AP Chemistry - Semester 1 : Stoichiometry of Formulas and Equations
Sections: Introduction   |  Section 1   |  Section 2   |  Section 3   |   lab Lab

3.2 Determining the Formula of an Unknown Compound

This process uses the same mathematical ideas to find the formula of an unknown compound.

A compound was analyzed and found to contain 13.5 g Ca, 10.8 g O, and 0.675 g H. What is the empirical formula of the compound?

Convert the mass of each element to moles using the molar mass from the periodic table.

= 0.337 mol Ca

= 0.675 mol O

= 0.668 mol H

Divide each mole value by the smallest number of moles calculated. Round to the nearest whole number.

= 0.337 mol Ca = mol Ca= 1.00

= 0.675 mol O = mol O = 2.00

= 0.668 mol H = mol H = 1.98 or 2.00

This is the mole ratio of the elements and is represented by subscripts in the empirical formula.

3.3 Writing and Balancing Chemical Equations

A chemical equation is a statement in formulas that expresses the identities and quantities of the substances involved in a chemical or physical change. The left side of the equation shows the amount of each substance present before the change, and the right side shows the amounts present afterward.

website reference You should have a pretty good idea on balancing equations from your first year of chemistry. Please read the text and visit this balancing chemical reactions web site, From Ohio State University.

website reference And even more practice: A balanced equation is essential for any calculations involving amounts of reactants or products. If you know the number of moles of one substance, the balanced equation tells you the number of moles of all other substances in the reaction.

practice Self check questions

1. Which is the correctly balanced equation?

a. C8H18 + O2 right arrow CO2 + H2O
b. 2C8H18 + 25O2 right arrow 16CO2 + H2O
c. 2C8H18(l) + 25O2(g) right arrow 16CO2(g) + 18H2O(g)
d. 2C8H18(l) + 12O2(g) right arrow 32CO2(g) + 16H2O(g)

solution

2. Which is the correctly balanced equation?

a. C7H6O2 + O2 right arrowCO2 + H2O
b. 2C7H6O2 + O2 right arrowCO2 + H2O
c. 2C7H6O2(s) + 15O2(s) right arrow2CO2(g) + 16H2O(l)
d. 2C7H6O2(s) + 15O2(s) right arrow14CO2(g) +6 H2O(l)

solution

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