Stoichiometry and Molar Mass Calculations in Chemistry

Mole Amounts of Pure Substances

4.003 g Helium Gas, 63.55 g Copper Wire, 200.59 g Liquid Mercury, 12.01 g Carbon Powder, 32.06 g Solid Sulfur

Molar Mass

Calculation for the Molar Masses of Compounds

Molar Mass of Sodium Chloride

MNaCl: (1 x 22.99 g/mol) + (1 x 35.45 g/mol) = 58.44 g/mol

Molar Mass of Glucose

MC6H12O6: (6 x 12.01 g/mol) + (12 x 1.01 g/mol) + (6 x 16.00 g/mol) = 180.18 g/mol

Molar Mass of Calcium Phosphate

MCa3(PO4)2: (3 x 40.08 g/mol) + (2 x 30.97 g/mol) + (8 x 16.00 g/mol) = 310.18 g/mol

Formula Mass

Mass of a single molecule (for molecular compounds) or a single formula unit (for ionic compounds)

Terminology: Formula Mass

Applies to ionic compounds

Terminology: Molecular Mass

Applies to molecular compounds

Formula Mass Units

Expressed in atomic mass units (amu)

Atomic Mass Examples

H (1.01 amu), O (15.9994 amu), C (12.0107 amu)

Percent Composition Calculation

Calculate the Percent Composition of Methane (CH4): Sum = 74.83 % + 25.17 % = 100 %

Percent Composition for Binary Compounds

The % composition of 1st element is first found, then subtract from 100 to find the % composition of the 2nd element.

Percent Composition for Compounds with 3 Elements

Find the % compositions of 2 of the 3 elements, then subtract their sum from 100 to find the % composition of the 3rd element.

Percent Composition of Glucose

Calculate the percent composition of glucose, C6H12O6

Percent Composition of Iron(III) Carbonate

Calculate the percent composition of iron(III) carbonate, Fe2(CO3)3

Empirical Formulas

The empirical formula of a compound can be determined from knowing the percent compositions of all the element types present in the compound.

Empirical Formula

The simplest whole number ratio of elements in a compound.

Molecular Formula

The formula that shows the actual number of each type of atom in a molecule of a compound.

Stoichiometry

The quantitative relationship between the reactants and products in a balanced chemical equation.

Limiting Reactant

The reactant that is completely consumed first in a chemical reaction.

Excess Reactant

The reactant(s) that is/are leftover unreacted once the limiting reactant has been completely consumed.

Theoretical Yield

The maximum amount of product possible in a chemical reaction from given amounts of reactants.

Actual Yield

The amount of product obtained from a chemical reaction, which is often less than the theoretical yield.

Percent Yield

Quantitative measure of the efficiency and success of a chemical reaction.

Steps for Determining Empirical Formulas

1. Assume 100 g of the compound. 2. Equate the % composition of each element type to mass (g). 3. For each element type, convert g to mol. 4. Divide each converted mole value to the smallest mole value in the set. 5. If there are common decimals or fractions present, convert the number to an improper fraction, then multiply all values by the least common integer.

Finding the Molecular Formula

The molecular formula can be determined by knowing the empirical formula and the molecular mass of the compound, then calculating the ratio of the compound's molecular mass to its empirical mass.

Sample Calculation for Benzene

Benzene is composed of 92.24 % C and 7.76 % H by mass. Calculate its empirical formula.

Sample Calculation for Ascorbic Acid

Ascorbic acid is composed of 40.92 % C, 4.58 % H, and 54.50 % O by mass. The molecular mass of ascorbic acid is 176.14 amu. Calculate its molecular formula.

Balanced Chemical Equation

A chemical equation where the number of atoms for each element is the same on both sides of the equation.

Haber-Bosch Process

A method for producing ammonia, NH3, from nitrogen and hydrogen.

Combustion of Butane

The chemical reaction of butane, C4H10, with oxygen to produce carbon dioxide and water.

Common Decimals in Empirical Formulas

Common decimals or fractions present include 0.25 (1/4), 0.33 (1/3), 0.5 (1/2), 0.67 (2/3), 0.75 (3/4).

Stoichiometric Calculation

A type of calculation that involves chemical formulas and chemical equations.

Chemical Reaction

A process that leads to the transformation of one set of chemical substances to another.

Empirical Mass

The mass of the empirical formula of a compound.

Molecular Mass

The mass of a molecule, calculated as the sum of the atomic masses of its constituent atoms.

Subscript in Empirical Formula

The number that indicates the number of atoms of each element in the empirical formula.