Chemical Formulas: Part 2

Chemical Formulas- Part 2

Basics

Parentheses & Groups

Coefficients

Counting Atoms in Chemical Formulas with Coefficients

A coefficient is the big number in front of a chemical formula. It tells you how many entire units (molecules or formula units) are present. The coefficient applies to every atom in the formula.

5 Mg₃(PO₄)₂

3 H₂O

The coefficient 3 means there are 3 molecules of H2O.

STEP 1: Identify elements Magnesium (Mg), Phosphorus (P), Oxygen (O) STEP 2: Apply the subscripts Mg₃ means 3 Mg atoms (PO₄)₂ means 2 phosphate (PO₄) groups, so: Phosphorus: 1 × 2 = 2 P atoms Oxygen: 4 × 2 = 8 O atoms STEP 3: Apply the coefficient 5 Mg₃(PO₄)₂ means 5 groups of Mg₃(PO₄)₂ (magnesium phosphate) so Multiply everything by 5: Magnesium: 5 × 3 = 15 Mg Phosphorus: 5 × 2 = 10 P Oxygen: 5 × 8 = 40 O STEP 4: Add up all atomsTotal atoms: 15 Mg + 10 P + 40 O = 65 atoms in 5 Mg₃(PO₄)₂ formula units

STEP 1: Identify elements Hydrogen (H), Oxygen (O) STEP 2: Apply the subscripts H₂O means each water molecule has: 2 Hydrogen (H) atoms 1 Oxygen (O) atom STEP 3: Apply the coefficient 3 H₂O means 3 groups of H₂O (water molecules), so: Multiply everything by 3: Hydrogen: 3 × 2 = 6 H atoms Oxygen: 3 × 1 = 3 O atoms STEP 4: Add up all atomsTotal atoms: 6 H + 3 O = 9 atoms in 3 H₂O molecules.

The coefficient 5 means there are 5 formula units of Mg₃(PO₄)₂.

How to Count Atoms in a Compound with Parentheses

Parentheses in a chemical formula group atoms together, often representing a polyatomic ion. When a subscript is placed outside the parentheses, it applies to all atoms inside the parentheses.

Iron(III) Sulfate

Fe₂(SO₄)₃

This means there are 3 SO₄ groups, so the subscript applies to every atom inside the parentheses.

STEP 1: Identify elementsIron (Fe), Sulfur (S), Oxygen (O) STEP 2: Apply subscriptsFe₂ → The subscript 2 means 2 Fe atoms.(SO₄)₃ → The 3 outside the parentheses applies to everything inside:Sulfur: 1 × 3 = 3 S atomsOxygen: 4 × 3 = 12 O atoms STEP 3: Add up all atomsIron: 2Sulfur: 3Oxygen: 12Total: 2 + 3 + 12 = 17 atoms in one formula unit of Fe₂(SO₄)₃

To determine the number of elements in a compound using a chemical formula: 1. Identify each unique element symbol in the formula. 2. Count the different symbols—each represents a distinct element. Example: Na2SO4 has three elements— Sodium (Na), Sulfur (S), and oxygen (O).

To count atoms in a compound using a chemical formula: 1. Identify each element in the formula. 2. Read the subscripts – they indicate how many atoms of each element are present. -If no subscript is shown, it means 1 atom. 3. Add up all the atoms to find the total number in the compound.

2 Sodium (Na) atoms 1 Sulfur (S) atom + 4 Oxygen (O) atoms

Na2SO4

Sodium Sulfate

7 atoms total in each formula unit of Na2SO4