Chy102-Lecture 03

Chy102-Lecture 03 - Dr Raafat Aly 3 rd lecture General...

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Unformatted text preview: Dr. Raafat Aly 3 rd lecture General Chemistry Chem102 for Engineering 07/11/2007 D o n e 2 Objectives Define atomic masses Atomic masses and isotopic abundance Define the mole Calculate the number of moles Calculate the molar mass of any compound Determine the percent composition of compounds Determine the formula of compounds Balance chemical equations Stoichiometry in chemical reaction Calculations involving a limiting reactants Calculation of percent yield of a product 3 Atomic masses; the Carbon-12 Scale The mass of an atom is measured most easily relative to the mass of a chosen atomic standard. The modern system for atomic masses, is based on C-12 as the standard. In this system, C-12 is assigned as a mass of exactly 12 atomic mass units (amu) , and the masses of all other atoms are given relative to this standard . mass of C-12 atom = 12 amu (exactly) Similarly, 1 amu = mass of one carbon-12 atom ≈ mass H atom = 1.66 x 10-24 g 12 1 4 Atomic masses and Isotopic Abundance The reason is atoms have isotopes , and these isotopes do not all have the same masses. The mass of the atoms in nature – what we use as the atomic mass - is a weighted average of all these different isotopes . Why isn't the atomic mass of natural carbon exactly 12? 01 . 12 ] 0034 . 13 100 11 . 1 [ ] 00 . 12 100 89 . 98 [ = × + × = carbon natural of mass atomic Average 1.11% 13.0034 C-13 98.89% 12.00 C-12 Abundance Atomic mass Carbon found on earth (natural carbon) as a mixture of the isotopes C C C 14 13 12 Where, y n : is the mass of any isotope of the element n 100 ) (y mass atomic )% ( ....... 100 ) (y mass atomic )% ( 100 ) (y mass atomic )% ( ) ( n 2 2 1 1 × + + × + × = n y y y y mass Atomic Generally: Generally: 5 The Mole : A unit of measure used in counting atoms T The mass of 1 mole (6.022 x 10 23 atoms) of an element is equal to its atomic mass expressed in grams. One mole of a substance contains AVOGADRO’S NUMBER of that substance i.e. 6.022 x10 23 units of that substance . A sample of 12.01 g carbon contains the same number of atoms as 4.003 g helium. Both samples contain 1 mole of atoms (6.022 x 10 23 ) Mass – mole – number conversions Moles Number of Particles Mass × 6 .0 2 2 x 1 2 3 ÷ 6 .0 2 2 x 1 2 3 × m o l a r m a s s ÷ m o l a r m a s s Molar mass = atomic weight of element Or molecular weight of compound 1 mole 243 g 9.96x10- 24 moles ? g Example Americium is an element that does not occur in nature. It is made by very small amounts in a devise named as a particle accelerator. Compute the mass in grams of a sample of americium containing six atoms. (Atomic mass of Am = 243) 1 mole 6.022x10 23 atoms ? moles 6 atoms g mole g moles atoms americium six of Mass 21 24 10 42 . 2 1 243 10 96 . 9-- × = × × = mol atoms mol atoms atoms Am contains that moles of no 24 23 10 96 . 9 10 022 . 6 1 6 6 ....
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Chy102-Lecture 03 - Dr Raafat Aly 3 rd lecture General...

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