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Warm-Up

- Write the formulas that go with the following names:
- TricarbonPentaoxide
- Osmium (III) Sulfide
- Magnesium Nitride
- DinitrogenTetraoxide

Empirical Formula and Molecular Formula

- Empirical formula is the simplest chemical formula
- Ex. CH4
- Molecular formula is the chemical formula that can be reduced.
- Molecular Formula- the formula for a compound in which the subscripts give the actual number of each element in the formulas it truly exists.
- Ex. C2H8

Notice two things: 1. The molecular formula and the empirical formula can be identical. 2. You scale up from the empirical formula to the molecular formula by a whole number factor.

Calculating the empirical formula from the percent composition

- Steps
- Assume the percent = grams.
- Convert each amount of grams to moles.
- Divide each number of moles by the smallest number of moles.
- If all the numbers are not whole numbers, multiple everything by a number that will get you a whole number. (This may take a few tries!)
- The whole numbers become subscripts in the formula.

Percent to mass

- Mass to mole
- Divide by small
- Multiply ‘til whole

Example

- A compound is 32.38% sodium, 22.65% sulfur, and 44.99% oxygen. What is the empirical formula?

Example 2

- A compound is 40.0% Carbon; 6.71% Hydrogen; 53.29% Oxygen. What is its empirical formula?

Practice

- 89.94% C; the rest is H
- 56.34% P; 43.66% O
- 43.64% P; 56.36% O
- 40.9% C; 4.58% H; 54.5% O

Now, finding the molecular formula

- Find the empirical formula
- Calculate the molar mass (formula mass)
- Divide the given mass of the substance by the molar mass
- Multiply each subscript of the empirical formula by the answer to #3

Example

- You have 56.106 grams of a substance with an empirical formula of CH2. What is the molecular formula?

CuSO4 ● 5H2O

- MgSO4 ● 7H2O
- SnCl2 ● 2H2O
- Na2CO3 ● 10H2O

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