Chapter 7 Notes, part III-% composition-empirical formulas-molecular formulas

In ReviewThe mole is an SI unit of measure for amount of particles6.02x1023 particles = 1 mole22.4 L = 1 moleMolar mass=1 mole

% CompositionThe percent composition of an element is the relative amount of that element in a compound.You find the percent composition of an element by dividing the elements mass by the mass of the entire compound. mass of elementmass of compoundx 100

For example:What is the % composition of carbon dioxide?Mass of oxygen = 2(16.0)=32.0 gMass of carbon = 12.0 gMass of carbon dioxide = 44.0 g 12.0 g C 44.0 g CO2

32.0 g O44.0 g CO2x100 =27.3% Carbonx100 =72.7% Oxygen

Try theseWhat is the percent of each element in phosphorus trichloride?

If 20.0 grams Ca react completely with 16.0 grams of S to form a compound, what is the percent of S?

Empirical FormulaThe empirical formula gives you the lowest, whole number ratio of elements in the compoundThe empirical formula may or may not be the same as the molecular formula

For example:For carbon dioxide, the molecular formula is CO2, and the empirical formula is CO2. One carbon and two oxygens are the lowest ratio of atoms.The molecular formula for dinitrogen tetrahydride is N2H4, but the empirical formula is NH2.

What is the empirical formula for:C6H12O6C6H12O2N2H2CH4

To Find the Empirical Formula from % Composition:Given the percentages, assume there are 100.0 grams of the compound.Convert the grams of each element to moles.Divide by the smaller amount of moles, then manipulate the ratio so that all numbers are whole.

An Example:What is the empirical formula of a compound that is 25.9% nitrogen and 74.1% oxygen?

More to tryCalculate the empirical formula of a compound that is 94.1% oxygen, 5.9% hydrogen.Calculate the empirical formula of a compound that is 79.8% carbon, 20.2% hydrogen.

From there, find its molecular formula:If given the molar mass (how many g/mol of the compound) then you can calculate the molecular formula from the empirical formula.Take the compounds empirical formula mass and compare to the molecular mass. The molecular mass will be a multiple of the empirical formulas mass.

Example:If the molecular mass of the first example (N2O5) problem is 216 g/mol, then what is the molecular formula for the compound?

Try these:What is the molecular formula of a compound whose molar mass is 60.0 g and whose empirical formula is CH4N?What is the molecular formula for a compound whose molar mass is 78 g and whose empirical formula is CH?