Lesson 10
Home Up (12) Weights Needed for Synthesis (13) Weights Formed by Decomposition (14) Weight of Compound Made

 

(13) Weight of Elements Formed by Decomposition

Obj. 13.  From the name of a compound, determine the weight of each element that could be obtained by decomposing a certain amount of it.

Although a decomposition reaction is the opposite of  a synthesis reaction, the thought process and clalculations are nearly identical to what was used int the previous section.

 

Exercises

How much of each element could be obtained by decomposing the indicated amount of each of the following compounds?

a. 3.0 moles of hydrogen chloride

b. 1.2 moles of dinitrogen trioxide

c. 7.00 g of aluminum chloride

d. 4.24 g of phosphorus pentachloride

e. 3.71 g of potassium iodide

f. 10.0 g of calcium carbonate

g. 22.2 g of nickel(II) oxide

 

Worked-Out Example (d)

(d) Decomposing 4.24 grams of phosphorus pentachloride. In this example, the balanced equation approach will be used. (The percentage approach could also be used because ther is only one reactant. The percentage approach would actually be quicker, but the balanced equation approach gives practice with more skills.)

 

The formula for phosphorus pentachloride is PCl5, which will decompose into the elements phosphorus and chlorine. For a skeleton equation we start with PCl5 rtarrow.gif (850 bytes) P + Cl2. To balance that, start with the chlorine, because that's the element that will cause the most trouble. The result is that the balanced equation is 2 PCl5 rtarrow.gif (850 bytes) 2 P + 5 Cl2.

The mole relationship is that two moles of PCl5 gives two moles of phosphorus and 5 moles of chlorine. The mole relationship and the formula weights can be used to figure out the weight relationships. Two moles of PCl5 will weigh 417.0 grams. Two moles of phosphorus will weigh 62.0 grams. Five moles of Cl2 will weigh 355.0 grams. Therefore, 417.0 grams of PCl5 will decompose to form 62.0 grams of phosphorus and 355.0 grams of chlorine.
Decomposition of phosphorus pentachloride
PCl5 rtarrow.gif (850 bytes) P + Cl2
2 PCl5 rtarrow.gif (850 bytes) 2 P + 5 Cl2
417.0 g 62.0 g 355.0 g

 

The question asks how much phosphorus and how much chlorine will we get from 4.24 grams of PCl5? The way to do that is to set up these calculations. Start with 4.24 grams of PCl5, and multiply by a conversion factor that has 62.0 grams of phosphorus over 417.0 grams of PCl5, to get 0.630 grams of phosphorus.
4.24 g PCl5 x 62.0 g P
417.0 g PCl5		 = 0.630 g P
To find out how much chlorine, start with 4.24 grams of PCl5, multiply by a conversion factor that has 355.0 grams of chlorine over 417.0 grams of PCl5, to get 3.61 grams of Cl2.
4.24 g PCl5 x 355.0 g Cl2
417.0 g PCl5		 = 3.61 g Cl2

 

Answers to Exercises

How much of each element could be obtained by decomposing the indicated amount of each of the following compounds?

a. 3.0 moles of hydrogen chloride yields 3.0 g of hydrogen and 1.1 x 102 g of chlorine

b. 1.2 moles of dinitrogen trioxide yields 34 g of nitrogen and 58 g of oxygen

c. 7.00 g of aluminum chloride yields 1.42 g of aluminum and 5.58 g of chlorine

d. 4.24 g of phosphorus pentachloride yields 0.630 g of phosphorus and 3.61 g of chlorine

e. 3.71 g of potassium iodide yields 0.874 g of potassium and 2.84 g of iodine

f. 10.0 g of calcium carbonate yields 4.01 g of calcium, 1.20 g of carbon and 4.80 g of oxygen

g. 22.2 g of nickel(II) oxide yields 17.4 g of nickel and 4.76 g of oxygen

 

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