# Calculating Empirical formulas - PowerPoint PPT Presentation

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Calculating Empirical formulas. EMPIRICAL FORMULA. “Won’t you gimme three steps, Gimme three steps mister, Gimme three steps towards the …”. Calculating Empirical Formula. Convert grams to moles Calculate simplest mole ratio Write empirical formula. Step 1: Convert grams to moles.

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Calculating Empirical formulas

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## CalculatingEmpiricalformulas

EMPIRICAL FORMULA

### Calculating Empirical Formula

• Convert grams to moles

• Calculate simplest mole ratio

• 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

10.89g Mg, 31.77gCl, 57.34g O

10.89 g Mg

1 mol Mg

mol Mg =

= 0.4481 mol Mg

24.3 g Mg

31.77 g Cl

1 molCl

molCl =

= 0.8949 molCl

35.5 g Cl

57.34 g O

1 molO

mol O =

= 3.584 mol O

16.0 g 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 number

• All 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 Mg

0.8949 molCl

3.584 mol O

0.4481 mol

Mg:

= 1.000

= 1

0.4481 mol

0.8949 mol

Cl:

= 1.999

= 2

0.4481 mol

3.584 mol

O:

= 7.998

= 8

0.4481 mol

### 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

Mg:

1

Mg Cl O

Cl:

2

O:

8

1

2

8

Mg(ClO4)2

MgCl2O8

Mg Cl O

1

2

8

### 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

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