University:
Montgomery College
Course:
CHEM 131: Principles of Chemistry I
Academic year:

2020
Views:

238

Pages:

2
Author:

do3ran7ut

Atomic Mass  One atomic mass unit (amu): the mass exactly equal to one-twelfth the mass of one carbon-12 atom that has six protons and six neutrons. 1 atom of carbon-12 = 12 amu 1 amu = mass of one C-12 atom 12 1 amu = 1.66054 x 10-24 g and 1 g = 6.02214 x 1023 amu  Average Atomic Mass: the weighted average of the masses of the naturally occurring isotopes of the element; the mass of the atom in atomic mass units Average Atomic Mass = ∑ (fractional abundance of isotope n) × (mass of isotope n)  Isotopes: atoms with identical atomic numbers but different mass numbers (that is, same number of protons but different numbers of neutrons) Average atomic mass of carbon = (0.09890)(12.00000 amu) + (0.0110)(13.00335 amu) = 12.0107 amu Atomic Number (number of protons) Mass number (number of protons plus neutrons) Carbon Isotopes Example If chlorine is 75.78 % Cl-35 with a mass of 34.9689 amu and the rest Cl-37 with a mass of 36.9659 amu, find chlorine’s atomic mass. Cl atomic mass = 0.7578 x 34.9689 + (1 - 0.7578) × 36.9659 = 35.45 amu Atomic Number Atomic Symbol Atomic Weight Atomic mass = (Fraction of isotope 1 x Mass of isotope 1) + (Fraction of isotope 2 x Mass of isotope 2) + (Fraction of isotope 3 x Mass of isotope 3) + …… Practice Problems 1. Three isotopes of silicon occur in nature. 28Si (92.23%), which has an atomic mass of 27.97693 amu; 29Si (4.68%), which has an atomic mass of 28.97649 amu; and 30 Si (3.09%), which has an atomic mass of 29.97377 amu. Calculate the atomic weight of silicon. 2. Gallium has two naturally occurring isotopes: Ga-69 with mass 68.9256 amu and a natural abundance of 60.11%, and Ga-71 with mass 70.9247 amu and a natural abundance of 39.89%. Calculate the atomic mass of gallium. 3. Bromine has two naturally occurring isotopes (Br-79 and Br-81) and an atomic mass of 79.904 amu. (a) If the natural abundance of Br-79 is 50.69%, what is the natural abundance of Br81? (b) If the mass of Br-81 is 80.9163 amu, what is the mass of Br-79? 4. Titanium has five common isotopes: 46Ti (8.25%), 47Ti (7.44%). 48Ti (73.72%), 49Ti (5.41%), 50Ti (5.18%). What is the average atomic mass of titanium? Isotope 46 Ti 47 Ti 48 Ti 49 Ti 50 Ti Aundance (fraction) 0.0825 0.0744 0.7372 0.0541 0.0518 Atomic mass (amu) 45.953 46.952 47.948 48.948 49.945 References: Tro, Chemistry: A Molecular Approach 2nd ed., Pearson Brown/LeMay/Bursten, Chemistry: The Central Science, 12th ed., Pearson 1. 28.09 amu 2. 69.72 amu 3. (a) 49.31% (b) 78.91 amu 4. 47.7 amu Answers