Accurately calculate the molecular weight of chemical compounds with our easy-to-use tool and comprehensive guide.
Molecular Weight Calculator
Enter the chemical formula (e.g., H2O, CO2, C6H12O6). Use standard element symbols and numbers for subscripts.
Calculation Results
Total Atoms:
Unique Elements:
Sum of Atomic Masses:
Formula Used:Molecular Weight (MW) = Σ (Number of Atoms of Element × Atomic Mass of Element)
Assumptions
Atomic Mass Data Source:Standard Atomic Weights (IUPAC)
Elemental Composition Breakdown
Elemental Atomic Masses
Atomic Masses of Elements in the Formula
Element
Symbol
Atomic Mass (u)
Count in Formula
Contribution (u)
What is Molecular Weight?
Molecular weight, often abbreviated as MW or M, is a fundamental property of a chemical compound. It represents the sum of the atomic weights of all atoms in a single molecule of that compound. This value is typically expressed in atomic mass units (u), where 1 u is approximately equal to the mass of a single proton or neutron. Understanding molecular weight is crucial in chemistry for stoichiometric calculations, determining molar mass, and predicting physical properties of substances.
Who should use it? This calculator and the concept of molecular weight are primarily used by students, researchers, chemists, pharmacists, and anyone involved in chemical analysis, synthesis, or formulation. It's essential for tasks like calculating reactant quantities in chemical reactions, determining the concentration of solutions, and characterizing unknown compounds.
Common Misconceptions:
Molecular Weight vs. Molar Mass: While numerically very similar, molecular weight is the mass of a single molecule (in u), whereas molar mass is the mass of one mole of a substance (in g/mol). For practical calculations, they are often used interchangeably.
Isotopes: The atomic weights used are typically the weighted averages of the naturally occurring isotopes of an element. Calculations using specific isotopes would require different values.
Formula Accuracy: The accuracy of the molecular weight calculation directly depends on the correctness of the chemical formula provided.
Molecular Weight Formula and Mathematical Explanation
The molecular weight of a compound is calculated by summing the atomic masses of each atom present in its molecular formula. Each element's contribution is determined by multiplying the number of atoms of that element by its average atomic mass.
Step-by-step derivation:
Identify all the unique elements present in the chemical formula.
Determine the number of atoms for each element in the molecule (indicated by subscripts in the formula).
Find the average atomic mass for each element from a reliable source (like the periodic table or IUPAC data).
For each element, multiply its number of atoms by its atomic mass.
Sum up the results from step 4 for all elements in the molecule.
Variables Explained:
Number of Atoms of Element: The count of a specific element's atoms within one molecule.
Atomic Mass of Element: The average mass of an atom of that element, expressed in atomic mass units (u).
Variables Table:
Molecular Weight Calculation Variables
Variable
Meaning
Unit
Typical Range
Ni
Number of atoms of element 'i'
Count (unitless)
1 to hundreds
AMUi
Average Atomic Mass of element 'i'
u (atomic mass units)
~1.008 (H) to >200 (Uup)
MW
Molecular Weight
u
Varies widely depending on compound
Practical Examples (Real-World Use Cases)
Understanding molecular weight calculations is vital across many scientific disciplines. Here are a couple of practical examples:
Example 1: Water (H₂O)
Inputs:
Chemical Formula: H2O
Calculation Steps:
Elements: Hydrogen (H), Oxygen (O)
Atomic Masses: H ≈ 1.008 u, O ≈ 15.999 u
Counts: H = 2, O = 1
Contributions: (2 × 1.008 u) + (1 × 15.999 u)
Outputs:
Total Atoms: 3
Unique Elements: 2
Sum of Atomic Masses: 18.015 u
Molecular Weight: 18.015 u
Interpretation: This means a single molecule of water has a mass of approximately 18.015 atomic mass units. This value is essential for calculating the molar mass of water (18.015 g/mol), which is used in nearly all quantitative chemical analyses involving water.
Example 2: Glucose (C₆H₁₂O₆)
Inputs:
Chemical Formula: C6H12O6
Calculation Steps:
Elements: Carbon (C), Hydrogen (H), Oxygen (O)
Atomic Masses: C ≈ 12.011 u, H ≈ 1.008 u, O ≈ 15.999 u
Sum of Atomic Masses: 72.066 u + 12.096 u + 95.994 u = 180.156 u
Molecular Weight: 180.156 u
Interpretation: A single molecule of glucose weighs approximately 180.156 u. This is critical in biology and biochemistry for understanding carbohydrate metabolism, as glucose is a primary energy source. Its molar mass (180.156 g/mol) is used to calculate how much glucose is present in biological fluids or consumed in metabolic processes.
How to Use This Molecular Weight Calculator
Our Molecular Weight Calculator is designed for simplicity and accuracy. Follow these steps to get your results:
Enter the Chemical Formula: In the "Chemical Formula" input field, type the formula of the compound you want to analyze. Use standard element symbols (e.g., H, O, C, Na, Cl) followed immediately by their subscript number if greater than 1. For example, for water, type H2O; for sulfuric acid, type H2SO4; for glucose, type C6H12O6. Ensure there are no spaces within the formula itself.
Click "Calculate Molecular Weight": Once you've entered the formula, click the "Calculate Molecular Weight" button.
View Results: The calculator will immediately display:
Molecular Weight: The primary result, shown in large, bold font.
Total Atoms: The total count of all atoms in the molecule.
Unique Elements: The number of different elements present.
Sum of Atomic Masses: The preliminary sum before multiplication by counts, useful for understanding intermediate steps.
Elemental Breakdown Table: A detailed table showing each element, its atomic mass, its count in the formula, and its contribution to the total molecular weight.
Chart: A visual representation of the elemental composition by mass.
Copy Results: Use the "Copy Results" button to copy all calculated values and key assumptions to your clipboard for easy pasting into reports or documents.
Reset: To clear the current calculation and start over, click the "Reset" button. It will restore the input field to a default state.
Decision-Making Guidance: The calculated molecular weight helps determine the molar mass, which is essential for any quantitative chemical work. For instance, if you need to prepare a specific molar concentration of a solution, you'll use the molar mass (derived from molecular weight) to weigh out the correct amount of solute.
Key Factors That Affect Molecular Weight Results
While the calculation itself is straightforward, several factors influence its interpretation and application:
Accuracy of the Chemical Formula: This is the most critical factor. An incorrect formula (e.g., H2O2 instead of H2O) will lead to a completely wrong molecular weight. Double-check the formula for typos or errors.
Atomic Mass Values Used: The calculator uses standard atomic weights from IUPAC. These are weighted averages of naturally occurring isotopes. If you need calculations based on specific isotopic compositions (e.g., for mass spectrometry), you would need to use precise isotopic masses, not average atomic weights.
Isotopes and Natural Abundance: The standard atomic weights reflect the average isotopic composition found on Earth. For elements with significant isotopic variations, the calculated molecular weight might slightly differ from samples with unusual isotopic ratios.
Hydration or Solvation: For compounds that exist in hydrated forms (e.g., CuSO₄·5H₂O), the molecular weight calculation needs to include the mass of the water molecules. Our calculator expects the precise formula including hydrates.
Units of Measurement: Ensure consistency. Atomic masses are in atomic mass units (u), and the resulting molecular weight is also in u. When converting to molar mass for laboratory use, remember it becomes grams per mole (g/mol), which is numerically equivalent.
Complexity of the Molecule: Very large or complex molecules (like proteins or polymers) can have extremely high molecular weights, and their formulas can be very long. The calculator is designed for common chemical formulas; extremely large ones might require specialized software.
Ionic Compounds: Strictly speaking, ionic compounds don't form discrete molecules but rather crystal lattices. What is calculated is the formula weight, which serves the same purpose for stoichiometric calculations as molecular weight does for covalent compounds.
Precision of Atomic Masses: While standard atomic weights are highly accurate, using more decimal places can increase the precision of the final molecular weight, especially for complex molecules. Our calculator uses commonly accepted precise values.
Frequently Asked Questions (FAQ)
What is the difference between molecular weight and molar mass?
Molecular weight is the mass of a single molecule in atomic mass units (u). Molar mass is the mass of one mole (approximately 6.022 x 10^23 particles) of a substance, expressed in grams per mole (g/mol). Numerically, they are often the same, but conceptually they refer to different scales.
Can this calculator handle chemical formulas with parentheses, like (NH₄)₂SO₄?
Yes, the calculator is designed to interpret standard chemical notation, including parentheses for polyatomic ions. For (NH₄)₂SO₄, it correctly calculates the counts for Nitrogen, Hydrogen, Sulfur, and Oxygen.
What atomic mass values does the calculator use?
The calculator uses standard atomic weights, which are the weighted averages of the naturally occurring isotopes of each element, as published by the International Union of Pure and Applied Chemistry (IUPAC).
How accurate is the molecular weight calculation?
The accuracy depends on the precision of the atomic mass values used and the correctness of the input chemical formula. The values used are highly precise, making the calculation very accurate for standard compounds.
What if I enter an invalid chemical formula?
The calculator will attempt to parse the input. If it encounters unrecognized symbols or an ambiguous structure, it may return an error or an unexpected result. It's best to use standard chemical notation.
Can this calculator determine the molecular formula from just the elements?
No, this calculator requires a specific chemical formula as input. It calculates the molecular weight based on the formula provided. To determine a molecular formula, you often need additional information like the empirical formula and the compound's molar mass.
What does the "Sum of Atomic Masses" value represent?
This is an intermediate step showing the sum of the atomic masses of each element multiplied by its count. It's essentially the final molecular weight before rounding or final summation, representing the total mass contribution from each element type in the molecule.
Why is molecular weight important in drug discovery or materials science?
Molecular weight influences properties like solubility, diffusion rates, viscosity, and how molecules interact with biological systems or form structures. It's a critical parameter for designing new drugs, polymers, and materials with specific functionalities.
// Data for atomic masses (simplified, common elements)
var atomicMasses = {
"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": 63.55, "Zn": 65.38,
"Br": 79.904, "I": 126.90, "Au": 196.97, "Ag": 107.87, "Pb": 207.2, "Hg": 200.59, "U": 238.03
// Add more elements as needed
};
var chartInstance = null;
function parseChemicalFormula(formula) {
var elements = {};
var totalAtoms = 0;
var formulaError = null;
var currentElement = ";
var currentNumber = ";
var i = 0;
formula = formula.replace(/\s+/g, "); // Remove all whitespace
while (i < formula.length) {
var char = formula[i];
if (char === '(') {
// Handle parentheses – find closing parenthesis and recursively parse
var balance = 1;
var j = i + 1;
while (j 0) {
if (formula[j] === '(') balance++;
if (formula[j] === ')') balance–;
j++;
}
if (balance !== 0) {
formulaError = "Mismatched parentheses.";
break;
}
var subFormula = formula.substring(i + 1, j – 1);
var multiplierStr = ";
var k = j;
while (k < formula.length && !isNaN(parseInt(formula[k]))) {
multiplierStr += formula[k];
k++;
}
var multiplier = multiplierStr === '' ? 1 : parseInt(multiplierStr);
if (isNaN(multiplier)) {
formulaError = "Invalid multiplier after parentheses.";
break;
}
var subElements = parseChemicalFormula(subFormula);
if (subElements.error) {
formulaError = subElements.error;
break;
}
for (var elem in subElements.elements) {
var count = subElements.elements[elem] * multiplier;
elements[elem] = (elements[elem] || 0) + count;
totalAtoms += count;
}
i = k; // Move index past the multiplier
continue;
} else if (char === ')') {
formulaError = "Unexpected closing parenthesis.";
break;
} else if (char === char.toUpperCase() && char !== char.toLowerCase()) {
// It's an uppercase letter, potential start of an element symbol
if (currentElement !== '') {
// Previous element had no number, assume 1
elements[currentElement] = (elements[currentElement] || 0) + (currentNumber === '' ? 1 : parseInt(currentNumber));
if (isNaN(elements[currentElement])) {
formulaError = "Invalid count for " + currentElement;
break;
}
totalAtoms += (currentNumber === '' ? 1 : parseInt(currentNumber));
currentNumber = '';
}
currentElement = char;
i++;
// Check for a second lowercase letter (e.g., Cl, Br)
if (i 0) {
return { error: "Could not parse formula. Please check format.", elements: {}, totalAtoms: 0 };
}
return { error: null, elements: elements, totalAtoms: totalAtoms };
}
function calculateMolecularWeight() {
var formulaInput = document.getElementById("chemicalFormula");
var formula = formulaInput.value;
var formulaErrorDiv = document.getElementById("chemicalFormulaError");
var resultsContainer = document.getElementById("resultsContainer");
var elementTableBody = document.getElementById("elementTableBody");
var mwResultDiv = document.getElementById("molecularWeightResult");
var numAtomsValueSpan = document.getElementById("numAtomsValue");
var numElementsValueSpan = document.getElementById("numElementsValue");
var atomicMassSumValueSpan = document.getElementById("atomicMassSumValue");
var chartCanvas = document.getElementById("mwChart");
var chartLegendDiv = document.getElementById("chartLegend");
// Clear previous results and errors
formulaErrorDiv.textContent = ";
resultsContainer.style.display = 'none';
elementTableBody.innerHTML = ";
mwResultDiv.textContent = ";
numAtomsValueSpan.textContent = ";
numElementsValueSpan.textContent = ";
atomicMassSumValueSpan.textContent = ";
chartLegendDiv.innerHTML = ";
if (formula.trim() === "") {
formulaErrorDiv.textContent = "Please enter a chemical formula.";
return;
}
var parsed = parseChemicalFormula(formula);
if (parsed.error) {
formulaErrorDiv.textContent = parsed.error;
return;
}
var elements = parsed.elements;
var totalAtoms = parsed.totalAtoms;
var molecularWeight = 0;
var elementData = [];
var chartData = [];
var colors = ['#004a99', '#28a745', '#ffc107', '#dc3545', '#6f42c1', '#fd7e14', '#20c997', '#6610f2', '#e83e8c', '#007bff']; // Example colors
for (var elementSymbol in elements) {
if (!atomicMasses.hasOwnProperty(elementSymbol)) {
formulaErrorDiv.textContent = "Atomic mass data not available for element: " + elementSymbol;
return;
}
var atomicMass = atomicMasses[elementSymbol];
var count = elements[elementSymbol];
var contribution = atomicMass * count;
molecularWeight += contribution;
elementData.push({
element: elementSymbol,
atomicMass: atomicMass,
count: count,
contribution: contribution
});
chartData.push({
element: elementSymbol,
mass: contribution
});
}
// Sort elements by contribution for chart and table (optional, but good for visualization)
elementData.sort(function(a, b) {
return b.contribution – a.contribution;
});
chartData.sort(function(a, b) {
return b.mass – a.mass;
});
// Display Results
mwResultDiv.textContent = molecularWeight.toFixed(4) + " u";
numAtomsValueSpan.textContent = totalAtoms;
numElementsValueSpan.textContent = Object.keys(elements).length;
atomicMassSumValueSpan.textContent = molecularWeight.toFixed(4) + " u"; // This is the same as MW in this context
resultsContainer.style.display = 'flex';
// Populate Table
for (var i = 0; i < elementData.length; i++) {
var row = elementTableBody.insertRow();
var data = elementData[i];
row.insertCell(0).textContent = data.element; // Element Name (simplified to symbol here)
row.insertCell(1).textContent = data.element; // Symbol
row.insertCell(2).textContent = data.atomicMass.toFixed(4); // Atomic Mass
row.insertCell(3).textContent = data.count; // Count
row.insertCell(4).textContent = data.contribution.toFixed(4); // Contribution
}
// Draw Chart
if (chartInstance) {
chartInstance.destroy(); // Destroy previous chart instance if it exists
}
var ctx = chartCanvas.getContext('2d');
var totalMass = molecularWeight;
var chartLabels = [];
var chartValues = [];
var legendHtml = '
';
for (var i = 0; i < chartData.length; i++) {
var item = chartData[i];
chartLabels.push(item.element);
chartValues.push(item.mass);
var percentage = (item.mass / totalMass) * 100;
var color = colors[i % colors.length];
legendHtml += '
';
chartLegendDiv.innerHTML = legendHtml;
chartInstance = new Chart(ctx, {
type: 'pie', // or 'bar'
data: {
labels: chartLabels,
datasets: [{
label: 'Mass Contribution',
data: chartValues,
backgroundColor: colors.slice(0, chartData.length),
borderColor: '#fff',
borderWidth: 1
}]
},
options: {
responsive: true,
maintainAspectRatio: true,
plugins: {
legend: {
display: false // Legend is handled by custom HTML
},
tooltip: {
callbacks: {
label: function(context) {
var label = context.label || ";
if (label) {
label += ': ';
}
if (context.parsed !== null) {
var percentage = ((context.raw / totalMass) * 100).toFixed(1);
label += context.raw.toFixed(4) + ' u (' + percentage + '%)';
}
return label;
}
}
}
}
}
});
}
function resetCalculator() {
document.getElementById("chemicalFormula").value = "";
document.getElementById("chemicalFormulaError").textContent = ";
document.getElementById("resultsContainer").style.display = 'none';
document.getElementById("elementTableBody").innerHTML = ";
if (chartInstance) {
chartInstance.destroy();
chartInstance = null;
}
document.getElementById("mwChart").getContext('2d').clearRect(0,0, chartCanvas.width, chartCanvas.height);
document.getElementById("chartLegend").innerHTML = ";
}
function copyResults() {
var mainResult = document.getElementById("molecularWeightResult").textContent;
var totalAtoms = document.getElementById("numAtomsValue").textContent;
var numElements = document.getElementById("numElementsValue").textContent;
var atomicMassSum = document.getElementById("atomicMassSumValue").textContent;
var formula = document.getElementById("chemicalFormula").value;
var massSource = document.getElementById("atomicMassSourceValue").textContent;
var tableRows = document.querySelectorAll("#elementTableBody tr");
var tableData = [];
tableRows.forEach(function(row) {
var cells = row.cells;
tableData.push([
cells[0].textContent, cells[1].textContent, cells[2].textContent, cells[3].textContent, cells[4].textContent
].join("\t")); // Tab-separated for table
});
var tableString = "Element\tSymbol\tAtomic Mass (u)\tCount in Formula\tContribution (u)\n" + tableData.join("\n");
var textToCopy = "Molecular Weight Calculation Results for: " + formula + "\n\n";
textToCopy += "— Primary Results —\n";
textToCopy += "Molecular Weight: " + mainResult + "\n";
textToCopy += "Total Atoms: " + totalAtoms + "\n";
textToCopy += "Unique Elements: " + numElements + "\n";
textToCopy += "Sum of Atomic Masses: " + atomicMassSum + "\n\n";
textToCopy += "— Elemental Breakdown —\n";
textToCopy += tableString + "\n\n";
textToCopy += "— Assumptions —\n";
textToCopy += "Atomic Mass Data Source: " + massSource + "\n";
// Use a temporary textarea to copy
var tempTextArea = document.createElement("textarea");
tempTextArea.value = textToCopy;
tempTextArea.style.position = "fixed";
tempTextArea.style.left = "-9999px";
document.body.appendChild(tempTextArea);
tempTextArea.focus();
tempTextArea.select();
try {
var successful = document.execCommand('copy');
var msg = successful ? 'Results copied successfully!' : 'Failed to copy results.';
// Optionally show a temporary message to the user
// console.log(msg);
} catch (err) {
// console.error('Fallback: Oops, unable to copy', err);
}
document.body.removeChild(tempTextArea);
}
function toggleFaq(element) {
var faqItem = element.parentElement;
faqItem.classList.toggle('open');
}
// Add a listener for Enter key on the formula input
document.getElementById("chemicalFormula").addEventListener("keypress", function(event) {
if (event.key === "Enter") {
event.preventDefault(); // Prevent default form submission if inside a form
calculateMolecularWeight();
}
});
// Load Chart.js library dynamically or ensure it's included in your theme's header
// For a self-contained HTML file, you'd typically include it via CDN or local file
// Example CDN inclusion (add this line inside the ):
//
// NOTE: For this code to work, Chart.js must be available in the global scope.
// If running this in an environment where Chart.js isn't loaded, it will fail.
// Assuming Chart.js is available for the sake of this example.
// You would typically have:
//
// before this script block in the HTML .