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Chemical Quantities: Interpreting Equations and Calculating Mass

Learn how to interpret chemical equations, calculate mole-mole relationships, perform mass calculations, identify limiting reactants, and calculate percent yield in chemistry.

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Chemical Quantities: Interpreting Equations and Calculating Mass

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  1. Chapter 9: Chemical quantities Chemistry 1020: Interpretive chemistry Andy Aspaas, Instructor

  2. Interpreting chemical equations • The coefficients in a chemical equation indicate relative numbers of molecules involved in the equation 2H2(g) + O2(g)  2H2O(g) • This equation can be interpreted many ways • 2 molecules H2 gas react with 1 molecule O2 gas to form 2 molecules H2O gas • 2 dozen molecules H2 gas react with 1 dozen molecules O2 gas to form 2 dozen molecules H2O gas • 2 mol H2 gas react with 1 mol O2 gas to form 2 mol H2O gas

  3. Mole-mole relationships in reactions 2H2(g) + O2(g)  2H2O(g) • If you want to produce 15.7 mol H2O, how many moles of O2 gas are required in this reaction? • Use the coefficients in the balanced chemical reaction to convert between moles of any components in a reaction • Mole ratio: 1 mol O2(g) for every 2 mol H2O(g) from the equation • Use that mole ratio as a dimensional analysis conversion factor

  4. Mass calculations • In chemistry, number of moles is the only unit that can be used to convert between quantities of constituents in a chemical reaction • Stoichiometry: conversion of masses of different reactants and products in a chemical reaction • If you’re given the mass of a reactant or product, you must first convert mass to moles • If you’re looking to find the mass of a reactant or product, you must first find the moles, and then convert to mass

  5. Practice mass calculation problems CO(g) + 2H2(g)  CH3OH(l) • What masses or carbon monoxide gas and hydrogen gas are required to produce 6.0 kg of methanol? LiOH(s) + CO2(g)  Li2CO3(s) + H2O(l) • What mass of CO2 gas can 1.00 kg of lithium hydroxide react with?

  6. Limiting reagent • Whenever you are given quantities of more than one reactant, you must decide which will be consumed first (the limiting reactant) • The cheese sandwich analogy • Decide quantity of products that each individual reactant is able to produce • The reactant that produces the smallest quantity of product is the limiting reactant

  7. Limiting reactant • Convert each reactant to moles • Use mole ratios to convert each to moles of product • Reactant which produces smallest value of product moles is limiting reactant • Use limiting reactant to calculate product moles and finally product mass

  8. Limiting reactant practice N2(g) + 3H2(g)  2NH3(g) • Say 25.0 g nitrogen gas and 5 g hydrogen gas are reacted to form ammonia. What mass of ammonia can be produced? • Convert reactants to moles • Convert each to moles products • Identify limiting reagent • Product moles from limiting reagent • Convert to mass of product

  9. Percent yield • All calculations so far have given theoretical yields of products • The maximum amount of product that can be formed, assuming all of the limiting reagent reacts, and all product can be collected • In reality, actual yield is almost always lower • Percent yield: measure of experimental efficiency = [(actual yield) / (theoretical yield)] x 100%

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