The molecular formula of a compound represents the actual number of atoms of each element present in a single molecule. Unlike the empirical formula, which provides the simplest whole-number ratio, the molecular formula provides the exact chemical makeup.
To calculate the molecular formula, you must first know two critical pieces of data: the empirical formula (and its mass) and the experimentally determined molar mass of the substance.
The Step-by-Step Process
Find the Empirical Formula Mass: Sum the atomic masses of all atoms in the empirical formula. (Example: For CH2, C=12.01 and H=1.008. Mass = 12.01 + 2(1.008) = 14.026 g/mol).
Divide Molar Mass by Empirical Mass: Take the total molar mass of the compound (usually provided in the problem) and divide it by the empirical mass calculated in step 1.
Determine the Multiplier (n): Round the result of your division to the nearest whole number. This is your "n" factor.
Multiply the Subscripts: Multiply every subscript in the empirical formula by "n" to get the molecular formula.
While many different compounds can share the same empirical formula (for example, formaldehyde, acetic acid, and glucose all share CH2O), their molecular formulas define their unique properties, structures, and functions. Calculating the molecular formula is a fundamental skill in analytical chemistry and stoichiometry.
Common Molecular Formula Multipliers
The multiplier 'n' is always a whole number. If your calculation results in a number like 1.99 or 2.01, it is safe to round to 2. If you get a number like 1.5, re-check your empirical mass calculations or the provided molar mass, as molecular formulas represent discrete whole atoms.
function calculateMolecularFormula() {
var empFormulaInput = document.getElementById('empFormula').value.trim();
var empMass = parseFloat(document.getElementById('empMass').value);
var molarMass = parseFloat(document.getElementById('molarMass').value);
var resultDiv = document.getElementById('molecularResult');
var multiplierDisplay = document.getElementById('multiplierResult');
var formulaDisplay = document.getElementById('finalFormula');
if (isNaN(empMass) || isNaN(molarMass) || empMass <= 0 || molarMass <= 0) {
alert("Please enter valid positive numbers for masses.");
return;
}
// Step 1: Calculate the multiplier n
var n = Math.round(molarMass / empMass);
var exactN = (molarMass / empMass).toFixed(3);
// Step 2: Logic to manipulate the formula string
var resultFormula = "";
if (empFormulaInput === "") {
resultFormula = "Multiplier (n) is " + n;
} else {
resultFormula = formatFormula(empFormulaInput, n);
}
// Display results
multiplierDisplay.innerHTML = "Multiplier (n) = " + exactN + " ≈ " + n + "";
formulaDisplay.innerHTML = "Molecular Formula: " + resultFormula;
resultDiv.style.display = "block";
}
function formatFormula(formula, n) {
if (n === 1) return formula;
// Regular expression to find element symbols and their following numbers
// This handles symbols like C, Mg, Cl and numbers
var regex = /([A-Z][a-z]*)(\d*)/g;
var match;
var newFormula = "";
while ((match = regex.exec(formula)) !== null) {
var element = match[1];
var count = match[2] === "" ? 1 : parseInt(match[2]);
var newCount = count * n;
newFormula += element + (newCount > 1 ? newCount : "");
}
if (newFormula === "") return "Error parsing formula. Check format (e.g., CH2).";
// Simple subscript formatting for web display
return newFormula.replace(/(\d+)/g, '$1');
}