Introduction to Stoichiometry and Conversion

Stoichiometry - quantitative relationships between the compounds in a chemical reaction

• measurement of the elements

• allow chemists to calculate the amounts of reactants needed for a reaction and to predict the quantity of product

• used to determine the formulas of compounds and simplify procedures in chemical analysis

Dimensional analysis method - predominant method taught for solving stoichiometry problems

• The goal of dimensional analysis is to use conversion factors to use conversion factors to calculate a desired piece of information. It helps to minimize memorization while applying basic chemical concepts like balanced equations, atomic mass, etc.

• Conversion factor - ratio equating two values of different units

• Conversions change the units of a value, not its magnitude.

Our modern version of the metric system is called the Systeme International, or S.I. The seven base units of the SI are defined in the table below.

Base units can be modified by the use of a metric prefix.

Metric prefix - number that multiplies the base unit

Mole - central unit of measurement in chemistry

• Avogadro’s Number - one mole numerically represents 6.02 x 10^23 units of a chemical substance

• can refer to one mole of atoms or one mole of molecules, depending on the substance

Gram-atomic mass - equal to one mole of an element

Gram-molar mass - equal to one mole of that compound

For a chemical compound, the gram-molar mass is equal to the sum of the gram-atomic masses of all atoms in the chemical formula.

Gram-molar masses of ions are determined by the atom(s) present in the ion since the gain or loss of electrons has virtually no effect on the total mass.

The formula mass is the same as the molar mass but the term molecular mass is only used to reference molecular substances.

Conversion factors can be derived from

• chemical formulas

• balanced equations

• concentration of a solution

  • Molarity - number of moles of a solute that are dissolved in one liter of solution

• density of a substance

• the relationship between the moles of a gas and the volume of a gas at STP

  • Standard temperature is 0 degrees Celsius and standard pressure is 1 atmosphere of pressure.

Stoichiometric coefficients are exact numbers.

All soichiometric conversions must obey the law of conservation of matter.

There are several quantities useful to remember:

1 cm^3 = 1 mL

1 L = 1000 mL = 1000 cm^3

1 cm^3 (H20) = 1 g H2O

Common Uses for Stoichiometry

Calculating the mass of a compound given the mass of another compound and the balanced chemical equation

1. Convert the starting mass to moles, using the molar mass of the given compound.

2. Convert the moles of compound A to moles of compound B, using the equivalencies derived from the balanced chemical reaction.

3. Convert from moles back to grams, using the molar mass of the requested compound.

Volume and molarity of of a reactant that forms a precipitate is given

1. Start with the given volume and convert to moles, using the molarity of the given solution.

2. Use the balanced chemical reaction to calculate the moles of product.

3. Convert the moles of product to grams, using the molar mass.

Limiting-reactant calculations

Limiting reactant - reactant that is totally consumed, stopping the reaction

• always completely used up

• amount of limiting reactant determines how much of the other reactant(s) react and how much of each product is formed

• always produces fewer moles or grams of product than the excess reactant

Excess reactant - reactant that is NOT the limiting reactant and is left over even after the reaction has stopped

Once a limiting reactant is identified, all further calculations are based on the amount of the limiting reactant given in the original statement of the problem.

Theoretical yield - maximum amount of product formed in a reaction based on the amounts of reactants used

• is theoretical since no lab work is done

• refers to the maximum mass of product that can be produced since in actual lab work, many things could result in a smaller yield

  • reactants don’t combine completely, side reactions, poor lab techniques, etc.

Titrations

Titration - technique used for chemical analysis that utilizes the reactions of two solutions

• One reactant solution is placed in a beaker and the other in a buret

• Buret - long, graduated tube with a stopcock

• Stopcock - valve that allows the chemist to add controlled amounts of solution from the buret to the beaker

• An indicator is added to the solution in the beaker.

• Indicator - compound that changes color when the reaction is complete

• A chemist reads the volume of solution in the buret at the start of the experiment and again at the point at which the indicator changes color. The difference of these volumes represents the volume of reactant used to complete the reaction.

• The crucial point about the titration experiment is that the indicator is designed to change color when the amount of reactant delivered from the buret is exactly the amount needed to react with a solution in the beaker.

Percent Composition

Percent composition - how much of each element or polyatomic ion is present in a compound on a percentage basis

• mass of an element or polyatomic ion that is present in 100 g of a chemical compound

Empirical Formulas

1. Determine the mass of each element in the compound. You might have to use one or more of the following techniques:

   1. Convert percentages to grams

   2. Convert the mass of compound, obtained experimentally, to the mass of an element

   3. Calculate the mass of a missing element

2. Convert the mass of each element to moles.

3. Divide moles in step two by the smallest number of moles.

4. If the results in step two are integers, use them as subscript.

5. If results in step two are not integers, multiply the appropriate number to obtain integers (usually a small number such as 2, 3, 4, or 5). Use trial and error or the decimal portion of the numbers in step two to determine the multiplier.

A molecular compound can either be the empirical formula or some whole-number multiple of the empirical formula.

The number of empirical formula units in the molecular formula of a compound is determined by dividing the molar mass of the compound by the empirical formula mass. This will result in a small whole number. Each and every subscript in the empirical formula and then multiplied by the small whole number to obtain the molecular formula.