Limiting quantity problems

In Secs. 8.1 and 8.2, there was always sufficient (or excess) of all reactants except the one whose quantity was given. The quantity of only one reactant or product was stated in the problem. In this section, the quantities of more than one reactant will be stated. This type of problem is called a limiting quantities problem. 

To solve a limiting quantities problem in which the reactant in excess is not obvious, you should ... 

If the quantities of both reactants are in exactly the correct ratio for the balanced chemical equation, then either reactant may be used to calculate the quantity of product produced. (If on a quiz or examination it is obvious that they are in the correct ratio, you should state that they are so that your instructor will understand that you recognize the problem to be a limiting quantities problem.)... 

Ans. The quantities of two reactants (or products) arc given in the problem. They might be stated in any terms—moles, mass, etc.—but they must be given for the problem to be a limiting quantities problem. 

Read each of the other supplementary problems and state which ones are limiting quantities problems. 

Net ionic equations are used in discussions of limiting quantities problems (Chapter 10), molarities of ions (Chapter 11), balancing oxidation-reduction equations (Chapter 16), acid-base theory (Chapter 19), and many other areas beyond the scope of this book. They make possible writing equations for halfreactions at the electrodes in electrochemical experiments (Chapter 17), which have electrons included explicitly in them. They make understandable the heat effects of many reactions such as those of strong acids with strong bases. 

Limiting quantities problems have the quantities of two (or more) reactants given. 

Explain why limiting quantities problems do not usually involve decomposition reactions. 

Fe is much easier to reduce to Fe than Fe is to reduce to Fe. Each is easier to reduce than water is to reduce to hydrogen. (Table 17.2) Thus iron(III) will be reduced to iron(II) first, and if it all gets reduced, then iron(II) will get reduced to iron. So this is a limiting quantities problem. 

In a limiting-quantities problem, you might be asked the number of moles of every substance remaining after the reaction. A useful way to calculate all the quantities is to use a table to do the calculations. 

These substances react, so we have a limiting-quantities problem. We use the net ionic eqnation, and we recognize that the number of millimoles of barium ion and the number of millimoles of chloride ion will not change. 

To solve limiting quantities problems, the first step is to recognize that it is such a problem. In these problems, the quantities of two (or more) reactants are given. Make sure that all the quantities are in moles, or convert them to moles. Select one of the quantities and calculate how much of the other(s) will react with that quantity, as we did in Section 5.1. If we calculated that we need more moles of the second reactant than is present, then the second reactant is in limiting quantity. If we calculated that we have more moles of the second reactant than is needed, the first reactant is in limiting quantity. We use the number of moles of limiting reactant to calculate the quantity of reaction that will occur. 

Before studying this chapter, review Section 5.4 on limiting quantities problems and, before Section 10.2, Acid-Base Equilibrium, review net ionic equations from the textbook. 

When 0.100 mol of NH3 and 0.150 mol of NH4CI are dissolved in enough water to make 1.00 L of solution, (a) is a limiting quantities problem being presented (b) What ions are present in greater than 0.010 M concentration (c) What other ions are present (d) What is the principal equilibrium reaction (e) What effect does each of the ions of part (b) have on the equilibrium of part (d) ( f) What is the hydroxide ion concentration of the solution (g) What is the pH of the solution ... 

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