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Calculating Empirical formulasPowerPoint Presentation

Calculating Empirical formulas

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Calculating Empirical formulas

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CalculatingEmpiricalformulas

EMPIRICAL FORMULA

- Convert grams to moles
- Calculate simplest mole ratio
- Write empirical formula

- 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

10.89g 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

- 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

- 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

- 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

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

- To calculate molecular formula:
molar mass of molecular formula

molar mass of empirical 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