How Is the Formula of a Compound Determined?

The look and fluidity of a compound are clues to the chemical formula.
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The chemical formula for water, H2O, is easily recognized, and if someone asked for a glass of H2O, most people would understand the request even though "water" sounds nothing like "H2O." Knowing how chemical formulas are created will help you understand why the formula looks the way it does and why there is a two after the hydrogen symbol, H, and not after the oxygen symbol, O.

Nonmetal Compounds: Using Chemical Name

Rules exist for naming compounds, and familiarity with them can help in determining a formula for a compound. For example, one chemical name for water is dihydrogen monoxide. Here, the prefix di- means two and mono-, one. This means there are two hydrogen atoms and one oxygen, or H2O.

The advantage of prefixes in naming nonmetal compounds offers a simple approach to creating chemical formulas: Note the prefix in front of the element, and then place the symbol for the element followed by the number in the prefix. If no prefix exists before the first element, it is assumed to be one.

Formula of a Compound: Example for Nonmetal Compounds

Create the formula for dinitrogen pentoxide.

Here, the prefix in front of nitrogen is di-, meaning two, and in front of nitrogen is penta-, meaning five.

The formula for dinitrogen pentoxide is N2O5.

Ionic Compound Formula

Compounds between metals and nonmetals create ionic compounds. The overall charge of an ionic compound must be neutral.

Knowing the charges of elements from the periodic table is helpful. Remember that a group of elements is a column on the periodic table:

  • Group 1 (e.g., hydrogen), +1
  • Group 2 (e.g., calcium): +2
  • Group 15 (e.g., nitrogen): +3
  • Group 16 (e.g., oxygen): -2
  • Group 17 (e.g., Fluorine): -1

The cation (metal, positively charged) is named first followed by the anion (nonmetal, negatively charged) in an ionic compound.

Formula of a Compound: Example for Ionic Compounds

Example 1: Create the ionic compound formula for sodium chloride.

Step 1: Write the symbol and charge: Na1+ and Cl1-.

Step 2: Use the crisscross method. Place the number (not the charge sign) of the positive charge as the subscript of the anion. Then add the number of the negative charge to the subscript for the cation: Na1Cl1.

Step 3: Reduce to lowest ratio and leave out subscripts of 1: NaCl.

Example 2: Create a formula for aluminum chlorate.

Step 1: Write the symbol and charge: Al3+ and ClO31-.

Step 2: Use crisscross method. Note that chlorate is a polyatomic ion. The atoms of a polyatomic ion must be kept together as one ion despite being composed of more than one atom: Al1(ClO)3.

Step 3: Reduce to lowest ratio and leave out subscripts of 1: Al(ClO)3.

Example 3: Create a formula for aluminum oxide.

Step 1: Write the symbol and charge: Al3+ and O2-.

Step 2: Use crisscross method: Al2O3.

Step 3: Reduce to lowest ratio and leave out subscripts of 1: Al2O3.

Formula of a Compound: Empirical Formula Example

An empirical formula will give the simplest whole-number ratio of atoms in a compound when the information given is the mass of each element and not the name of the compound.

Step 1: Note the number of grams given in the problem:

40.50 g Ca; 32.40 g O; 2.00 g H

Note: If percentages are given, assume it is the mass of each element.

Step 2: Convert mass to molar mass. Find the molar mass*es* on the periodic table for each element; for example, calcium is 40.08 grams/mol. Then multiply the molar mass ratio with the amount given in the problem:

40.50 g Ca × 1 mol Ca / 40.08 g Ca = 1.01 mol Ca

32.40 g O × 1 mol O / 16.00 g O = 2.03 mol O

2.00 g H × 1 mol H / 1.01 g H = 1.98 mol H

Step 3: Divide each of the values calculated above by the smallest number of moles calculated. In this problem, this is the 1.01 mol Ca:

Ca: 1.01 / 1.01 = 1

O: 2.03 / 1.01 = 2

H: 1.98 / 1.01 = 1.96 or 2

Step 4: Use the mole ratio of whole numbers to create the formula:

CaO2H2 or Ca(OH)2 creates calcium hydroxide.

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