CALCULATINGEMPIRICALFORMULAS

“Won’t you gimme three steps,Gimme three steps mister,Gimme three steps towards the …”

EMPIRICAL FORMULA

Calculating Empirical Formula1. Convert grams to moles

2. Calculate simplest mole ratio

3. Write empirical formula

Step 1: Convert grams to moles.

When given percentages, assume a 100.00-gram sample of the unknown compound is available

Use the numbers given as percentages with the unit grams

Convert grams to moles using molar mass

10.89% Mg, 31.77% Cl, 57.34% O

10.89 g Mg 1 mol Mg

24.3 g Mg= 0.4481 mol Mg mol Mg =

10.89 g Mg, 31.77 g Cl, 57.34 g O

= 0.8949 mol Cl

mol Cl =

= 3.584 mol O mol O =

31.77 g Cl

57.34 g O

35.5 g Cl1 mol Cl

16.0 g O

1 mol O

Step 2: Calculate simplest mole ratio. Divide the number of moles of each element

by the smallest value calculated in Step 1 If the result from the division is not a whole

number, multiply it by the lowest factor that will result in a whole numberAll other results must be multiplied by this same

factor Rounding, rather than multiplying, can only

occur when the result is very close to a whole number

0.4481 mol

0.4481 mol

0.4481 mol

0.4481 mol Mg0.8949 mol Cl3.584 mol O

3.584 mol

0.4481 mol

0.8949 mol

= 1.000

= 1.999

= 7.998

= 1

= 2

= 8

Mg:

Cl:

O:

Step 3: Write empirical formula.

The answers from Step 2 become the subscripts for each element in the empirical formula

Do not write subscripts of 1 The empirical formula represents the

simplest whole number ratio of elements in a compound

1Mg:

2Cl:

8O:

Mg Cl O1 2 8

Mg Cl O1 2 8

MgCl2O8Mg(ClO4)2

Determine the empirical formula for: A compound containing 58.81%

barium, 13.73 % sulfur, and 27.46% oxygen.BaSO4

A compound containing 36.5% sodium, 25.4% sulfur, and 38.1% oxygen.Na2SO3

Empirical Formula HW

p. 349: #144 and #145

Due Friday, 02/03

Molecular Formula

Given:Empirical formula (or data to calculate)Molecular mass

To calculate molecular formula:

molar mass of molecular formula

molar mass of empirical formula

Calculating Molecular Formula

The empirical formula of a compound is CH4. What is the molecular formula when the molecular mass is 64.0 g/mol?C4H16

A compound with a molar mass of 90.0 g/mol has the empirical formula CH2O. What is its molecular formula?C3H6O3