Atomic Mass Micro World atoms molecules Macro World

Atomic Mass

Micro World atoms & molecules Macro World grams Atomic mass is the mass of an atom in atomic mass units (amu) By definition: 1 atom 12 C “weighs” 12 amu On this scale 1 H = 1. 008 amu 16 O = 16. 00 amu 2

The average atomic mass is the weighted average of all of the naturally occurring isotopes of the element. 3

Average Atomic mass of an element = ( % abundance of Isotope #1 x mass of Isotope #1 ) + ( % abundance of Isotope #2 x mass of Isotope #2 ) + 4 …

Example Silver has two naturally occurring isotopes. Ag-107 has an abundance of 51. 82% and mass of 106. 9 amu. Ag-109 has a relative abundance of 48. 18% and a mass of 108. 9 amu. Calculate the average atomic mass of silver.

Example Rubidium is a soft, silvery-white metal that has two common 85 87 isotopes, 37 Rb and 37 Rb. If the abundance of 85 Rb is 72. 2% with an atomic mass of 84. 91 amu and the abundance of 87 Rb is 27. 8% with an atomic mass of 86. 91 amu, what is the average atomic mass of rubidium?

Example Copper, a metal known since ancient among other things. times, is used in electrical cables and pennies, The atomic masses of its two stable isotopes, (69. 09 percent) and percent), are 62. 93 amu and 64. 9278 amu, respectively. (30. 91 Calculate the average atomic mass of copper. The relative abundances are given in parentheses.

Average atomic mass (63. 55) 8

Heavy Light Mass Spectrometry 9

Heavy Light Mass Spectrum of Ne 10

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Formula mass is the sum of the atomic masses (in amu) in a formula unit of an ionic compound. 1 Na Molecular mass (or molecular weight) is the sum of the atomic masses (in amu) in a molecule. 1 S 22. 99 amu 32. 07 amu 2 O + 2 x 16. 00 amu 64. 07 amu SO 2 1 Cl + 35. 45 amu Na. Cl 58. 44 amu SO 2 Na. Cl 1 molecule SO 2 = 64. 07 amu 1 formula unit Na. Cl = 58. 44 amu 12

Example Calculate the molecular masses (in amu) of the following compounds: (a)sulfur dioxide (SO 2), a gas that is responsible for acid rain (b)caffeine (C 8 H 10 N 4 O 2), a stimulant present in tea, coffee, and cola beverages

The Mole (mol): SI base unit to measure the amount of a substance (count numbers of particles) Dozen = 12 Pair = 2 The mole (mol) is the SI base unit to measure the amount of a substance (used to count numbers of particles) 1 mol = NA = 6. 022 x 1023 Avogadro’s number (NA) 14

atoms Molar mass is the mass of 1 mole of molecules in grams formula units 1 mole 12 C atoms = 6. 022 x 1023 atoms = 12. 00 g 1 12 C atom = 12. 00 amu For any element atomic mass (amu) = molar mass (grams) 1 atom of Li = 6. 941 amu 1 mole of Li atoms = 6. 941 g Li For any ionic compound 1 formula unit Na. Cl = 58. 44 amu formula mass (amu) = molar mass (grams) 1 mole Na. Cl = 58. 44 g Na. Cl For any molecule 1 molecule SO 2 = 64. 07 amu molecular mass (amu) = molar mass (grams) 1 mole SO 2 = 64. 07 g SO 2 15

One Mole of: S C Hg Cu Fe 16

M = molar mass in g/mol NA = Avogadro’s number 17

Example Helium (He) is a valuable gas used in industry, low-temperature research, deepsea diving tanks, and balloons. How many moles of He atoms are in 6. 46 g of He?

Example Zinc (Zn) is a silvery metal that is used in making brass (with copper) and in plating iron to prevent corrosion. How many grams of Zn are in 0. 356 mole of Zn?

Example Sulfur (S) is a nonmetallic element that is present in coal. When coal is burned, sulfur is converted to sulfur dioxide and eventually to sulfuric acid that gives rise to the acid rain phenomenon. How many atoms are in 16. 3 g of S?

Example A sample of aluminum chloride (Al. Cl 3) has a mass of 35. 6 g. How many chloride ions are present?

Example How many oxygen atoms are in 37. 5 grams of the molecular compound sulfur trioxide (SO 3)