Understanding Molecular Formulas
The molecular formula of a compound provides the exact number of atoms of each element present in a single molecule of that compound. For example, the molecular formula for glucose is C6 H12 O6 , indicating six carbon atoms, twelve hydrogen atoms, and six oxygen atoms.
Empirical vs. Molecular Formula
It's important to distinguish between the empirical formula and the molecular formula:
Empirical Formula: Represents the simplest whole-number ratio of atoms in a compound. For glucose, the empirical formula is CH2 O (dividing all subscripts by 6).Molecular Formula: Represents the actual number of atoms of each element in a molecule. It is always a whole-number multiple of the empirical formula.
How to Calculate the Molecular Formula
To determine the molecular formula from the empirical formula, you typically need two pieces of information:
The empirical formula of the compound.
The molecular weight (or molar mass) of the compound.
The calculation involves these steps:
Calculate the Empirical Formula Weight (EFW): Sum the atomic weights of all atoms in the empirical formula.Determine the 'n' factor: Divide the given molecular weight by the empirical formula weight. This 'n' factor should be a whole number (or very close to one, allowing for minor rounding due to atomic weight precision).Multiply Subscripts: Multiply each subscript in the empirical formula by the 'n' factor to obtain the molecular formula.
Example:
Let's say a compound has an empirical formula of CH2 O and a molecular weight of 180.16 g/mol.
Calculate Empirical Formula Weight (EFW) for CH2 O: Determine 'n' factor: Multiply Subscripts: 2 O) x 6 = C(1×6) H(2×6) O(1×6) = C6 H12 O6
Thus, the molecular formula is C6 H12 O6 .
Molecular Formula Calculator
Enter the empirical formula and the molecular weight of the compound to find its molecular formula.
Empirical Formula:
Molecular Weight (g/mol):
Calculate Molecular Formula
// Atomic weights lookup table (common elements)
var atomicWeights = {
"H": 1.008, "He": 4.0026, "Li": 6.94, "Be": 9.0122, "B": 10.81, "C": 12.011, "N": 14.007, "O": 15.999, "F": 18.998, "Ne": 20.180,
"Na": 22.990, "Mg": 24.305, "Al": 26.982, "Si": 28.085, "P": 30.974, "S": 32.06, "Cl": 35.453, "Ar": 39.948, "K": 39.098, "Ca": 40.078,
"Sc": 44.956, "Ti": 47.867, "V": 50.942, "Cr": 51.996, "Mn": 54.938, "Fe": 55.845, "Co": 58.933, "Ni": 58.693, "Cu": 63.546, "Zn": 65.38,
"Ga": 69.723, "Ge": 72.63, "As": 74.922, "Se": 78.971, "Br": 79.904, "Kr": 83.798, "Rb": 85.468, "Sr": 87.62, "Y": 88.906, "Zr": 91.224,
"Nb": 92.906, "Mo": 95.96, "Tc": 98, "Ru": 101.07, "Rh": 102.91, "Pd": 106.42, "Ag": 107.87, "Cd": 112.41, "In": 114.82, "Sn": 118.71,
"Sb": 121.76, "I": 126.90, "Xe": 131.29, "Cs": 132.91, "Ba": 137.33, "La": 138.91, "Ce": 140.12, "Pr": 140.91, "Nd": 144.24, "Pm": 145,
"Sm": 150.36, "Eu": 151.96, "Gd": 157.25, "Tb": 158.93, "Dy": 162.50, "Ho": 164.93, "Er": 167.26, "Tm": 168.93, "Yb": 173.05, "Lu": 174.97,
"Hf": 178.49, "Ta": 180.95, "W": 183.84, "Re": 186.21, "Os": 190.23, "Ir": 192.22, "Pt": 195.08, "Au": 196.97, "Hg": 200.59, "Tl": 204.38,
"Pb": 207.2, "Bi": 208.98, "Po": 209, "At": 210, "Rn": 222, "Fr": 223, "Ra": 226, "Ac": 227, "Pa": 231.04, "Th": 232.04,
"Np": 237, "U": 238.03, "Am": 243, "Pu": 244, "Cm": 247, "Bk": 247, "Cf": 251, "Es": 252, "Fm": 257, "Md": 258,
"No": 259, "Rf": 261, "Lr": 262, "Db": 262, "Bh": 264, "Sg": 266, "Mt": 268, "Rg": 272, "Hs": 277
};
function parseEmpiricalFormula(formula) {
var elements = {};
// Regex to match element symbols (e.g., C, Cl) followed by an optional number (subscript)
var regex = /([A-Z][a-z]*)(\d*)/g;
var match;
var hasMatches = false;
while ((match = regex.exec(formula)) !== null) {
hasMatches = true;
var elementSymbol = match[1];
var count = parseInt(match[2] || '1', 10); // If no number, count is 1
if (!atomicWeights[elementSymbol]) {
return { error: "Unknown element symbol: '" + elementSymbol + "'. Please use standard element symbols (e.g., C, H, O, Cl)." };
}
elements[elementSymbol] = (elements[elementSymbol] || 0) + count;
}
if (!hasMatches && formula.trim() !== "") {
return { error: "Invalid empirical formula format. Please check element symbols and subscripts. (e.g., C2H6O, not C2h6o)" };
}
if (Object.keys(elements).length === 0 && formula.trim() === "") {
return { error: "Empirical formula cannot be empty." };
}
return { elements: elements };
}
function calculateEmpiricalFormulaWeight(parsedElements) {
var efw = 0;
for (var symbol in parsedElements) {
if (parsedElements.hasOwnProperty(symbol)) {
efw += atomicWeights[symbol] * parsedElements[symbol];
}
}
return efw;
}
function formatMolecularFormula(elements, nFactor) {
var molecularFormula = "";
for (var symbol in elements) {
if (elements.hasOwnProperty(symbol)) {
var count = elements[symbol] * nFactor;
molecularFormula += symbol;
if (count > 1) {
molecularFormula += "
" + count + " ";
}
}
}
return molecularFormula;
}
function calculateMolecularFormula() {
var empiricalFormulaInput = document.getElementById("empiricalFormula").value.trim();
var molecularWeightInput = document.getElementById("molecularWeight").value;
var resultDiv = document.getElementById("molecularFormulaResult");
resultDiv.innerHTML = ""; // Clear previous results
if (empiricalFormulaInput === "") {
resultDiv.innerHTML = "Please enter the empirical formula.";
return;
}
var molecularWeight = parseFloat(molecularWeightInput);
if (isNaN(molecularWeight) || molecularWeight tolerance && nFactor > 0.1) {
resultDiv.innerHTML = "The ratio of molecular weight to empirical formula weight is not a whole number. Please check your inputs. (Calculated n-factor: " + nFactor.toFixed(2) + ")";
return;
}
if (roundedNFactor < 1) {
resultDiv.innerHTML = "The molecular weight (" + molecularWeight.toFixed(2) + ") is less than the empirical formula weight (" + empiricalFormulaWeight.toFixed(3) + "). Please check your inputs.";
return;
}
var molecularFormula = formatMolecularFormula(parsed.elements, roundedNFactor);
resultDiv.innerHTML = "
Calculations: " +
"Empirical Formula Weight (EFW): " + empiricalFormulaWeight.toFixed(3) + " g/mol" +
"n-factor (Molecular Weight / EFW): " + molecularWeight.toFixed(2) + " / " + empiricalFormulaWeight.toFixed(3) + " = " + nFactor.toFixed(2) + " (rounded to " + roundedNFactor + ")" +
"
Molecular Formula: " +
"
" + molecularFormula + " ";
}
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