Calculations Using Chemical Equations

Chapter 10

Chemical EquationsBalanced chemical equations tell us the ratio of

the moles of reactants and products.

Calculating the Volumes of Reacting Gases

According to Avogadro’s Law, the volume of a gas in a reaction is proportional to the number of moles.

A chemical equation gives the ratio of the moles of substances.

Hence, it also gives the ratio of the volumes of gases.

Limiting ReagentsChemical equation:

Mg(s) + H2SO4(aq) MgSO4(aq) + H2(g)

From the equation, 1 mole of magnesium reacts with 1 mole of sulfuric acid. At the end of the reaction, both reactants will be completely used up.

IF 1 mole of magnesium is mixed with 2 moles of sulfuric acid:• insufficient magnesium to react with all the sulfuric acid• 1 mole of magnesium will only react with 1 mole of sulfuric

acid: 1 mole of sulfuric acid will be left unreacted• magnesium is the limiting reagent, sulfuric acid is in excess

Percentage Yield During the preparation of a chemical, some of the

chemical is always lost.evaporate into the air be left in the solution reaction simply did not go to completion

Percentage Yield All reactants are converted to products in a complete reaction:

the calculated amount of products that would be obtained if the reaction is completed is known as the theoretical yield or maximum possible mass of product

Amount of products that is actually produced in the experiment is known as the actual yield or experimental mass of the product

Percentage yield = × 100%

Experimental mass of

product

Maximum possible mass of

product

Let’s Try P178 Q1

Zinc reacts with sulphur according to the equation:

Zn(s) + S(s) ZnS(s)

In an experiment, 6.5g of zinc was reacted with sulphur to make zinc sulphide, ZnS. [tbc..]

9.0g of zinc sulphide was obtained. Calculate the percentage yield.

Percentage Purity Often, the percentage yield is less than 100% because the

reactants are not pure.

The more impure the reactants, the lower the actual yield of the products.

For an impure sample of substance:

Percentage purity = × 100%

Mass of pure substance

present

Mass of sample