6/28/11. Avogadro s Number and the Mole. The Mole. The Mole. The Mole (mol)

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1 Avogadro s Number and the Mole Molecular weight: The sum of atomic weights of all atoms in a molecule. Formula weight: The sum of atomic weights of all atoms in one formula unit of any compound. Mole: One mole of any substance is the amount whose mass in grams (molar mass) is numerically equal to its molecular or formula weight Avogadro s number: The number of molecules or formula units in a mole. N A = x A mole is a count of things, like a dozen eggs (only many, many, many more than a dozen!) The Mole Now we know what we have to do if we sit down and do the reaction one molecule at a time (this is very tedious) Chemists have a measure that allows us to package lots of atoms or molecules or whatever: The Mole (mol) A mole is just a big pile of stuff (whatever we are counting) a mole, whether it is a mole of iron atoms, a mole of octane molecules, or a mole of sodium ions (or eggs or elephants) always contains the same number of formula units The Mole Mole (mol) A mole of the amount of substance that contains as many atoms, molecules, or ions as are in exactly 12 g of carbon-12 Its value is x formula units per mole the number of formula units in a mole is known as Avogadro s number (know this number!) A mole, whether it is a mole of iron atoms, a mole of methane molecules, or a mole of sodium ions, always contains the same number of formula units Weighing vs. Counting Moles (numbers of things) can be associated with mass through the molar mass Molar mass is determined by the formula (or molecular) weight This conversion allows us to weigh things out and know how many atoms, ions or molecules we are working with 1

2 Review: Formula Weight Formula weight: the sum of the atomic weights in atomic mass units (amu) of all atoms in a compound s formula Molar Mass Molar Mass We can use molar mass to convert from grams to moles, and from moles to grams Calculate the number of moles of water in 36.0 g water 2

3 Grams to Moles Calculate the number of moles of sodium ions, Na +, in 5.63 g of sodium sulfate, Na 2 SO 4 first we find the how many moles of sodium sulfate the formula weight of Na 2 SO 4 is 2(23.0) (16.0) = amu therefore, 1 mol of Na 2 SO 4 = g Na 2 SO 4 the formula Na 2 SO 4 tells us there are two moles of Na + ions per mole of Na 2 SO 4 The molar mass of water is 18.0 g. The conversion factor between moles of water and mass of water is 18.0 g/mol and the conversion factor between mass of water and moles of water is 1 mol/18.0 g: Copyright 2010 Pearson Education, Inc. Chapter Six 8 Mole Relationships and Chemical Equations The coefficients in a balanced chemical equation tell how many molecules, and thus how many moles, of each reactant are needed and how many molecules, and thus moles, of each product are formed. 3

4 The coefficients can be put in the form of mole ratios, which act as conversion factors when setting up factor-label calculations. In the ammonia synthesis the mole ratio of H 2 to N 2 is 3:1, the mole ratio of H 2 to NH 3 is 3:2, and the mole ratio of N 2 to NH 3 is 1:2 leading to the following conversion factors: (3 mol H 2 )/(1 mol N 2 ) (3 mol H 2 )/(2 mol NH 3 ) (1 mol N 2 )/(2 mol NH 3 ) 13 Easy example: how many grams of ethylene oxide are required to produce 500 g of ethylene glycol? (and why should I care?) First, do we have a balanced equation? 4

5 C 2 H 4 O H 2 O C 2 H 6 O 2 Mass? 500 g MW 44 g/mol 18 g/mol 62 g/mol Moles So let s try something a little harder: How much oxygen does it take to burn 100 g of octane? How much water is produced? Mass 100 g?? MW 114 g/mol 32 g/mol 18 g/mol Moles