A Study in Scarlet In solving a problem of this sort the grand thing is to be able to reason backward This is a very useful accomplishment and a very easy one but people do not practice it much ID: 1044568
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1. StoichiometrySherlock Holmes, in Sir Arthur Conan Doyle’s A Study in Scarlet“In solving a problem of this sort, the grand thing is to be able to reason backward. This is a very useful accomplishment, and a very easy one, but people do not practice it much.”
2. Review: Chemical EquationsChemical change involves a reorganization of the atoms in one or more substances.C2H5OH + 3O2 ® 2CO2 + 3H2Oreactantsproducts1 mole of ethanol reacts with 3 moles of oxygen to produce 2 moles of carbon dioxide and 3 moles of waterWhen the equation is balanced it has quantitative significance:
3. Calculating Masses of Reactants and ProductsBalance the equation.Convert mass or volume to moles, if necessary.Set up mole ratios.Use mole ratios to calculate moles of desired substituent.Convert moles to mass or volume, if necessary.
4. Working a Stoichiometry Problem6.50 grams of aluminum reacts with an excess of oxygen. How many grams of aluminum oxide are formed.1. Identify reactants and products and write the balanced equation.Al+O2Al2O3b. What are the reactants?a. Every reaction needs a yield sign!c. What are the products?d. What are the balanced coefficients?432
5. Working a Stoichiometry Problem6.50 grams of aluminum reacts with an excess of oxygen. How many grams of aluminum oxide are formed?4 Al + 3 O2 2Al2O3=6.50 g Al? g Al2O31 mol Al26.98 g Al4 mol Al2 mol Al2O31 mol Al2O3101.96 g Al2O36.50 x 2 x 101.96 ÷ 26.98 ÷ 4 =12.3 g Al2O3
6. Limiting Reactant The limiting reactant is the reactant that is consumed first, limiting the amounts of products formed.