Chemistry 101 10 01 15 10 08 15 Chapter 4 Mass Relationships in Chemical Reactions Stoichiometry o Balanced equations shows quantitative relationship between reactants and products On submicroscopic scale stoichiometric coefficients in the equation o Rules for Stoichiometry refer to atoms molecules or formula units In macro level refer to number of moles 1 Need a balanced equation 2 Convert known unit to moles 3 Use ratio to calculate moles of sought quantity 4 Convert moles of sought into desired units o Grams to moles to moles to grams Coefficients allow use to relate the amount of one substance involved in a chemical reaction to the amount of another substance involved o Stoichiometric amounts exact proportions indicated by the balanced chemical equation Reactions in which one reactant is present in limited supply o Carried out w an excess of 1 reactant to ensure the other is completely o Limiting reactant amount determines or limits the amount of product consumed formed Percent Yield Used up first and completely o Theoretical yield maximum mass of product that can be obtained from a o Actual yield actual amount chemical reaction Always less than theoretical Why 1 o Reaction condition specific o Human error cop out Chemical Equations and Chemical Analysis Loss of product usually in isolation purification o Percent yield actual yield theoretical yield x 100 o Quantitative Analysis of Mixture Depends on basic ideas 10 01 15 10 08 15 completely with sodium hydroxide An unknown amount of a substance can be allowed to react w a known number of another substances If ratio is known unknown can be determined o Ex vinegar with unknown acetic acid reacts readily and A material of unknown composition can be converted to one or more substances of known compositions thus can be identified amounts determined and amounts related to the amount of the original unknown substance o Ex thenardite which is largely sodium sulfate which is soluble in water The sample is crushed and dissolved reacted with barium chloride to precipitate and barium sulfate is filtered o Determining the Formula of a compound by combustion Where does percent composition come from Analysis by combustion o Each element in the compound combines with oxygen to produce the appropriate oxide Ex In the experiment gaseous carbon dioxide and water are Ratio gives the empirical formula separated and masses are determined If molar mass is known from a separate experiment the molecular formula can also be determined Measuring concentrations of Compounds in solution o Solution concentration relates the volume of solution in liters to the Solute any substance solid liquid or gas that is dissolved in a amount of solute solvent o Solution Concentration Molarity Per liter of solution not per liter of solvent Molarity amount of solute per liter of solution Unit of solute concentration c Molarity of x M amount of solute x mol volume of solution L M moles per liter Notation NaCl 1 00 mol L or 1 00M Concentration amount of solute given in a quantity of solvent When making solutions dissolve solute in a volume of solvent smaller than desired volume and then add more solvent once solute has been dissolved 2 10 01 15 10 08 15 o Preparing Solutions of Known Concentrations Combining a weighed solute w the solvent Diluting a more concentrated solution Weigh the required quantity as accurate as possible Add solvent and dissolve Dilution procedure for preparing a less concentration solution Begin with a concentrated solution and add more solvent until from a more concentrated solution the desired lower concentration is reached o Moles of solute same before and after M1V1 M2V2 only for dilution pH a concentration scale for acids and bases pH log H3O o pH negative of the base 10 logarithm of the hydronium ion concentration o pOH log OH o H OH Kw 1 0 x 10 14 autoionizatoin of water o pH pOH 14 00 safety net check answers o 2 decimal places o As pH increases H concentration decreases o For aqueous solutions at 25oC Acids have a pH less than 7 Bases have a pH greater than 7 H OH H 1 x 10 7 H OH H 1 x 10 7 neutral solution At 7 H OH H 1 x 10 7 Stoichiometry of Reactions in Aqueous Solutions o Solution Stoichiometry 1 Write balanced equation 2 Find amount of reactant in moles 3 Relate back to other reactant 4 Conversation Definition standardized known to find an unknown species with o Titration a method of chemical analysis unknown concentration Indicators substance that changes color at or near the equivalence point Doesn t react by environment specific conditions Equivalence point amount of OH added exactly equals the amount of H3O that can be supplied by the acid o Standardizing an acid or base The procedure by which the concentration of an analytical reagent is determined accurately standardization 3 10 01 15 10 08 15 2 approaches 1 Weigh accurately a sample of pure solid acid base primary standard and titrate this sample with s solution of base acid to be standardized 2 Titrate it with another already standardized solution o Determining molar mass by titration o Gravimetric Analysis 1 Write balanced chemical equation 2 Amount of base mol volume L molarity mol L 3 Use a stoichiometric factor to relate base acid 4 Molar mass mass of acid in sample amount of acid in sample 1 Dissolve in water 2 React with known to form a precipitation 3 Filter and dry 4 Weigh 5 Use chemical equation and mass to determine amount of unknown ion o Titrations using oxidation reduction reactions titration 1 Use volume and concentration to calculate amount used in 2 Stoichiometric factor to find the other amount 3 Molar mass 4 Mass percentage Spectrophotometry an important analytical method o Measures quantitatively the extent of light absorption and to relate this to concentration Most frequently used o Every substance absorbs or transmits certain wavelengths of radiant o Color intensity is a measure of concentration of the material in the energy but not others solution Transmittance Absorbance and the Beer Lambert Law o Transmittance T ratio of the amount of light transmitted by or passing through the sample relative to the light that initially fell T intensity of transmitted light intensity of incident light o Absorbance negative logarithm of its transmittance Inverse relationship with transmittance Absorbance log T Absorbance increases as path length increases Absorbance path length l x concentration c Linear relationship between a sample s absorbance and its o Beer
View Full Document
Unlocking...