.3 amu