Writing Correct Formulae for Reactants and Products

Introduction

Balancing cannot begin responsibly until each substance has the right formula. Ionic salts use charge-neutral ratios; molecular substances require their actual atom counts; elemental gases may be diatomic. This page gathers those decisions into a formula-checking workflow so that a neat row of symbols still corresponds to the named chemistry.

Core explanation

For a simple ionic compound, write the ions and balance charge. Magnesium is commonly Mg²⁺ and chloride Cl⁻, giving MgCl₂. Aluminium oxide uses Al³⁺ and O²⁻, giving Al₂O₃. Reduce to the simplest whole-number ratio; a formula such as Mg₂O₂ is not the conventional simplest representation for magnesium oxide.

Polyatomic ions must remain intact. Calcium nitrate contains Ca²⁺ and NO₃⁻, giving Ca(NO₃)₂. The parentheses indicate two complete nitrate ions; replacing this with CaNO₆ would lose one nitrogen and fail to represent the named compound. Sulfate, carbonate, hydroxide and ammonium similarly have established internal formulas and charges.

Molecular formulas describe discrete species. Water is H₂O, methane CH₄ and carbon dioxide CO₂. Simple valency patterns help check these formulas, but they do not justify creating an arbitrary compound whenever a tally looks convenient. Carbon monoxide CO is also a real species, so product identification must come from the specified reaction rather than assuming every carbon oxide is CO₂.

Elemental forms require care. Ordinary hydrogen, oxygen and nitrogen gases are represented as H₂, O₂ and N₂. Chlorine and the other common elemental halogens are also diatomic in the introductory context. A lone element symbol may represent a metal solid or, in another context, an isolated atom, so the state and intended species matter.

Variable-charge metals need explicit information. Iron(II) oxide is FeO from Fe²⁺ and O²⁻, while iron(III) oxide is Fe₂O₃ from Fe³⁺ and O²⁻. Both names are chemically distinct. If the prompt merely says “iron oxide” without observations, it may not supply enough information to choose one unique formula.

Step-by-step reasoning

1. Classify each named species as elemental, ionic or molecular in the intended context. 2. For ionic compounds, write complete ion formulas and find the neutral ratio. 3. For molecules and elemental substances, use established molecular identities and check familiar bond patterns. 4. Match every chosen formula back to the exact name before introducing balancing coefficients.

Visual explanation

Draw three formula gates. “Ionic?” leads to charge balance, “molecular?” leads to actual atom count and “elemental?” leads to physical form such as O₂ or Fe(s). Feed each named substance through one gate before placing it into an equation.

Real-world analogy

A shipping label must identify the correct package before its quantity is entered on an order form. Doubling the quantity of the wrong package does not make the order correct. Equation coefficients similarly cannot repair the use of an incorrect chemical formula.

Real-world example

The reaction calcium hydroxide with hydrochloric acid gives calcium chloride and water in the simple neutralisation pattern. Correct formulas are Ca(OH)₂, HCl, CaCl₂ and H₂O. Only after these are fixed do the two acid molecules needed in Ca(OH)₂ + 2HCl → CaCl₂ + 2H₂O become clear.

Why?

Why do ionic formulas usually use the simplest ratio rather than the literal count of ions in a sample? An ionic solid contains many repeating ions, not one small molecule with a fixed finite group size. Its formula expresses the smallest charge-neutral composition ratio.

Common misconception

“An element's group number always gives the formula of every compound it makes.” Familiar charge and valency patterns help, but variable oxidation states, covalent structures and real product identity can require additional information.

Worked example

Choose formulas for iron(III) chloride and iron(II) chloride. Fe³⁺ needs three Cl⁻, giving FeCl₃. Fe²⁺ needs two Cl⁻, giving FeCl₂. Changing the Roman numeral changes the salt formula. If an equation names iron(III) chloride, replacing it with FeCl₂ to simplify a balance would describe the wrong product.

Quick check

1. What formula follows from Ca²⁺ and two nitrate ions in a neutral compound? Answer: Ca(NO₃)₂, with the outside two multiplying the entire nitrate group.

Exam focus

Write ion charges in working, preserve polyatomic groups and check diatomic elemental gases. Treat variable-charge names as formula information, not decoration.

Advanced insight

The empirical formula of a compound and a molecular formula answer different questions. Glucose has molecular formula C₆H₁₂O₆ but empirical ratio CH₂O. Confusing these representations during equation writing can silently change the entities counted by coefficients.

Summary

Correct formulas come from identified species: charge-balanced ratios for ionic solids, actual atom counts for molecules and appropriate elemental forms for free elements. Variable charges and compound ions require care. Once formulas are fixed, coefficients can balance the complete reaction.

Practice questions

1. Give the formula of aluminium sulfate from Al³⁺ and SO₄²⁻. Answer: Al₂(SO₄)₃, because +6 and −6 balance. 2. Why can FeO not replace Fe₂O₃ in an equation explicitly naming iron(III) oxide? Answer: FeO corresponds to iron(II) oxide and therefore changes the product's specified chemical identity. 3. What is the difference between glucose's molecular and empirical formulas? Answer: C₆H₁₂O₆ gives its actual molecular atom counts; CH₂O gives only the simplest ratio.