FORMULAS, EQUATIONS AND MOLES

Chemical Equation

Stoichiometry

Chemical Formula

CHEMICAL REACTION

• Rearrangement of the atoms present in the reacting substances to give new chemical combinations present in the product substances.

CHEMICAL EQUATION

• Shorthand symbolic notation for a chemical reaction

• Qualitative aspect – identity of reactants [R] and products [P]; use

study of nomenclature to write equations– Identify the state of matter for each [R] and [P]– identify reaction type

CHEMICAL EQUATION (2)

• Quantitative aspect– how much reactant is consumed and how much

product is formed– coefficients must be consistent with the Law of

Conservation of Mass; atoms are neither created nor destroyed in a chemical reaction.

– i.e. chemical equation must be balanced

FORMULA UNIT

• Qualitative description of elements which comprise the chemical unit

• Describes an atom, ion, molecular or ionic compound: Mg, NH4

+, C12H22O11, KCl

• Quantitative description of relative numbers of atoms of each element

•FORMULA or MOLECULAR MASS

• Sum of atomic masses of constituent atoms in the formula unit.

• Average mass of one formula unit of substance (atom, molecule, ion, formula unit) in amu

• Average mass of one mole of formula units of substance in grams

MOLE

• The number of C-12 atoms in exactly 12 grams of pure C-12

• 6.022E+23 items

• Avogadro’s Number, N

• A mole of an element has a mass equal to its average atomic mass

CONVERSIONS

• Grams to Moles Divide by Molar Mass*

* = Atomic, Molar or Formula Mass• Moles to Grams x by Molar Mass• Grams to amu x by N• amu to Grams Divide by N• Moles to #Units** x by N• #Units to Moles Divide by N

** = Atoms, Molecules, Formula Units

STOICHIOMETRY

• Quantitative relationships based on a BALANCED chemical reaction.

• Relationships between R(eactant)1 and R2 or R1 and P(roduct)2 or P1 and P2

C(s) + 2H2(g) CH4(g)

• One atom of solid carbon reacts with two molecules of gaseous hydrogen to produce one molecule of gaseous methane.

• One mole of solid carbon reacts with two moles of hydrogen gas to produce one mole of methane gas.

• 12.0 g of C reacts with 4.0 g of H2 to produce 16.0 g of CH4. Note conservation of mass.

STOICHIOMETRIC COEFFICIENTS

• We will use mole interpretation for stoich. coefficients

• represent number of moles of each R and P

• Provide quantitative (i.e. mole) relationships between R and P.

• Can be used to determine amount of mass of each R and P

MOLE RATIOS

• Use as conversion factors from R1 to R2, P2 to R1, etc

• Depend on a balanced chemical equation

C(s) + 2H2(g) CH4(g)1 mol 2 mol 1 mol12.0 g 2.0 g 16.0 g

• How many g of carbon are needed to react with 10 g of hydrogen? How much CH4is formed

• g-H2 mol-H2 mol-C g-C

• [10.0g H2 /2.0g H2/mol]*[1 molC/2 mol H2] * [12.0g C/mol] = 30 g C

• [10.0g H2 /2.0g H2/mol]*[1 molCH4/2 mol H2] * [16.0g CH4/mol] = 40 g CH4

• Is mass conserved?

REACTION YIELD

• In the previous example, say that only 32.0 g of CH4 were produced due to side reactions and waste.

• We define the percent or reaction yield as [actual yield/theoretical yield]*100

• This gives % yield = [32/40] * 100 = 80%

LIMITING REACTANT

• Find the actual mole ratios of the reactant. Compare this ratio to the stoichiometric mole ratio to determine the limiting reactant or reagent (LR); the other is the excess reactant (xs)

• Use the LR to solve the stoichiometric problem to find the amount of product formed.

• Calculate the amount of xs left over.

SOLUTIONS

• Most chem rxns occur in a solution

• Solution - homogenous mixture – Solvent - compound in solution present in

greatest amount– Solute - other compounds present in soln

• Concentration– Measure of quantity of solute in solvent– Molarity = M = #moles solute per l liter soln

SOLUTIONS(2)

• # moles = molarity * volume(L) = M * V

• Dilution Mc * Vc = Md * Vd where c = concentrated and d = dilute

• Stoichiometric calculations for reactions taking place in aqueous solutions require: VA, MA #mol A #mol B VB if MA and MB are known

TITRATION

• Determination of the concentration of a solution (Mu) by allowing it to react with a standard solution of the titrant of known concentration (Mt).

• Start with a known volume (Vu) of the unknown and an indicator that changes color when the rxn is complete.

• Deliver titrant to the unknown from a buret until the rxn is over. Record Vt.

TITRATION (con’t)

• Mt, Vt #mol titrant --# mol unknown Mu since Vu is known.

• Note that this analysis is based on stoichiometry.

CHEMICAL FORMULA

• A chemical formula can be determined from the– Mass of each element in the formula – % Mass of each element in the formula (%

Composition)– Number of moles of each element in the

formula– Elemental analysis by combustion

CHEMICAL FORMULAS

• EMPIRICAL - Contains constituent elements in their correct but smallest integer ratios .

• MOLECULAR - Shows all the elements in actual amounts; determined from the empirical formula and molecular mass.