Chapter 3: Stoichiometry - lmtsd.org

1 Chapter 3 10, 2015 Oct 6 2:25 AMChapter 3: StoichiometryOct 6 2:25 AMThe Meaning of the WordThe word Stoichiometry derives from two Greek words: stoicheion (meaning "element") and metron (meaning "measure"). Stoichiometry deals with calculations about the masses (sometimes volumes) of reactants and products involved in a chemical reaction. It is a very mathematical part of chemistry, so be prepared for lots of calculator is Stoichiometry ? Chapter 3 10, 2015 Oct 3 3:44 Counting by Weighing Solve problems related to determining the number of atoms you have based on total mass and average can be counted by Atomic Mass Calculate average atomic masses from isotope data Calculate relative isotope abundance from pertinent mass spectral Spectrometer. 12C is assigned a mass of exactly 12 atomic mass units(amu), and the masses of all other atoms are given relative to this 6 2:31 AMExample ACalculation of Average Atomic Masses from Isotopic sample of metal "M" is vaporized and injected into a mass spectrometer.

2 The mass spectrum tells us that of the metal is present as 69M and is present as 71M. The mass value for 69M and 71M are amu and amu respectively. What is the average atomic mass of the element? What is the element?To calculate Weighted Average use: Chapter 3 10, 2015 Oct 6 3:14 AMThe element indium exists naturally as two isotopes. 113In has a mass of amu, and 115In has a mass of amu. The average atomic mass of indium is amu. Calculate the percent relative abundance of the two isotopes of B Calculation of Relative Isotope AbundanceOct 3 4:00 THE MOLE Interconvert between mole, mass, and the number of particles of a given mole is the key to many chemical calculations. A mole is defined as the number equal to the number of carbon atoms in exactly 12 grams of pure 12C. 1mole of anything = x 1023 units of that thingChemistry Lovers Celebrate Mole DayOne mole of ANY substance contains Avogardo's number of problem solving relationship: The average mass of one atom of a substance expressed in amu is the same number as the mass of one mole of a substance expressed in grams.

3 1 atom of 20Ne = amu1 mole of 20Ne = grams1 mole of 20Ne = x 1023 atoms of NeThe mole unit song Chapter 3 10, 2015 Oct 17 12:00 PMOct 23 10:03 PMSamples containing one mole each of copper, aluminum, iron, sulfur, iodine, and mercuryExample AConversion Among Grams, Atoms, and AMUsHow many grams does a sample containing 34 atoms of neon weight?Example BConversion Between Atoms, Moles, and MassA sample of elemental silver (Ag) has a mass of many moles of silver are in the sample?How many atoms of silver are in the sample?Example C Practice with ConversionsWhat is the weight of x 1022 atoms of calcium? Chapter 3 10, 2015 Oct 6 7:13 AM Calculate the molar masses. Interconvert among molar mass, moles, mass, and number of particles in a given Molar MassThe Molar Mass of a substance is the mass in grams of one mole of the ACalculations of the Molar mass of a compoundCalculate the molar mass of potassium many micrograms are in x 10 7 moles of pyrogallol, C6H6O3?

4 Example BConverting Among Molar Mass, Moles, Mass, and Number of ParticlesOct 6 3:23 AMFreon 12, which has the formula of CCl2F2, is used as a refrigerant in air conditions and as a propellant in aerosol cans. Given a mg sample of Freon 12: a. Calculate the number of molecules of Freon 12 in that How many mg of chlorine are in the sample?Example C Practice with ConversionsChapter 3 10, 2015 Oct 10 10:19 Percent Composition of Compounds Calculate the mass percent of each element in the the mass percent of each element in Glucose, C6H12O6 Example ADetermination of the mass percent in a CompoundThere are 3 steps to calculating the mass percent of each element in a Compute the molecular mass of the Calculate how much the molecular mass comes from each elementc. Divide each element's mass contribution by the total molecular mass, and multiply by 100 to convert to Learning to Solve Problems Solving problems in a flexible, creative way is called conceptual problem solving Become independent problem solver.

5 With a new problem you need: a. Decide on a final goal b. work backwards from the final goal c. Ask" Does the answer makes sense?"Oct 10 10:19 AMExample B Practice with Mass PercentCalculate the mass percent of each element in potassium ferricyanide, K3Fe(CN)6?Checking the FiguresAdding up the percentages does not add up to 100% due to the rounding to 2 sig. figs after the decimal point for the molar mass which leads to a loss of In such cases we say the answers are correct "within round off error" Chapter 3 10, 2015 Oct 10 10:19 Determining the Formula of a Compound Determine the empirical formula of a compound Calculate the molecular formula of a compoundThe empirical formula is represented by the simplest whole number ratio of atoms in a compound. Examples: CH, CH4, CH2O, K2Cr2O7 The molecular formula is the actual ratio of atoms in a : C6H6, C6H12O6, N2H4 The empirical and molecular formulas can be the of substances whose empirical and molecular formulas differ.

6 Notice that: molecular formula = (empirical formula)n, where n is an 11 12:37 PMExample ADetermination of the Empirical Formula of a CompoundThe analysis of rocket fuel showed that it contained nitrogen and hydrogen by weight. Mass spectral analysis showed the fuel to have a molar mass of g/mol. What are the empirical and molecular formulas of the compound?To solve empirical formula problems that do not involve combustion (added oxygen)1. Assume the compound has a mass of exactly 100 grams. You can therefore convert percentage to Calculate the moles of each kind of atom present3. Determine the simplest whole number ratios by dividing the moles of each element by the smallest calculated mole valueRead the summary of methods for obtaining empirical and molecular formulas. p. 96 Numbers very close to whole numbers, such as and , should be rounded to whole numbers. Numbers such as , , and should NOT be rounded to whole 3 10, 2015 Oct 12 12:31 PMDetermine the empirical formula and molecular formulas for a deadly nerve gas that gives the following mass percent analysis:C = H = O = P = F = total mass is known to be BDetermination of the Empirical Formula of a CompoundDetermination of the Empirical Formula of a HydrateIn an experiment, g of sodium carbonate hydrate are heated.

7 Following the heating the dry remains (called anhydrate) weigh g. What is the formula for the hydrate sodium carbonate?Oct 12 12:31 PMA combustion device was used to determine the empirical formula of a compound containing ONLY carbon , hydrogen and oxygen. A g sample of the unknown produced of CO2, and g of H2O. Determine the empirical formula of the CDetermining the Empirical Formula From Combustion DataSolution tips:1. Assume that the combustion was complete. Therefore, all the carbon was converted to CO2 and all the hydrogen was converted to Calculate the grams of carbon in the given mass of CO2 and the grams of hydrogen in the given mass of water. 3. Based on the law of conservation of mass and the given mass of the original compound you know that grams O = total grams (grams C + grams H)4. Follow the steps for finding the empirical formula Chapter 3 10, 2015 Oct 10 10:19 Chemical Equations State the meaning of each part of a chemical reaction State a variety of different relationships that are inferred from a chemical equation are all of the form CH4 + 2O2CO2 + 2H2 OReactants ProductsIn a chemical reactions atoms have been reorganized.

8 Bonds have been broken and new ones have been formed. Atoms are neither created nor destroyed. All atoms present in the reactants must be accounted for among the 13 8:50 PM Chapter 3 10, 2015 Oct 13 8:50 PMExample A Interpreting a Chemical EquationState, completely, what is happening in the reaction given below. Include in your answer the physical states of each component of the BRelating Reactions and Products of a Chemical EquationsShow, by means of molar masses, that the matter is neither created nor destroyed in the equation given (aq) + 2 HCl(aq) CO2(g) + H2O(l) + 2 NaCl(aq)C2H6(l) + 3O2(g) 2CO2(g) + 3H2O(g) Oct 13 8:50 PM Balance many chemical Balancing Chemical EquationsA balanced chemical equation means that the number of atoms of each element on the reactants' side equals the number of atoms of each element of atoms of each element on the products' an Equation with the JigsawChapter 3 10, 2015 Oct 13 8:50 PMGUIDELINES FOR BALANCING A CHEMICAL EQUATION1.

9 Use Correct formulas. Never change a molecular structure. Only use Place a coefficient in front of each formula. The coefficients must be whole numbers. 3. Find the atoms that are in only one compound on the reactants' side. Balance those Balance polyatomic ions as a single unit. 5. In general leave O and H until the very end. 6. Always double check AFTER balancing!!!NaOH(aq) + H3PO4(aq) Na3PO4(aq) + H2O(l) Balance the following chemical Balancing Chemical EquationExample Practice with Balancing Balance the following chemical (g) + O2(g) CO2(g) + H2O(g)Oct 13 8:50 Stoichiometric Calculations: Amounts of Reactants & Products Calculate the amount of products obtained from a given amount of reactant Calculate the amount of reactants required to generate a desired amount of Balance the equation for the Convert the known mass of the reactant or the product to moles of that Use the balanced equation to set up the appropriate mole Use the appropriate mole ratio to calculate the number of moles of the desired reactant or Convert from moles back to grams if required by the Masses of Reactants and Products in Chemical Double check the balanced chemical equation.

10 Don't multiply the molar mass of a substance by the coefficient in the problem BEFORE using it in one of the steps above. For example, if the formula says 2H2O, DON'T use g/mol, use g/mol. Round off only once after all calculations are done. Chapter 3 10, 2015 Jan 31 5:49 PMAnalogies in Stoichiometry Bike by Parts Consider the following Analogy: 2 Wheels + 1 Body + 1 Handle bar + 1 Gear Chain = 1 bike How many bikes can be produce given, 8 wheels, 5 bodies, 6 handle bar and 5 gear chain. Oct 13 8:50 PMExample Amount of product from a Given Amount of ReactantGiven the following reaction:Na2S(aq) + AgNO3(aq) Ag2S(g) + NaNO3(aq) How many grams of Ag2S can be generated from the reaction of g of AgNO3 with excess Na2S?Strategy1. Always balance the chemical equation!The mole ratio conversion factor acts as the bridge between AgNO3 and Ag2S. This leads to the following strategy: molar massmole ratiomolar massmoles Ag2Sg Ag2Sg AgNO3moles AgNO3 Chapter 3 10, 2015 Oct 15 10:01 AMC7H6O3 + C4H6O3 C9H8O4 + HC2H3O2 Aspirin is prepared by the reaction of salicylic acid (C7H6O3) and acetic acid anhydride (C4H6O3).