Chapter 11 Notes, part II -% composition. In Review The mole is an SI unit of measureThe mole is an SI unit of measure for amount of particles for amount.

Chapter 11 Notes, part II

Chapter 11 Notes, part II-% composition-% composition

In ReviewIn Review• The mole is an SI unit of measure The mole is an SI unit of measure

for amount of particlesfor amount of particles• 6.02x106.02x102323 particles = 1 mole particles = 1 mole• Molar mass=1 moleMolar mass=1 mole

% Composition% Composition• The percent composition of an element The percent composition of an element

is the relative amount of that element is the relative amount of that element in a compound.in a compound.

• You find the percent composition of an You find the percent composition of an element by dividing the element’s mass element by dividing the element’s mass by the mass of the entire compound.by the mass of the entire compound.

mass of elementmass of element

mass of mass of compoundcompound

x 100x 100

For example:For example:• What is the % composition of carbon What is the % composition of carbon

dioxide?dioxide?

• What is the % composition of carbon What is the % composition of carbon dioxide?dioxide?

Mass of oxygen = 2(16.0)=32.0 gMass of oxygen = 2(16.0)=32.0 gMass of carbon = 12.0 gMass of carbon = 12.0 gMass of carbon dioxide = 44.0 gMass of carbon dioxide = 44.0 g 12.0 g C12.0 g C 44.0 g CO44.0 g CO22

32.0 g O32.0 g O44.0 g CO44.0 g CO22

For example:For example:

x100 =x100 =27.3% 27.3% CarbonCarbon

x100 =x100 =72.7% Oxygen72.7% Oxygen

Try these…Try these…

• What is the percent of each What is the percent of each element in phosphorus trichloride?element in phosphorus trichloride?

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

Empirical FormulaEmpirical Formula

• The empirical formula gives you The empirical formula gives you the lowest, whole number ratio of the lowest, whole number ratio of elements in the compound.elements in the compound.

• The empirical formula may or may The empirical formula may or may not be the same as the molecular not be the same as the molecular formula.formula.

For example:For example:

• For carbon dioxide, the molecular For carbon dioxide, the molecular formula is CO2, and the empirical formula is CO2, and the empirical formula is CO2. One carbon and two formula is CO2. One carbon and two oxygens are the lowest ratio of oxygens are the lowest ratio of atoms.atoms.

• The molecular formula for dinitrogen The molecular formula for dinitrogen tetrahydride is N2H4, but the tetrahydride is N2H4, but the empirical formula is NH2.empirical formula is NH2.

What is the empirical What is the empirical formula for:formula for:

•CC66HH1212OO66

•CC66HH1212OO22

•NN22HH22

•CHCH44

To Find the Empirical To Find the Empirical Formula from % Formula from %

Composition:Composition:• If given the percentages, assume If given the percentages, assume

there are 100.0 grams of the there are 100.0 grams of the compound.compound.

• Convert the grams of each element to Convert the grams of each element to moles.moles.

• Divide by the smaller amount of Divide by the smaller amount of moles, then manipulate the ratio so moles, then manipulate the ratio so that all numbers are whole.that all numbers are whole.

Two Examples:Two Examples:

• What is the empirical formula of a What is the empirical formula of a compound that is 27.3% carbon compound that is 27.3% carbon and 72.7% oxygen?and 72.7% oxygen?

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

More to try…More to try…

• Calculate the empirical formula of Calculate the empirical formula of a compound that is 94.1% oxygen, a compound that is 94.1% oxygen, 5.9% hydrogen.5.9% hydrogen.

• Calculate the empirical formula of Calculate the empirical formula of a compound that is 79.8% carbon, a compound that is 79.8% carbon, 20.2% hydrogen.20.2% hydrogen.

From there, find it’s From there, find it’s molecular formula:molecular formula:

• If given the molar mass (how many If given the molar mass (how many g/mol of the compound) then you g/mol of the compound) then you can calculate the molecular formula can calculate the molecular formula from the empirical formula.from the empirical formula.

• Take the compound’s empirical Take the compound’s empirical formula mass and compare to the formula mass and compare to the molecular mass. The molecular molecular mass. The molecular mass will be a multiple of the mass will be a multiple of the empirical formula’s mass.empirical formula’s mass.

Example:Example:

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

Try these:Try these:

• What is the molecular formula of a What is the molecular formula of a compound whose molar mass is 60.0 compound whose molar mass is 60.0 g and whose empirical formula is g and whose empirical formula is CH4N?CH4N?

• What is the molecular formula for a What is the molecular formula for a compound whose molar mass is 78 g compound whose molar mass is 78 g and whose empirical formula is CH?and whose empirical formula is CH?