Stoichiometry

The mass relationships in a chemical reaction can be determined from a balanced equation for that reaction. Once those mass relationships have been determined the masses of elements needed to synthesize any given amount of that compound can be calculated. You can also do other types of stoichiometric calculation along these same lines.

Objective 11: From the name of a compound, determine the mass of each element needed to make a certain amount of it

The specified amount of the compound to be made might be in moles or in grams. To solve the problem write a balanced equation for the formation of the compound (see previous page), determine the mass ratios from the equation, and do the stoichiometry.

Exercises

What mass of each element is needed to synthesize the indicated amount of each of the following compounds?

a. 0.24 moles of magnesium bromide

b. 3.2 moles of sulfur trioxide

c. 17.0 g of sodium chloride

Answers

a. 0.24 moles of magnesium bromide requires 5.8 g of magnesium and 38 g of bromine

b. 3.2 moles of sulfur trioxide requires 1.0 x 10² g of sulfur and 1.5 x 10² g of oxygen

c. 17.0 g of sodium chloride requires 6.68 g of sodium and 10.3 g of chlorine

Objective 12: 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 calculations are nearly identical to what was used in 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

Answers

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

Objective 13: From the name of a compound, determine the weight of the compound that can be made from specified amounts of each element

Again we are dealing with synthesis. But now we need to find out how much product can be made from a certain amount of reactants - the elements. The tricky part here is that we are given two amounts of elements that are reacting with one another and those may or may not be the proper amounts to react with one another to make the product. In other words, we are dealing with limiting reactant problems.

Exercises

Although they don't say so, the following questions are limiting reagent problems.

a. How much copper (II) sulfide can be made from 4.20 g of copper and 2.40 g of sulfur?

b. How much sulfur dioxide can be made by burning 0.14 g of sulfur in a flask that contains 0.24 g of oxygen?

c. How much potassium chloride can be made from 2.50 g of potassium and 2.50 g of chlorine?

d. How much potassium bromide can be made from 2.50 g of potassium and 2.50 g of bromine?

Answers

a. 6.32 g CuS

b. 0.28 g SO₂

c. 4.76 g KCl

d. 3.72 g KBr