CHEM 101 Chapter 4
CHEM 101 Chapter 4 Chem 101
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Date Created: 01/27/16
10/01/1510/08/15 Chemistry 101 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 refer to atoms, molecules, or formula units In macro level, refer to number of moles o Rules for Stoichiometry 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 consumed o Limiting reactant: amount determines or limits the amount of product formed Used up first and completely Percent Yield o Theoretical yield: maximum mass of product that can be obtained from a chemical reaction o Actual yield: actual amount Always less than theoretical Why? 1 10/01/1510/08/15 o Reaction condition specific o Human error (cop out) Loss of product usually in isolation/purification o Percent yield = actual yield/theoretical yield x 100% Chemical Equations and Chemical Analysis o Quantitative Analysis of Mixture Depends on basic ideas: 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 completely with sodium hydroxide 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 separated and masses are determined Ratio gives the empirical formula 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 amount of solute Solute: any substance (solid, liquid or gas) that is dissolved in a solvent o Solution Concentration: Molarity Molarity: amount of solute per liter of solution Per liter of solution not per liter of solvent 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 2 10/01/1510/08/15 When making solutions, dissolve solute in a volume of solvent smaller than desired volume and then add more solvent once solute has been dissolved o Preparing Solutions of Known Concentrations Combining a weighed solute w/ the solvent Weigh the required quantity as accurate as possible Add solvent and dissolve Diluting a more concentrated solution Dilution: procedure for preparing a less concentration solution from a more concentrated solution Begin with a concentrated solution and add more solvent until the desired, lower concentration is reached o Moles of solute: same before and after M 1 =1 V 2on2y for dilution) pH, a concentration scale for acids and bases o pH: negative of the base-10 logarithm of the hydronium ion concentration + pH=-log [H O3] o pOH: -log[OH ] - o [H ][OH ]=K =1.w x 10 -1(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 25 C: o Acids have a pH less than 7 [H ] > [OH ] - + -7 [H ] > 1 x 10 Bases have a pH greater than 7 [H ] < [OH ] - + -7 [H ] < 1 x 10 At 7, neutral solution + - [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 o Titration: a method of chemical analysis Definition: standardized known to find an unknown species with unknown concentration Indicators: substance that changes color at (or near) the equivalence point Doesn’t react, by environment specific (conditions) 3 10/01/1510/08/15 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 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 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 o Gravimetric Analysis 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 1) Use volume and concentration to calculate amount used in titration 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 energy but not others o Color intensity is a measure of concentration of the material in the 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 o Beer-Lambert Law 4 10/01/1510/08/15 Absorbance = path length (l) x concentration (c) Linear relationship between a sample’s absorbance and its concentration for a given path length Spectrophotometric Analysis o 1) Record the absorption spectrum of the substance to be analyzed. o 2) Choose the wavelength for the measurement o 3) Prepare a calibration o 4) Determine the concentration of the species of interest in other solutions 5