Lecture 5

THE MOLE Avogadro's number The mole is used when we're talking about numbers of atoms and molecules (tiny particles).moleatomsmolecules The mole is the “chemist’s dozen 1 dozen particles = 12 particles 1 mole particles = x particles Mole is the amount of a substance which contains x “items” “items” could be: atoms, molecules, ions, The mole is used to describe a huge amount of any extremely small particle.

= 6.02 x C atoms = 6.02 x H 2 O molecules = 6.02 x NaCl “molecules” (technically, ionics are compounds not molecules so they are called formula units) 6.02 x Na + ions and  6.02 x Cl – ions A Mole of Particles A Mole of Particles Contains 6.02 x particles 1 mole C 1 mole H 2 O 1 mole NaCl Note that the NUMBER is always the same, but the MASS is very different!

Avogadro's number  The number of atoms per mole is known as Avogadro's number N A, 6.02 x  Number of atoms in 12 g of carbon-12 is Avagadro’s number (N A ) which equals 6.02 x VariableMeaningUnit Nnumber of “particles”atoms, ions, formula units, molecules NANA Avogadro’s number 6.02 x “ particles” per 1 mole = 6.02 x “ particles” per 1 mole nnumber of molesmol N A = N n N = N A. n n= N N A

Mole Calculations 1. To convert from atoms (or molecules) to moles, divide the # of atoms (or molecules) by Avogadro’s # Example: How many moles are 1.0x10 24 atoms? 2. To convert from moles to atoms (or molecules), multiply the # of moles by Avogadro’s # Example: How many molecules are in 2.5 moles? n= N N A N = N A. n

(MM) (g/mol) Molar Mass (MM) (g/mol) MOLE IS quantity of substance that contains the same number of molecules as exactly 12 g of carbon Molar mass is the mass of one mole of any element or compound. (in grams) The molar mass for elements is found by looking at the atomic masses on the Periodic Table. The units for molar mass are “grams per mole”

Periodic Table- The number below any element symbol on the periodic chart in grams is 6.02 x [avogadro’s number] of that element.

Molar Mass for Compounds Molar Masses are equal to the sums of the molar masses of all atoms in one molecule of that chemical compound Ex. NaCl Contains one atom of sodium and one atom of chlorine. (1 atom x 23.0 g/mole Na) + (1 atom x 35.5 g/mole Cl) = 58.5 g/mole NaCl  Formula Weight (FW) = 58.5 amu/f.u. The mass of 1 mole of Glucose (C 6 H 12 O 6 ) is _180.15g/mole. MM of C = 6 x 12= 72g MM of H = 12x 1= 12g MM of O = 6x 16 = g/ mole Molecular Weight (MW) = amu/molecule

Molar Mass of Compounds  Another example: H 2 O H: 2 x = O: 1 x = The molar mass of H 2 O =  Example: CaSO 4 1 Ca: x 40.1g = 40.1 g 1 S: x 32.1g = 32.1 g 4 O: x 16.0g = 64.0g Then add masses of all elements together g/mol g g 136 g/mol

Mole-Mass Calculations 1. To convert from moles to grams, multiply the # of moles by atomic mass Example: How many grams in 2.5 moles of carbon? 2. To convert from grams to moles, divide the mass in grams by atomic mass Example: How many moles are in 2.5 g of lithium? Mole-Mass Relationship n = m M g molg/mol g = mol x g/mol mol = g  g/mol

Multi Step Example  How many magnesium atoms are contained in 5.00 g of Mg?  5.00g x 1mol x 6.02 x atoms g 1 mol = 30.1 x = 3.01 x divide by = x = 1.24 x atoms of magnesium