# Daily Science pg. 74 - PowerPoint PPT Presentation

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Daily Science pg. 74. How many atoms are in 3.6 mol of calcium? How many moles are in 1.45 x 10 24 atoms of sodium? What is the molar mass of K 2 SO 4 ? 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 10 23 atoms of KF?

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Daily Science pg. 74

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### 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?

## Empirical and mOlecular Formulas

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.