Enter the chemical formula (e.g., H2O, C6H12O6). Case-insensitive. Use numbers for subscripts.
Provide custom atomic masses for elements in JSON-like format (Element: Mass). If empty, default values are used.
Result
—
Formula Used: Molecular Weight = Σ (Number of Atoms of Element × Atomic Mass of Element)
Molecular Weight Calculation Details
Atomic Mass Breakdown
Element
Atomic Mass (g/mol)
Count
Total Mass (g/mol)
Atomic Mass Contribution Chart
What is Molecular Weight?
Molecular weight, often referred to as molar mass, is a fundamental property of a chemical compound. It represents the mass of one mole of a substance. A mole is a unit of measurement in chemistry that corresponds to approximately 6.022 x 10^23 elementary entities (like atoms, molecules, ions, or electrons). Essentially, molecular weight tells you how much a specific molecule weighs in terms of grams per mole (g/mol). Understanding the molecular weight is crucial for stoichiometry, determining reaction yields, and analyzing chemical compositions.
Who should use it? This calculator and the concept of molecular weight are primarily used by students of chemistry, researchers, laboratory technicians, pharmacists, material scientists, and anyone involved in chemical analysis, synthesis, or formulation. It's a cornerstone calculation in fields ranging from drug development to industrial chemical production.
Common Misconceptions:
Confusing with Atomic Weight: Atomic weight refers to the mass of a single atom of an element, while molecular weight is the mass of a molecule (composed of multiple atoms).
Ignoring Isotopes: Standard atomic weights used in most calculations are average values considering the natural isotopic abundance. Highly specific calculations might require isotopic masses.
Units: While often called "molecular weight," the correct SI unit is "molar mass" in kg/mol. However, g/mol is far more common in practice and understood universally in chemistry.
Molecular Weight Formula and Mathematical Explanation
The formula to calculate molecular weight (or molar mass) is straightforward. It involves summing up the atomic masses of all the atoms present in the chemical formula of a compound.
The Formula:
Molecular Weight (M) = Σ (Number of Atoms of Element × Atomic Mass of Element)
This formula essentially means you take each unique element in the chemical formula, multiply its atomic mass by the number of times it appears in the formula, and then add all these values together.
Step-by-Step Derivation:
Identify Elements: Parse the chemical formula to identify all the unique elements present.
Count Atoms: Determine the number of atoms for each element. Subscripts in the formula indicate these counts. If no subscript is present, it implies one atom of that element. For polyatomic ions within parentheses, the subscript outside the parenthesis multiplies all atoms inside.
Find Atomic Masses: Look up the standard atomic mass for each element from the periodic table. These are typically found in units of atomic mass units (amu), but numerically they are equivalent to grams per mole (g/mol) for molar mass calculations.
Calculate Contribution: For each element, multiply its atomic mass by its count.
Sum Contributions: Add up the results from step 4 for all elements to get the total molecular weight.
Variable Explanations:
Variables Used in Molecular Weight Calculation
Variable
Meaning
Unit
Typical Range/Notes
Element Symbol
Abbreviation for a chemical element (e.g., H, O, C).
N/A
Standard symbols from the periodic table.
Atomic Mass
The average mass of atoms of an element, calculated using the relative abundance of isotopes.
g/mol (grams per mole) or amu (atomic mass units)
Varies by element; available on the periodic table.
Count (Subscript)
The number of atoms of a specific element in one molecule of the compound.
Unitless
Integer ≥ 1. If no subscript, count is 1.
Molecular Weight (M)
The sum of the atomic masses of all atoms in a molecule.
g/mol
Generally positive and depends on the compound.
Practical Examples (Real-World Use Cases)
Example 1: Water (H₂O)
Water is essential for life and a common chemical compound.
Inputs:
Chemical Formula: H2O
Calculation Steps:
Elements: Hydrogen (H), Oxygen (O)
Counts: H = 2, O = 1
Atomic Masses (approximate): H = 1.008 g/mol, O = 15.999 g/mol
Formula Interpretation: One molecule of H₂O contains 2 Hydrogen atoms and 1 Oxygen atom.
Interpretation: One mole of water molecules weighs approximately 18.015 grams. This value is critical for calculations involving chemical reactions where water is a reactant or product.
Example 2: Glucose (C₆H₁₂O₆)
Glucose is a simple sugar, a primary source of energy for living organisms.
Inputs:
Chemical Formula: C6H12O6
Calculation Steps:
Elements: Carbon (C), Hydrogen (H), Oxygen (O)
Counts: C = 6, H = 12, O = 6
Atomic Masses (approximate): C = 12.011 g/mol, H = 1.008 g/mol, O = 15.999 g/mol
Formula Interpretation: One molecule of C₆H₁₂O₆ contains 6 Carbon atoms, 12 Hydrogen atoms, and 6 Oxygen atoms.
Interpretation: One mole of glucose molecules weighs approximately 180.156 grams. This knowledge is vital in biochemistry and metabolic studies. For instance, understanding how much glucose is consumed or produced in a biological process relies on this molar mass. If you're working with a solution of glucose, knowing its molecular weight allows you to accurately calculate molarity.
How to Use This Molecular Weight Calculator
Our Molecular Weight Calculator is designed for simplicity and accuracy. Follow these steps to get your results quickly:
Enter Chemical Formula: In the "Chemical Formula" field, type the formula of the compound you want to analyze. Use standard element symbols (e.g., H, O, C, Na, Cl). For subscripts, simply type the number after the element symbol (e.g., H2O for water, C6H12O6 for glucose). The calculator is case-insensitive.
(Optional) Provide Custom Atomic Masses: If you need to use specific isotopic masses or non-standard atomic weights, use the "Atomic Masses" textarea. Enter them in a format like `Element: Mass` (e.g., `H: 1.00784, O: 15.99491`). If this field is left blank, the calculator will use standard, widely accepted atomic masses from the periodic table.
Calculate: Click the "Calculate Molecular Weight" button.
Reading the Results:
Main Result (Molecular Weight): This is the primary output, displayed prominently. It shows the calculated molecular weight in grams per mole (g/mol).
Total Atomic Mass Sum: This confirms the total mass calculated.
Element Counts: This breaks down how many atoms of each element were found in the formula.
Formula Interpretation: A plain language description of the molecule's composition.
Calculation Table: Provides a detailed breakdown for each element, showing its atomic mass, its count in the formula, and its contribution to the total molecular weight.
Chart: Visually represents the contribution of each element's total mass to the overall molecular weight.
Decision-Making Guidance: Use the calculated molecular weight for accurate stoichiometric calculations in chemical reactions, preparing solutions of specific molar concentrations, and determining empirical formulas. The detailed breakdown helps in verifying the calculation and understanding the composition of complex molecules.
While the calculation itself is deterministic based on the chemical formula and atomic masses, several factors influence its practical application and interpretation:
Accuracy of Atomic Masses: The standard atomic masses listed on the periodic table are averages based on isotopic abundance. For highly precise work, especially in fields like mass spectrometry, using specific isotopic masses might be necessary. Our calculator uses standard values by default but allows custom inputs.
Chemical Formula Precision: An incorrect or incomplete chemical formula will lead to an incorrect molecular weight. This is especially true for complex organic molecules or coordination compounds where parentheses and polyatomic ions are involved. Double-checking the formula is crucial.
Isotopic Variations: Natural variations in isotopic abundance can slightly alter the average atomic mass. While generally negligible for most common calculations, it can be a factor in high-precision analytical chemistry or nuclear science.
Hydration (Water of Crystallization): Many solid compounds crystallize with water molecules incorporated into their structure (e.g., CuSO₄·5H₂O). The molecular weight calculation must include these water molecules if the hydrated form is specified. The calculator can handle this if the formula is entered correctly (e.g., `CuSO4.5H2O` or `CuSO4*5H2O`).
Temperature and Pressure Effects: Molecular weight itself is an intrinsic property and doesn't change with temperature or pressure. However, properties derived from it, like density or molar volume, are affected by these conditions.
Purity of the Sample: The calculated molecular weight assumes a pure substance. Impurities in a sample will mean the measured mass doesn't perfectly align with the theoretical molecular weight. This is often used in gravimetric analysis to determine purity.
Anions and Cations in Salts: For ionic compounds (salts like NaCl), there isn't a discrete "molecule" in the same sense as covalent compounds. We calculate the formula weight based on the empirical formula unit. The concept is analogous, summing the atomic masses of the ions in the formula unit.
Frequently Asked Questions (FAQ)
Q1: What is the difference between molecular weight and molar mass?
Technically, molecular weight refers to the mass of a single molecule (in amu), while molar mass refers to the mass of one mole of a substance (in g/mol). However, in practice, these terms are often used interchangeably, and the calculated value in g/mol is universally understood as the molar mass.
Q2: Can I calculate the molecular weight for ions?
Yes, you can calculate the "formula mass" for ions. For example, for the sulfate ion (SO₄²⁻), you would calculate the mass of one sulfur atom plus four oxygen atoms. The charge doesn't affect the mass calculation itself.
Q3: What if the chemical formula has parentheses, like Ca(OH)₂?
The subscript outside the parentheses multiplies all atoms inside. For Ca(OH)₂, you have 1 Calcium (Ca) atom, 2 Oxygen (O) atoms (1 inside * 2 outside), and 2 Hydrogen (H) atoms (1 inside * 2 outside). So, the calculation would be Ca + 2*O + 2*H. Our calculator handles these standard notations.
Q4: Why are the atomic masses on the periodic table often decimal numbers?
Atomic masses are typically weighted averages of the masses of all naturally occurring isotopes of an element. Since isotopes have different numbers of neutrons, their masses vary slightly. The decimal value reflects the average mass considering the relative abundance of each isotope.
Q5: What units should I use for atomic masses?
For molecular weight calculations, atomic masses are used in grams per mole (g/mol). Numerically, this value is very close to the atomic mass unit (amu), but g/mol is the correct unit for molar mass.
Q6: Does the calculator handle common chemical notations like hydrates (e.g., CuSO₄·5H₂O)?
Yes, our calculator is designed to interpret common notations. For hydrates, you can often enter them as CuSO4.5H2O or CuSO4*5H2O. The calculator will parse this to include the mass of 5 water molecules.
Q7: Can I use this calculator for mixtures?
No, this calculator is designed for individual chemical compounds or ions. For mixtures, you would need to calculate the molecular weight of each component separately and then use them in mixture calculations based on their proportions.
Q8: How accurate are the results?
The accuracy depends on the precision of the atomic masses used. The calculator utilizes standard, highly accurate atomic masses. If you input custom atomic masses, the accuracy of your input determines the output accuracy. For most general chemistry purposes, the results are more than sufficient.
Related Tools and Internal Resources
Stoichiometry Basics ExplainedLearn how molecular weight is fundamental to understanding chemical reactions and their proportions.
Molarity CalculatorUse molecular weight to prepare solutions of precise concentrations.
Understanding the Periodic TableExplore element properties, including atomic masses, essential for molecular weight calculations.
Empirical Formula CalculatorDetermine the simplest whole-number ratio of elements in a compound, often derived from percentage composition.
Types of Chemical BondsUnderstand how atoms join to form molecules, influencing molecular structure and weight.
Chemistry GlossaryFind definitions for key terms like mole, atom, molecule, and isotope.
var defaultAtomicMasses = {
"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, "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": 72.630, "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, "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
};
function parseAtomicMasses(massesString) {
var customMasses = {};
if (!massesString || massesString.trim() === "") {
return defaultAtomicMasses;
}
var lines = massesString.split('\n');
for (var i = 0; i 0) {
customMasses[element] = mass;
} else {
throw new Error("Invalid mass value for " + element);
}
} else {
throw new Error("Invalid format in custom masses line: " + line);
}
}
}
// Merge custom with defaults, ensuring custom overrides
var finalMasses = Object.assign({}, defaultAtomicMasses, customMasses);
return finalMasses;
}
function getElementSymbol(formula, index) {
if (index >= formula.length) return null;
var symbol = formula[index];
if (index + 1 = 'a' && formula[index + 1] <= 'z') {
symbol += formula[index + 1];
}
return symbol.toUpperCase();
}
function parseChemicalFormula(formula, atomicMasses) {
var elements = {};
var totalMass = 0;
var i = 0;
var formulaUpper = formula.toUpperCase().replace(/[\s.]/g, ''); // Remove spaces and periods
while (i < formulaUpper.length) {
var elementSymbol = getElementSymbol(formulaUpper, i);
if (!elementSymbol) break;
if (!atomicMasses.hasOwnProperty(elementSymbol)) {
throw new Error("Unknown element symbol: " + elementSymbol);
}
i += elementSymbol.length;
var count = 1;
var countStr = "";
while (i 0) {
count = parseInt(countStr);
}
if (elements[elementSymbol]) {
elements[elementSymbol].count += count;
} else {
elements[elementSymbol] = {
atomicMass: atomicMasses[elementSymbol],
count: count
};
}
}
// Handle multiplication for parentheses (e.g., Ca(OH)2)
var finalElements = {};
for (var symbol in elements) {
if (elements.hasOwnProperty(symbol)) {
finalElements[symbol] = elements[symbol];
}
}
var processedFormula = formulaUpper;
var parenthesisRegex = /\((\w+)\)(\d+)/g;
var match;
while ((match = parenthesisRegex.exec(processedFormula)) !== null) {
var innerElements = {};
var innerFormula = match[1];
var multiplier = parseInt(match[2]);
var j = 0;
while (j < innerFormula.length) {
var innerSymbol = getElementSymbol(innerFormula, j);
if (!innerSymbol) break;
j += innerSymbol.length;
var innerCount = 1;
var innerCountStr = "";
while (j 0) {
innerCount = parseInt(innerCountStr);
}
var actualCount = innerCount * multiplier;
if (finalElements[innerSymbol]) {
finalElements[innerSymbol].count += actualCount;
} else {
if (!atomicMasses.hasOwnProperty(innerSymbol)) {
throw new Error("Unknown element symbol within parenthesis: " + innerSymbol);
}
finalElements[innerSymbol] = {
atomicMass: atomicMasses[innerSymbol],
count: actualCount
};
}
}
}
// Remove elements that were part of parenthesis expansion if they appear standalone outside too
// (This is a simplification, a more robust parser would be needed for complex cases)
// For now, assume the primary loop handles the main elements and parenthesis loop adds multipliers
for (var symbol in finalElements) {
if (finalElements.hasOwnProperty(symbol)) {
totalMass += finalElements[symbol].atomicMass * finalElements[symbol].count;
}
}
return { elements: finalElements, totalMass: totalMass };
}
function updateChart(elementsData) {
var canvas = document.getElementById('molecularWeightChart');
var ctx = canvas.getContext('2d');
ctx.clearRect(0, 0, canvas.width, canvas.height); // Clear previous drawing
var labels = [];
var dataValues = [];
var backgroundColors = [];
var totalMolecularWeight = 0;
var colorPalette = [
'rgba(54, 162, 235, 0.6)', // Blue
'rgba(255, 99, 132, 0.6)', // Red
'rgba(75, 192, 192, 0.6)', // Green
'rgba(255, 206, 86, 0.6)', // Yellow
'rgba(153, 102, 255, 0.6)', // Purple
'rgba(255, 159, 64, 0.6)', // Orange
'rgba(70, 130, 180, 0.6)', // Steel Blue
'rgba(34, 139, 34, 0.6)', // Forest Green
'rgba(128, 0, 128, 0.6)', // Purple
'rgba(218, 112, 214, 0.6)' // Orchid
];
var colorIndex = 0;
for (var symbol in elementsData) {
if (elementsData.hasOwnProperty(symbol)) {
var element = elementsData[symbol];
var elementTotalMass = element.atomicMass * element.count;
labels.push(symbol);
dataValues.push(elementTotalMass);
backgroundColors.push(colorPalette[colorIndex % colorPalette.length]);
totalMolecularWeight += elementTotalMass;
colorIndex++;
}
}
if (dataValues.length === 0) return;
var chartWidth = canvas.clientWidth;
var chartHeight = canvas.clientHeight;
var centerX = chartWidth / 2;
var centerY = chartHeight / 2;
var radius = Math.min(chartWidth, chartHeight) / 2 * 0.8; // 80% of half the smaller dimension
var totalAngle = 2 * Math.PI;
var startAngle = 0;
// Draw Pie Chart (using Canvas API directly for simplicity)
ctx.lineWidth = 1;
ctx.strokeStyle = '#fff'; // White border for slices
for (var i = 0; i < dataValues.length; i++) {
var sliceAngle = (dataValues[i] / totalMolecularWeight) * totalAngle;
ctx.fillStyle = backgroundColors[i];
ctx.beginPath();
ctx.moveTo(centerX, centerY);
ctx.arc(centerX, centerY, radius, startAngle, startAngle + sliceAngle);
ctx.closePath();
ctx.fill();
ctx.stroke();
startAngle += sliceAngle;
}
// Add legend
var legendHtml = '';
colorIndex = 0;
for (var symbol in elementsData) {
if (elementsData.hasOwnProperty(symbol)) {
var element = elementsData[symbol];
var elementTotalMass = element.atomicMass * element.count;
legendHtml += ' ' + symbol + ' (' + element.count + ' atoms): ' + elementTotalMass.toFixed(3) + ' g/mol';
colorIndex++;
}
}
legendHtml = 'Total Molecular Weight: ' + totalMolecularWeight.toFixed(3) + ' g/mol' + legendHtml;
document.getElementById('legendTotalMass').innerHTML = 'Total: ' + totalMolecularWeight.toFixed(3) + ' g/mol';
document.getElementById('legendElementMasses').innerHTML = legendHtml;
}
function calculateMolecularWeight() {
var formulaInput = document.getElementById("chemicalFormula");
var formula = formulaInput.value.trim();
var atomicMassesInput = document.getElementById("atomicMasses");
var atomicMassesString = atomicMassesInput.value.trim();
// Clear previous errors
document.getElementById("chemicalFormulaError").style.display = "none";
document.getElementById("atomicMassesError").style.display = "none";
document.getElementById("result").style.display = "none";
document.getElementById("calculationTable").style.display = "none";
document.getElementById("chartContainer").style.display = "none";
if (formula === "") {
document.getElementById("chemicalFormulaError").textContent = "Please enter a chemical formula.";
document.getElementById("chemicalFormulaError").style.display = "block";
return;
}
var selectedAtomicMasses;
try {
selectedAtomicMasses = parseAtomicMasses(atomicMassesString);
} catch (e) {
document.getElementById("atomicMassesError").textContent = "Error parsing custom atomic masses: " + e.message;
document.getElementById("atomicMassesError").style.display = "block";
return;
}
var parseResult;
try {
parseResult = parseChemicalFormula(formula, selectedAtomicMasses);
} catch (e) {
document.getElementById("chemicalFormulaError").textContent = "Error parsing formula: " + e.message;
document.getElementById("chemicalFormulaError").style.display = "block";
return;
}
var molecularWeight = parseResult.totalMass;
var elementsData = parseResult.elements;
// Display Main Result
document.getElementById("molecularWeightResult").textContent = molecularWeight.toFixed(3) + " g/mol";
document.getElementById("totalAtomicMassSum").innerHTML = "Total Atomic Mass Sum: " + molecularWeight.toFixed(3) + " g/mol";
// Display Element Counts and Interpretation
var elementCountsHtml = "Element Counts: ";
var formulaInterpretationHtml = "Formula Interpretation: ";
var tableBody = document.querySelector("#calculationTable tbody");
tableBody.innerHTML = "; // Clear previous table rows
var elementList = [];
for (var symbol in elementsData) {
if (elementsData.hasOwnProperty(symbol)) {
elementList.push(symbol);
}
}
elementList.sort(); // Sort elements alphabetically for consistent display
var interpretationParts = [];
for (var i = 0; i 1 ? "s" : ""));
// Populate Table
var row = tableBody.insertRow();
row.insertCell(0).textContent = symbol;
row.insertCell(1).textContent = atomicMass.toFixed(3);
row.insertCell(2).textContent = count;
row.insertCell(3).textContent = totalElementMass.toFixed(3);
}
elementCountsHtml = elementCountsHtml.slice(0, -2); // Remove trailing comma and space
formulaInterpretationHtml += interpretationParts.join(", ") + ".";
document.getElementById("elementCounts").innerHTML = elementCountsHtml;
document.getElementById("formulaInterpretation").textContent = formulaInterpretationHtml;
// Show results section
document.getElementById("result").style.display = "block";
document.getElementById("calculationTable").style.display = "table";
document.getElementById("chartContainer").style.display = "block";
// Update Chart
updateChart(elementsData);
}
function resetCalculator() {
document.getElementById("chemicalFormula").value = "H2O";
document.getElementById("atomicMasses").value = "";
document.getElementById("chemicalFormulaError").style.display = "none";
document.getElementById("atomicMassesError").style.display = "none";
document.getElementById("result").style.display = "none";
document.getElementById("calculationTable").style.display = "none";
document.getElementById("chartContainer").style.display = "none";
// Optionally, trigger calculation after reset
// calculateMolecularWeight();
}
// Initial calculation on page load with default values
document.addEventListener('DOMContentLoaded', function() {
resetCalculator(); // Set defaults
calculateMolecularWeight(); // Perform initial calc
});