Unit 8: The Mole

Formula weight vs.Molecular weight Formula weight – sum of the atomic

weights of each atom in its chemical formula The formula weight of sulfuric acid (H2SO4) is

98.1 amu If the chemical formula is that of a

molecule, then the formula weight is also called the molecular weight The molecular weight of glucose (C6H12O6) is

180.0 amu

Percentage Composition from Formulas To find the percentage of each element

that is in a compound, the following equation is used:

% element = (number of atoms of that element)(atomic weight of element) formula weight of compound

x 100

The Mole

(the real mole, not the chemistry one

The Mole The term, mole, is used to express the

number of atoms, ions, or molecules (mol) A mole is the amount of matter that

contains as many objects (atoms, molecules, or whatever objects we are considering) as the number of atoms in exactly 12 g of 12C

From experiments, scientists have determined that number to be 6.0221421 x 1023

Avogadro’s Number The number, 6.022 x 1023 is called

Avogadro’s number This number too large to imagine Spreading 6.02 x 1023 marbles over the

entire surface of the Earth would produce a layer about 3 miles thick

If you lay 6.02 x 1023 pennies side by side in a straight line, they would encircle the Earth 300 trillion (3 x 1014) times

Molar Mass What is molar mass?

Molar mass is the amount of the mass (in grams) of 1 mol of a certain element

Cl has a molar mass of 35.5 g/mol NaCl has a molar mass of 58.5 g/mol Molar mass is always numerically equal

to the formula weight

MoleLiters of a

gasparticle

Mass (grams)

22.4 L1 mol

1 mol22.4 L

6.02 x 1023 particle 1 mol

1 mol6.02 x 1023 particle

1 mol MM

MM1 mol

Empirical Formula Shows the relative number of atoms of

each element in a formula Steps to determine empirical formula:

1. If given % of element, change the % to g2. Determine the moles of each element

(divide by the MM)3. Divide by the lowest mole value

Empirical Formula Problem Ascorbic acid (vitamin C) contains

40.92% C, 4.58% H, and 54.50% O by mass. What is the empirical formula of ascorbic acid?

Molecular Formula Molecular formula can be obtained only

if you are given the molecular weight The subscripts in the molecular formula

of a substance are always a whole number multiple of the corresponding subscripts in its empirical formula

Whole-number multiple =

molecular weightempirical formula

weight

Molecular Formula Problem What is the molecular formula of

ethylene glycol, substance used in automobile antifreeze, which is composed of 38.7% C, 9.7%H, and 51.6% O by mass?