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### Empirical and mOlecular Formulas

Daily Sciencepg. 74

- How many atoms are in 3.6 mol of calcium?
- How many moles are in 1.45 x 1024 atoms of sodium?
- What is the molar mass of K2SO4?
- How many grams of CO are in 2.9 moles?
- How many moles is 236 g of NaOH?
- What is the mass of 7.6 x 1023 atoms of KF?
- How many atoms are in 192 g of MgCl2?

Pg. 73

Determining moles in a compound

- Formulas tell us ratios of the number of atoms in the compound
- Ex. Freon is CCl2F2
- There is 1 C, 2 Cl, and 2 F

- We can figure out how many moles of one element there are in a given compound.
- Moles of compound x # of atoms of element
1 mole of compound

Ex.

- How many moles of F are in 5.50 moles of CCl2F2?
- Calculate the number of moles of each element in 1.25 moles of C6H12O6.

Percent composition

- First, find the molar mass of the compound
- Multiply the mass of one element by its subscript
- Divide the mass of one element by the total mass in the compound
- Multiply by 100
- Ex. H2O

Ex.

- Determine the percent by mass of each element in the compound CaCl2.

Empirical Formulas

- Empirical formula = the formula with the smallest whole number ratio of the elements
- If you know the percent composition of the element, divide the amount of that element by its molar mass.
- This tells you # moles of that element.
- If it is written as a percent, assume there are 100 g of the compound
- If the numbers aren’t whole numbers, divide by the smaller value
- If the numbers still aren’t whole numbers, multiply by the smallest factor that will make them whole

Ex.

- Compound with 40.05% S and 59.95% O
- Determine the empirical formula for methyl acetate which has the following chemical make up: 48.64% C, 8.16% H, and 43.20% O.

Molecular formulas

- Sometimes the empirical formula is the same for two different compounds
- When that happens, you take the experimentally determined molar mass of the compound and divide it by the mass of the empirical formula
- This gives you a whole number (n) that must be multiplied into the empirical formula

Ex.

- Succinic acid is composed of 40.68% C, 5.08% H and 54.24% oxygen and has a molar mass of 118.1 g/mol. Determine the empirical and molecular formulas for succinic acid.

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