Stoichiometry of Formulas and Equations AP CHEMISTRY NOTES

What is a mole ?

Mole Concept  A mole contains the same # of particles as 12 g of C-12.  A mole of any substance has fixed # of particles i.e. Avogadro # of particles – x 10^23  The mole maintains the same mass relationship between macroscopic samples as exists between individual chemical entities.  Relates the # of chemical entities to the mass of the sample.

A mole of a substance contains Avogadro # of particles – atoms, molecules or ions.

Molar conversions

Mass % from chemical formula (percentage composition of elements in a compound) ‏  Mass % of element X = Atoms of element x atomic mass (amu) / formula mass of the compound (amu) ‏  Alternate formula = Atoms of element x molar mass (g/mol) / molar mass of compound (1 mol) ‏

Empirical and Molecular Formulas  Simplest whole # ratio of the moles of the elements present in a compound.  Molecular formula is the actual representation of the compound.  Empirical suggests that the formula of the unknown compound is found using experimental techniques, observations and calculations.

Steps to Find Empirical Formula of a Compound

How to Arrive at the Molecular Formula from the Empirical formula  n = whole number multiple  n = formula weight of the compound / empirical formula weight  Molecular formula = (empirical formula)n

Combustion analysis can be used as a technique used to find the empirical formula of compounds (particularly organic compounds) ‏

Chemical Equations  Is a statement in formulas which expresses identities and quantities of substances involved in a chemical or physical change.  Balancing equations with the correct coefficients is important. Reasons:  In a chemical process, atoms are not created or destroyed but rearranged into different combinations.  A formula represents a fixed ratio of elements in a compound, so a different ratio represents a different compound.

Information from a Chemical Equation

Quantitative information from Balanced Equations The coefficients in a balanced equation can be interpreted both as relative number of molecules in the reaction and also as the relative number of moles in the reaction.

How many grams of water are produced in the oxidation of 1 g of glucose?

Additional Practice The decomposition of potassium chlorate is commonly used to prepare small amounts of oxygen in the lab. How many grams of oxygen can be prepared from 4.5 grams of the compound?

Fundamentals of Solution Stoichiometry  Solution = Solute + Solvent  Concentration is usually expressed as the amount of solute dissolved in a given solution.  Molarity (M) expresses concentration in units of moles per liter of the solution.  Molarity = moles of solute / liters of solution  Sample problem 3.15 on page 121

Preparing Solutions

Dilution – moles of solute in concentrated solution = moles of solute in dilute solution

Dilution contd. A concentrated solution (higher molarity) can be converted to a dilute solution (lower molarity) by adding solvent to it. The dilute solution has fewer solute particles per unit volume compared to the concentrated solution which has more solute particles per unit volume. M(dil) x V(dil) = M(conc) x V(conc) ‏ Sample problem 3.17 – page 124

Stoichiometry of chemical reactions in solutions – sample problem 3.19 – page 125

AP CHEMISTRY Limiting Reagents in Chemical Reactions

Recipe for a Great Ice-Cream Sundae 2 scoops vanilla ice- cream 1 Ripe Cherry 50 mL syrup

How many sundaes will 8 scoops of vanilla ice-cream produce ?

How many sundaes will 6 cherries make? 6

How many sundaes can you make using 100 mL syrup ? 100 mL syrup

How many sundaes can you make using the following combination ? 8 SCOOPS OF VANILLA ICE CREAM 6 CHERRIES 100 mL SYRUP

Which reagent was responsible for making the smallest amount of product ? (Hint: Refer to the previous slides) Chocolate sauce is the “limiting reagent”. The amount available (100 mL) is sufficient to make only two sundaes.

How much of each reagent is left behind after making the sundaes ? 4 scoops left behind after using up 4 scoops 4 cherries left behind after using 2 cherries No chocolate sauce is left behind as all 100 mL was used in making the two sundaes. Limiting Reagents are completely consumed at the end of the reaction.

Properties of Limiting Reagents Limiting Reagent is a reactant in a chemical reaction. Limiting Reagent compared to other reactants produces the smallest amount of product. Limiting Reagents are completely consumed at the end of a chemical reaction.