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Chapter 3 Stoichiometry

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Section 3.1 Atomic Masses Mass Spectrometer – a device used to compare the masses of atoms Average atomic mass – calculated as a weighted average of the masses of all known isotopes. To calculate the average atomic mass – follow example on page 85

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Section 3.2 The Mole Mole – the unit of matter for counting atoms. The mole is the number equal to the number of carbon atoms in exactly 12 grams of pure carbon-12. Avogadro’s number = 6.02 x 10 23 which is the number of particles contained in one mole of a substance. The mole is often expressed in grams based on the atomic mass of a substance. Calculations of moles found on pages 88 - 90

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Section 3.3 Molar Mass Molar mass is the mass in grams of one mole of a compound. To calculate the molar mass from a chemical formula follow the steps below: List the number of atoms of each element List the atomic mass of each of these elements Multiply the number of atoms of each element by their respective atomic masses Add the result from the third step. This gives a gram per mole result. Calculations dealing with moles and molar mass are found on pages 90 – 93.

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Section 3.4 Percent Composition of Compounds Percent composition, also called mass percent, is calculated by dividing the mass of the individual elements calculated mass by the total mass of the compound Calculations for mass percent are found on pages 94 - 95

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Section 3.5 Determining the Formula of a Compound Empirical formula – the lowest whole number ratio for a formula. Empirical formulas are the exact formulas for ionic compounds. Molecular formula – the actual formula for covalently bonded compounds. To determine these formulas, the molar mass of the entire formula must be given. To determine an empirical formula use the following method. 1. Convert percentages to grams. 2. Convert grams to moles. 3. Divide each mole value by the lowest calculated mole value. 4. Write the formula if in whole numbers. 5. If the numbers obtained in the previous step are not whole numbers, multiply each number by an integer so that the results are all whole numbers. To determine a molecular formula use the following method. Calculate the molar mass of the empirical formula by the given molar mass. This will result in a whole number factor. Multiply the empirical formula by this factor and write the formula.

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Section 3.6 Chemical Equations Chemical equation – a representation of a chemical reaction. In a chemical equation: Reactants – written on left Products – written on right To balance a chemical equation, the law of the conservation of matter must be observed. Often symbols are used in chemical equations to identify the states of matter of substances, to determine if the substance is in solution, to show equilibrium, and if it is a precipitate reaction or if a gas is given off.

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Section 3.7 Balancing Equations To write and balance the equation for a chemical reaction follow the steps below: 1. Determine what reaction is occurring. What are the reactants, the products, and the physical states involved? 2. Write the unbalanced equation that summarizes the reaction described in step 1. 3. Balance the equation by inspection, starting with the most complicated molecule(s). Determine what coefficients are necessary so that the same number of each type of atom appears on both reactant and product sides. Do no change the identities (formulas) of any of the reactants or products by deleting elements or adding/subtracting subscripts. Practice for balancing equations are found on pages 106 -108.

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Section 3.8 Stoichiometric Calculations Mole ratio – a conversion factor that compares substances within a balanced equation. To solve stoichiometric problems follow the steps below: 1. Balance the equation for the reaction. 2. Convert the known mass of the reactant or product to moles of that substance. 3. Use the balanced equation to set up the appropriate mole ratios. 4. Use the appropriate mole ratios to calculate the number of moles of the desired reactant or product. 5. Convert from moles back to grams if required by the problem.

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Stoichiometric Calculations Slide 2 To the right is a diagram for summarizing the steps on the previous slide. Calculations for stoichiometric problems are on pages 111 - 112 Balanced Chemical Equation Moles of desired substance Moles of known substance Mass of known substance Moles of known substance Mass of desired substance Moles of desired substance Use mole ratio to convert Convert to moles Find appropriate mole ratios Convert to grams

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Section 3.9 Calculations involving a Limiting Reactant Limiting reactant (often called a limiting reagent) – the reactant that is consumed first and that therefore limits the amounts of products that can be formed. To determine a limiting reactant, the reactants must be compared using the mole ratios and the amount of each reactant given. Calculations for limiting reactants are on pages 116 - 118

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Section 3.9 Percent Yield Percent yield is derived by dividing the actual yield by the theoretical yield times 100. Actual yield – the amount that was obtained through experimentation Theoretical yield – the amount that was calculated using stoichiometric operations. Practice problems – page 119 - 120

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