UnitsConversions,3Ed.pdf

Constants, Units and Conversions (? S. O. Kasap, 1990 - 2001: v.1.0) 1 An e-Booklet Constants, Units and Conversions Safa Kasap Department of Electrical Engineering University of Saskatchewan Canada 1. Atomic Mass and Avogadro’s Number Atomic number (Z) is the number of protons in the nucleus of an atom. Atomic mass number (A ) is the number of protons and neutrons in the nucleus. If Z is the atomic number and N is the neutron number then A = Z + N. Atomic mass unit (amu or u) is a convenient atomic mass unit that is equal to 1 / 12 of the mass of a neutral carbon atom which has a mass number A = 12 (6 protons and 6 neutrons). It has been found that u = 1.66054 × 10 ?27 kg. It is equivalent to 10 -3 /N A where N A = 6.022 × 10 23 is called Avogadro’s number. Atomic mass (or relative atomic mass or atomic weight, M at ) of an element is the average atomic mass, in atomic mass units (u), of all the naturally occurring isotopes of the element. Atomic masses are listed in the Periodic Table. The amount of an element which has 6.022 × 10 23 atoms (Avogadro’s number of atoms) has a mass in grams equal to the atomic mass. This means we can express the atomic mass as grams per unit mole, g mol -1 . Avogadros number (N A ) is the number of atoms in exactly 12 grams of Carbon-12. It is 6.022 × 10 23 . Since atomic mass is defined as 1 / 12 of the mass of the carbon-12 atom, N A number of atoms of any substance has a mass equal to the atomic mass M at in grams. Mole of a substance is that amount of the substance which contains N A number of atoms (or molecules) where N A is Avogadros number (6.022 × 10 23 ). One mole of a substance has a mass as much as its atomic (molecular) mass in grams. For example, 1 mole of copper contains 6.022 × 10 23 number of copper atoms and has a mass of 63.55 grams. Problem: Molecular mass and mass of molecule Find the molecular mass and the mass of one molecule of carbon dioxide (CO 2 ) given that the atomic mass of O is