Empirical and Molecular Formula Calculator
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Empirical and Molecular Formula Calculator
Results
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Understanding Empirical and Molecular Formulas
In chemistry, understanding the composition of a compound is fundamental. This involves determining its empirical formula and molecular formula.
What are Empirical and Molecular Formulas?
-
Empirical Formula: The simplest whole-number ratio of atoms of each element present in a compound. It represents the relative proportions of elements, not necessarily the actual number of atoms in a molecule.
-
Molecular Formula: The actual number of atoms of each element in one molecule of a compound. It is a multiple of the empirical formula.
How to Calculate the Empirical Formula
The process typically starts with the percent composition of a compound, which can be obtained through experimental analysis (like combustion analysis).
-
Assume a 100g sample: This converts percentages directly into grams. For example, 40.0% Carbon becomes 40.0 grams of Carbon.
-
Convert grams to moles: Divide the mass of each element by its atomic mass (found on the periodic table).
Moles = Mass (g) / Atomic Mass (g/mol)
-
Find the simplest mole ratio: Divide the number of moles of each element by the smallest number of moles calculated in the previous step.
-
Obtain whole numbers: If the ratios obtained are not whole numbers, multiply all ratios by the smallest integer that will convert them into whole numbers. This gives you the empirical formula.
How to Calculate the Molecular Formula
To determine the molecular formula, you need the empirical formula and the molecular weight of the compound.
-
Calculate the empirical formula mass: Sum the atomic masses of all atoms in the empirical formula.
-
Determine the multiplier: Divide the molecular weight of the compound by the empirical formula mass.
Multiplier (n) = Molecular Weight / Empirical Formula Mass
-
Obtain the molecular formula: Multiply the subscript of each element in the empirical formula by the multiplier (n). If the multiplier is very close to a whole number (e.g., 1.99 or 2.01), round it to the nearest whole number.
Molecular Formula = (Empirical Formula)n
Example Calculation:
Let's consider a compound with the following composition:
- Carbon (C): 40.0%
- Hydrogen (H): 6.7%
- Oxygen (O): 53.3%
And a molecular weight of 180.16 g/mol.
- Assume 100g: 40.0g C, 6.7g H, 53.3g O
- Convert to moles:
- C: 40.0g / 12.01 g/mol ≈ 3.33 moles
- H: 6.7g / 1.008 g/mol ≈ 6.65 moles
- O: 53.3g / 16.00 g/mol ≈ 3.33 moles
- Smallest mole ratio: Divide by 3.33 (smallest value):
- C: 3.33 / 3.33 = 1
- H: 6.65 / 3.33 ≈ 1.99 ≈ 2
- O: 3.33 / 3.33 = 1
- Empirical Formula: CH2O
- Empirical Formula Mass: 12.01 + (2 * 1.008) + 16.00 = 30.03 g/mol
- Multiplier (n): 180.16 g/mol / 30.03 g/mol ≈ 6
- Molecular Formula: (CH2O)6 = C6H12O6 (Glucose)
This calculator automates these steps to quickly determine the empirical and molecular formulas.
function getAtomicMass(symbol) {
// Simplified atomic masses for common elements
var atomicMasses = {
"H": 1.008, "He": 4.003, "Li": 6.94, "Be": 9.012, "B": 10.81,
"C": 12.01, "N": 14.01, "O": 16.00, "F": 19.00, "Ne": 20.18,
"Na": 22.99, "Mg": 24.31, "Al": 26.98, "Si": 28.09, "P": 30.97,
"S": 32.07, "Cl": 35.45, "Ar": 39.95, "K": 39.10, "Ca": 40.08,
"Sc": 44.96, "Ti": 47.87, "V": 50.94, "Cr": 51.99, "Mn": 54.94,
"Fe": 55.84, "Co": 58.93, "Ni": 58.69, "Cu": 63.55, "Zn": 65.38,
"Ga": 69.72, "Ge": 72.63, "As": 74.92, "Se": 78.97, "Br": 79.90,
"Kr": 83.80, "Rb": 85.47, "Sr": 87.62, "Y": 88.91, "Zr": 91.22,
"Nb": 92.91, "Mo": 95.95, "Tc": 98.00, "Ru": 101.07, "Rh": 102.91,
"Pd": 106.42, "Ag": 107.87, "Cd": 112.41, "In": 114.82, "Sn": 118.71,
"Sb": 121.76, "Te": 127.60, "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": 145.00, "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": 209.00, "At": 210.00,
"Rn": 222.00, "Fr": 223.00, "Ra": 226.00, "Ac": 227.00, "Th": 232.04,
"Pa": 231.04, "U": 238.03, "Np": 237.00, "Pu": 244.00, "Am": 243.00,
"Cm": 247.00, "Bk": 247.00, "Cf": 251.00, "Es": 252.00, "Fm": 257.00,
"Md": 258.00, "No": 259.00, "Lr": 266.00, "Rf": 267.00, "Db": 268.00,
"Sg": 271.00, "Bh": 270.00, "Hs": 269.00, "Mt": 277.00, "Ds": 270.00,
"Rg": 272.00, "Cn": 285.00, "Nh": 286.00, "Fl": 289.00, "Mc": 290.00,
"Lv": 293.00, "Ts": 294.00, "Og": 294.00
};
return atomicMasses[symbol.toUpperCase()] || null;
}
function roundToNearestFraction(number, maxDenominator = 10) {
if (Math.abs(number – Math.round(number)) < 0.001) {
return Math.round(number);
}
for (var denominator = 1; denominator <= maxDenominator; denominator++) {
if (Math.abs(number – Math.round(number / denominator) * denominator) < 0.01) {
return Math.round(number / denominator);
}
}
return number; // Return original if no simple fraction found
}
function formatFormula(elements, ratios) {
var formula = "";
var sortedKeys = Object.keys(elements).sort(); // Sort elements alphabetically
for (var i = 0; i 1) {
formula += ratio;
}
}
return formula;
}
function calculateFormula() {
var elementsData = [];
var totalPercent = 0;
// Collect data for up to 3 elements (extendable)
for (var i = 1; i 0) {
elementsData.push({ symbol: symbol, percent: percent });
totalPercent += percent;
}
}
var molecularWeight = parseFloat(document.getElementById("molecularWeight").value);
var resultDiv = document.getElementById("result");
resultDiv.style.backgroundColor = "var(–success-green)"; // Reset color
if (elementsData.length 1.0) { // Allow a small tolerance
resultDiv.innerHTML = "The sum of percentages must be close to 100%.";
return;
}
var moles = {};
var minMoles = Infinity;
// Step 1 & 2: Convert percentage to moles
for (var j = 0; j < elementsData.length; j++) {
var element = elementsData[j];
var atomicMass = getAtomicMass(element.symbol);
if (!atomicMass) {
resultDiv.innerHTML = "Unknown element symbol: " + element.symbol + ". Please check periodic table.";
return;
}
var mass = element.percent; // Assume 100g sample
var elementMoles = mass / atomicMass;
moles[element.symbol] = elementMoles;
if (elementMoles 3/2)
var wholeRatio = roundToNearestFraction(ratio);
empiricalRatios[elementSymbol] = wholeRatio;
empiricalFormulaMass += wholeRatio * getAtomicMass(elementSymbol);
}
// Step 4: Obtain whole numbers (already handled by roundToNearestFraction for simplicity)
empiricalFormulaStr = formatFormula(elementsData.reduce((acc, el) => { acc[el.symbol] = true; return acc; }, {}), empiricalRatios);
var outputHTML = "
Empirical Formula: " + empiricalFormulaStr + "";
// Molecular Formula Calculation
if (molecularWeight && !isNaN(molecularWeight) && molecularWeight > 0) {
if (empiricalFormulaMass === 0) {
outputHTML += "
Molecular Formula: Cannot determine without valid empirical formula mass.";
} else {
var multiplier = Math.round(molecularWeight / empiricalFormulaMass);
if (multiplier < 1) multiplier = 1; // Ensure multiplier is at least 1
var molecularFormulaStr = "";
var sortedEmpiricalKeys = Object.keys(empiricalRatios).sort();
for (var k = 0; k 1) {
molecularFormulaStr += molecularRatio;
}
}
outputHTML += "
Molecular Weight: " + molecularWeight + " g/mol";
outputHTML += "
Multiplier (n): " + multiplier + "";
outputHTML += "
Molecular Formula: " + molecularFormulaStr;
}
} else {
outputHTML += "
Molecular Formula: Enter Molecular Weight to determine.";
}
resultDiv.innerHTML = outputHTML;
}
function resetCalculator() {
document.getElementById("element1").value = "C";
document.getElementById("percent1").value = "40.0";
document.getElementById("element2").value = "H";
document.getElementById("percent2").value = "6.7";
document.getElementById("element3").value = "O";
document.getElementById("percent3").value = "53.3";
document.getElementById("molecularWeight").value = "";
document.getElementById("result").innerHTML = "Enter data above to see results.";
document.getElementById("result").style.backgroundColor = "var(–success-green)";
}