Accurately calculate the molar mass and elemental composition of any chemical compound.
Enter a valid chemical formula. Case sensitive (e.g., use 'Co' for Cobalt, 'CO' for Carbon Monoxide).
Please enter a valid chemical formula.
Molecular Weight (Molar Mass)
0.00
grams per mole (g/mol)
Total Atoms
0
Heaviest Element
–
Element Count
0
Formula Used: Molecular Weight = Σ (Atomic Mass of Element × Number of Atoms)
Elemental Breakdown
Element
Atoms
Atomic Mass (g/mol)
Total Mass (g/mol)
Mass %
Table 1: Detailed breakdown of mass contribution by element.
Mass Composition Chart
Figure 1: Visual representation of elemental mass percentages.
What is How to Calculate Weight of Molecules?
Understanding how to calculate weight of molecules—scientifically known as molecular weight or molar mass—is a fundamental skill in chemistry, physics, and pharmacology. It represents the sum of the atomic masses of all atoms in a molecule. This calculation is critical for stoichiometry, preparing solutions in a laboratory, and understanding the physical properties of substances.
The molecular weight is typically expressed in grams per mole (g/mol) or Daltons (Da). Whether you are a student balancing chemical equations or a professional chemist formulating a compound, knowing the precise weight of a molecule ensures accuracy in reactions and dosage calculations.
Common misconceptions include confusing "molecular weight" with "molecular mass" (though often used interchangeably, weight implies an average of isotopes found in nature) or forgetting to account for subscripts in chemical formulas.
Molecular Weight Formula and Mathematical Explanation
To master how to calculate weight of molecules, one must use the periodic table of elements. The formula is a summation of the atomic weights of constituent elements multiplied by their abundance in the molecule.
Our tool simplifies the process of how to calculate weight of molecules by automating the periodic table lookups. Follow these steps:
Enter the Formula: Type the chemical formula in the input field (e.g., "NaCl" or "C8H10N4O2").
Check Case Sensitivity: Ensure you use uppercase for the first letter of an element symbol (e.g., "Ca" for Calcium, not "ca").
Click Calculate: The tool will parse the string, identify elements, and sum their weights.
Analyze Results: View the total molar mass, the percentage composition chart, and the detailed table breakdown.
Key Factors That Affect Molecular Weight Results
When learning how to calculate weight of molecules, consider these factors that influence the final figure:
Isotopic Abundance: Standard atomic weights are averages based on Earth's natural isotope distribution. Specific isotopes (e.g., Carbon-13 vs. Carbon-12) have different masses.
Formula Accuracy: A single incorrect subscript changes the entire result. "CO" (Carbon Monoxide) is vastly different from "CO2" (Carbon Dioxide).
Hydration: Many compounds absorb water. Copper Sulfate Pentahydrate (CuSO₄·5H₂O) is heavier than anhydrous Copper Sulfate (CuSO₄).
Purity: In practical lab settings, impurities can affect the measured weight, though the theoretical calculation remains constant.
Significant Figures: The precision of your atomic weight data (e.g., 1.01 vs 1.00784) affects the precision of the final molecular weight.
Polymerization: For polymers, the weight is often an average (Mn or Mw) rather than a fixed number, as chain lengths vary.
Frequently Asked Questions (FAQ)
What is the difference between molecular weight and molar mass?
Technically, molecular weight is the mass of a single molecule (measured in amu), while molar mass is the mass of one mole of substance (g/mol). However, numerically they are usually identical for practical purposes.
Does this calculator handle parentheses?
Yes, for simple groups. However, for complex nested structures, it is best to expand the formula manually (e.g., convert Ca(OH)2 to CaO2H2) to ensure absolute accuracy.
Why is case sensitivity important?
In chemistry, "Co" stands for Cobalt, while "CO" stands for Carbon and Oxygen. Case sensitivity distinguishes between single-letter and two-letter element symbols.
How do I calculate the weight of a hydrate?
Add the water molecules to your formula. For example, for CuSO₄·5H₂O, you can enter the atoms directly if you sum them up, or calculate the anhydrous part and add 5 times the weight of water (18.015).
What unit is used for molecular weight?
The standard unit is grams per mole (g/mol). In physics and biochemistry, Daltons (Da) are also frequently used.
Can I calculate the weight of ions?
Yes. The mass of an electron is negligible compared to protons and neutrons, so the molecular weight of an ion (e.g., SO₄²⁻) is effectively the same as the neutral group of atoms.
Why are atomic weights not whole numbers?
Atomic weights are weighted averages of all naturally occurring isotopes of an element. For example, Chlorine is ~35.45 because it exists as both Cl-35 and Cl-37.
Is this tool useful for stoichiometry?
Absolutely. Calculating the molar mass is the first step in converting grams to moles, which is essential for stoichiometric calculations in chemical reactions.
// Periodic Table Data (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": 35.45, "K": 39.098, "Ar": 39.948, "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": 78.96, "Br": 79.904, "Kr": 83.798, "Rb": 85.468, "Sr": 87.62, "Y": 88.906, "Zr": 91.224,
"Nb": 92.906, "Mo": 95.95, "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, "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, "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, "Th": 232.04, "Pa": 231.04,
"U": 238.03
};
var chartInstance = null;
function calculateWeight() {
var formulaInput = document.getElementById('chemicalFormula').value.trim();
var errorDiv = document.getElementById('formulaError');
var resultsArea = document.getElementById('resultsArea');
if (!formulaInput) {
errorDiv.style.display = 'block';
errorDiv.innerText = "Please enter a formula.";
resultsArea.style.display = 'none';
return;
}
// Basic parsing logic
var elements = parseFormula(formulaInput);
if (!elements || Object.keys(elements).length === 0) {
errorDiv.style.display = 'block';
errorDiv.innerText = "Invalid formula or unknown elements. Ensure correct casing (e.g., 'Cl' not 'cl').";
resultsArea.style.display = 'none';
return;
}
errorDiv.style.display = 'none';
resultsArea.style.display = 'block';
// Calculate totals
var totalWeight = 0;
var totalAtoms = 0;
var heaviestElem = { name: ", mass: 0 };
var breakdownData = [];
for (var symbol in elements) {
if (elements.hasOwnProperty(symbol)) {
var count = elements[symbol];
var atomicMass = atomicWeights[symbol];
if (!atomicMass) {
// Fallback for unknown elements if parser var them through
continue;
}
var totalElemMass = count * atomicMass;
totalWeight += totalElemMass;
totalAtoms += count;
if (atomicMass > heaviestElem.mass) {
heaviestElem = { name: symbol, mass: atomicMass };
}
breakdownData.push({
symbol: symbol,
count: count,
atomicMass: atomicMass,
totalMass: totalElemMass
});
}
}
// Update UI
document.getElementById('totalWeightDisplay').innerText = totalWeight.toFixed(3);
document.getElementById('totalAtomsDisplay').innerText = totalAtoms;
document.getElementById('heaviestElementDisplay').innerText = heaviestElem.name + " (" + heaviestElem.mass + ")";
document.getElementById('elementCountDisplay').innerText = Object.keys(elements).length;
// Update Table
var tableBody = document.getElementById('breakdownTableBody');
tableBody.innerHTML = ";
// Sort breakdown by mass contribution descending
breakdownData.sort(function(a, b) { return b.totalMass – a.totalMass; });
for (var i = 0; i < breakdownData.length; i++) {
var item = breakdownData[i];
var percent = (item.totalMass / totalWeight) * 100;
var row = '
' +
'
' + item.symbol + '
' +
'
' + item.count + '
' +
'
' + item.atomicMass.toFixed(3) + '
' +
'
' + item.totalMass.toFixed(3) + '
' +
'
' + percent.toFixed(2) + '%
' +
'
';
tableBody.innerHTML += row;
}
// Draw Chart
drawChart(breakdownData, totalWeight);
}
function parseFormula(formula) {
// Regex to match Element (Upper + optional Lower) and Count (optional digits)
// This is a flat parser. It does not handle parentheses recursively in this simple version,
// but handles basic formulas like C6H12O6 or NaCl well.
// To support parentheses simply, we would need a more complex stack-based parser.
// For this implementation, we will expand simple parentheses if possible or stick to flat parsing
// to ensure stability without external libraries.
// Let's try a robust flat parser first.
var regex = /([A-Z][a-z]?)(\d*)/g;
var counts = {};
var match;
var valid = true;
// Check for invalid characters (anything not A-Z, a-z, 0-9)
// Allowing () for future expansion, but currently ignoring logic for them in simple regex
if (/[^A-Za-z0-9]/.test(formula)) {
// If parentheses exist, this simple parser fails.
// We will return null to trigger error message about complexity.
// Or we can strip them if we want to be lenient, but that's scientifically wrong.
return null;
}
while ((match = regex.exec(formula)) !== null) {
var element = match[1];
var countStr = match[2];
var count = countStr === "" ? 1 : parseInt(countStr, 10);
if (!atomicWeights[element]) {
valid = false;
break;
}
if (counts[element]) {
counts[element] += count;
} else {
counts[element] = count;
}
}
// Verify the whole string was consumed (approximate check)
var reconstructed = "";
// This check is hard with regex exec loop.
// Instead, we check if we found any elements.
if (Object.keys(counts).length === 0 || !valid) return null;
return counts;
}
function resetCalculator() {
document.getElementById('chemicalFormula').value = ";
document.getElementById('resultsArea').style.display = 'none';
document.getElementById('formulaError').style.display = 'none';
}
function copyResults() {
var weight = document.getElementById('totalWeightDisplay').innerText;
var formula = document.getElementById('chemicalFormula').value;
var text = "Molecular Weight Calculation for " + formula + ":\n" +
"Total Weight: " + weight + " g/mol\n" +
"Calculated using Molecular Weight Calculator.";
var tempInput = document.createElement("textarea");
tempInput.value = text;
document.body.appendChild(tempInput);
tempInput.select();
document.execCommand("copy");
document.body.removeChild(tempInput);
var btn = event.target;
var originalText = btn.innerText;
btn.innerText = "Copied!";
setTimeout(function() { btn.innerText = originalText; }, 2000);
}
function drawChart(data, total) {
var canvas = document.getElementById('compositionChart');
var ctx = canvas.getContext('2d');
// Clear canvas
ctx.clearRect(0, 0, canvas.width, canvas.height);
// Set dimensions
canvas.width = 400;
canvas.height = 300;
var centerX = canvas.width / 2;
var centerY = canvas.height / 2;
var radius = 100;
var startAngle = 0;
var colors = ['#004a99', '#28a745', '#ffc107', '#dc3545', '#17a2b8', '#6610f2', '#fd7e14'];
for (var i = 0; i < data.length; i++) {
var item = data[i];
var sliceAngle = (item.totalMass / total) * 2 * Math.PI;
var endAngle = startAngle + sliceAngle;
// Draw slice
ctx.beginPath();
ctx.moveTo(centerX, centerY);
ctx.arc(centerX, centerY, radius, startAngle, endAngle);
ctx.closePath();
ctx.fillStyle = colors[i % colors.length];
ctx.fill();
// Draw Legend (Simple)
var legendX = 320;
var legendY = 50 + (i * 25);
// Only draw legend if it fits or limit items
if (i < 8) {
ctx.fillStyle = colors[i % colors.length];
ctx.fillRect(10, 20 + (i * 25), 15, 15);
ctx.fillStyle = '#333';
ctx.font = '12px Arial';
ctx.fillText(item.symbol + " (" + ((item.totalMass/total)*100).toFixed(1) + "%)", 30, 32 + (i * 25));
}
startAngle = endAngle;
}
// Draw center hole for Donut Chart look (optional, looks professional)
ctx.beginPath();
ctx.arc(centerX, centerY, radius * 0.5, 0, 2 * Math.PI);
ctx.fillStyle = '#ffffff';
ctx.fill();
}
// Initialize with default value
window.onload = function() {
calculateWeight();
};