Chemistry Topic 3

The Mathematics of Formulas and Equations

• Mass of atom is based on the mass of a Carbon-12 atom

• Formula Mass – sum of the atomic masses of all the atoms present.

• Gram Formula Mass – formula mass expressed in grams instead of atomic mass units.

• Use 35.5 for Cl

Find the Gram Formula Masses

• Na

• O2

• NaCl

• HBr

• NaOH

• Mg3N2

• Al2(SO4)3

• Percent Composition – composition as a percentage of each element compared to the total mass of the compound.

Find the Percent Composition

• Na

• O2

• NaCl

• HBr

• NaOH

• Mg3N2

• Al2(SO4)3

• Hydrates – crystals that contain water.

• Anhydrous – substances without water

• To calculate % water, treat the water molecule as a single unit.

Calculate % Water

• CuSO4∙5H2O

11/6 Chemistry

Aim: What calculations can we perform with formulas?

Obj: SWBAT calculate moles, find molecular formulas, and set up mole ratios

Do Now: What is the formula mass of NaOH?

Homework: Worksheet

Moles

• Mole = number of atoms of carbon present in 12.000 g of C-12

• Avogadro’s number = 6.022 x 1023

– Number of particles in a mole of a substance

• Moles = grams / gram formula mass

• Grams = moles x gram formula mass

• What is the mass in grams of a 8.4 mole sample of iron?

• Convert 0.45 g of sodium hydroxide, NaOH to moles.

• How many moles of potassium nitrate, KNO3 are present in a sample with a mass of 85.2 g.

• What is the mass in grams of 0.94 moles of sodium bicarbonate, NaHCO3?

Finding Molecular Formulas from Empirical Formulas

• Find the formula mass of the empirical formula

• Divide mass of compound by the mass of the empirical formula.

• Multiply your empirical by that answer.

• A compound has an empirical formula of CH2 and a molecular mass of 56 amu. What is its molecular formula?

• A compound has an empirical formula of CH and a molecular mass of 78 amu. What is the molecular formula?

Mole Relations in Balanced Equations

• Set up ratios

• Also works for volumes of gases