Accurately determine the molecular weight of any chemical compound.
Calculate Molecular Weight
Enter the chemical formula below. You can specify the count of each element using subscripts (e.g., H2O for water, C6H12O6 for glucose). For complex compounds, list each element and its count.
Use standard element symbols and numbers for counts (e.g., Ca2+ or SO4^2- are not supported for detailed calculation, only the atomic composition).
{/* Dynamically generated input fields for elements will go here */}
Molecular Weight
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Molecular Weight (MW) is calculated by summing the atomic masses of all atoms in a molecule.
MW = Σ (Number of Atoms of Element × Atomic Mass of Element)
What is Chemistry Molecular Weight?
The chemistry molecular weight calculator is a fundamental tool in chemistry, used to determine the mass of a single molecule of a substance. It's also often referred to as molar mass, expressed in grams per mole (g/mol), which is numerically equivalent to molecular weight in atomic mass units (amu). Understanding molecular weight is crucial for various chemical calculations, including stoichiometry, solution preparation, and reaction analysis. This calculator simplifies the process of obtaining this vital value.
Who should use it?
Students learning chemistry
Researchers in academic or industrial labs
Chemists and chemical engineers
Anyone performing chemical calculations
Common Misconceptions:
Confusing molecular weight with atomic weight: Atomic weight refers to a single atom of an element, while molecular weight refers to a compound formed by multiple atoms.
Assuming molecular weight is always a whole number: Atomic masses are often not whole numbers due to isotopes and precise measurements.
Using molecular weight interchangeably with molar mass without context: While numerically similar (amu vs. g/mol), the units and precise definitions differ.
Chemistry Molecular Weight Formula and Mathematical Explanation
The calculation of molecular weight for a chemical compound relies on the precise atomic masses of the constituent elements and the number of atoms of each element present in the molecule. The formula is straightforward but requires accurate data.
Formula:
Molecular Weight (MW) = Σ (Number of Atoms of Element × Atomic Mass of Element)
Step-by-step derivation:
Identify all the unique elements present in the chemical formula.
Determine the number of atoms of each element from the chemical formula (subscripts).
Find the standard atomic mass for each element from the periodic table. These values are typically given in atomic mass units (amu).
For each element, multiply its number of atoms by its atomic mass.
Sum the results obtained in step 4 for all elements to get the total molecular weight of the compound.
Variable Explanations:
Number of Atoms of Element: This is the count of a specific element in one molecule of the compound, as indicated by the subscript following the element's symbol in the chemical formula. If no subscript is present, it is assumed to be 1.
Atomic Mass of Element: This is the average mass of atoms of an element, calculated using the relative abundance of isotopes. It is typically found on the periodic table and is expressed in atomic mass units (amu).
Variables Table
Variable
Meaning
Unit
Typical Range (Approx.)
Number of Atoms of Element
Count of a specific element in a molecule
Unitless
1 to a large integer (e.g., 100+)
Atomic Mass of Element
Average mass of an element's atoms
amu (atomic mass units)
1.008 (H) to 294 (Og)
Molecular Weight (MW)
Total mass of a molecule
amu or g/mol
Varies widely, from simple molecules (e.g., H₂O ≈ 18 amu) to large biomolecules (e.g., proteins > 10,000 amu)
Result: The molecular weight of glucose (C₆H₁₂O₆) is approximately 180.156 amu.
Interpretation: A single molecule of glucose weighs about 180.156 amu. This is a foundational value for understanding its role in metabolic pathways and for performing quantitative chemical analysis related to carbohydrates.
How to Use This Chemistry Molecular Weight Calculator
Enter the Chemical Formula: In the "Chemical Formula" input field, type the exact chemical formula of the compound you want to analyze. Use standard element symbols (e.g., H, O, C, Na, Cl) and follow them immediately with the number of atoms of that element if it's greater than one (e.g., H2O for water, C6H12O6 for glucose). If an element appears only once, no number is needed after its symbol.
Parse Formula (Optional but Recommended): After entering the formula, click the "Parse Formula" button if available (or it may auto-parse). This step helps to break down the formula into its constituent elements and their counts, displaying them in separate fields. If the formula is complex or ambiguous, manually inputting the element counts might be necessary.
Verify Element Counts: Review the list of elements and their corresponding atom counts that appear. Ensure they accurately reflect the chemical formula you entered.
Calculate: Click the "Calculate" button. The calculator will use a built-in database of atomic masses to compute the total molecular weight.
Read the Results: The primary result, the "Molecular Weight," will be displayed prominently. You will also see intermediate values such as the elemental composition breakdown, the total count of atoms, and the sum of atomic masses before final calculation.
Understand the Formula: A brief explanation of the molecular weight formula is provided below the results to clarify how the value was obtained.
Copy Results: If you need to use the calculated values elsewhere, click the "Copy Results" button. This will copy the main molecular weight, intermediate values, and key assumptions (like the atomic masses used) to your clipboard.
Reset: To start over with a new calculation, click the "Reset" button. This will clear all fields and reset the calculator to its default state.
Decision-Making Guidance:
Accuracy Check: Always double-check the chemical formula you enter. A single incorrect symbol or number can lead to a significantly different molecular weight.
Units: Remember that the result is typically in atomic mass units (amu). For molar calculations in practical chemistry, this value is used directly as grams per mole (g/mol).
Complex Formulas: For very large or complex molecules (like polymers or proteins), this calculator provides an estimate based on the input formula. Exact masses might require more specialized tools or experimental data.
Key Factors That Affect Chemistry Molecular Weight Results
While the calculation of molecular weight itself is deterministic based on a given chemical formula, several factors and considerations influence the accuracy and interpretation of the results:
Accuracy of Atomic Masses: The primary factor is the precision of the atomic masses used for each element. Standard atomic weights from the periodic table are averages based on isotopic abundance and can vary slightly depending on the source or the level of precision required. For highly precise work, using more decimal places for atomic masses is crucial.
Isotopic Variations: Natural samples of elements are mixtures of isotopes, each with a different mass. The standard atomic weights are averages. If you are working with a sample enriched in a specific isotope (e.g., Deuterium instead of Hydrogen), the molecular weight will differ. Our calculator uses standard, naturally occurring atomic masses.
Chemical Formula Precision: The most common source of error is an incorrect chemical formula. Typos (e.g., 'CO' instead of 'CO2'), missed subscripts, or incorrect elemental symbols will lead to a wrong molecular weight. Always verify the formula against reliable chemical databases or literature.
Hydration/Solvation: Many compounds exist as hydrates (containing water molecules within their crystal structure, e.g., CuSO₄·5H₂O) or are dissolved in solvents. The molecular weight calculation should account for these associated molecules if they are considered part of the "entity" being weighed or reacted. Our calculator calculates the weight of the anhydrous compound unless the formula explicitly includes water (e.g., H2O).
Ionic Compounds vs. Molecular Compounds: Technically, ionic compounds (like NaCl) don't form discrete molecules but rather crystal lattices. The term "formula weight" is more appropriate for them, representing the sum of atomic weights in the empirical formula unit. For practical purposes in stoichiometry, it's often calculated similarly to molecular weight. This calculator computes the formula weight for ionic compounds.
Mixtures and Impurities: A calculated molecular weight applies to a pure substance. Real-world samples often contain impurities. The measured mass of an impure substance will not match the calculated molecular weight of the pure compound. Understanding the purity of your sample is key to interpreting experimental results involving mass.
Significant Figures: The number of significant figures in the atomic masses used will dictate the number of significant figures in the final molecular weight. Using atomic masses with more significant figures allows for a more precise molecular weight calculation, which is important in high-precision analytical chemistry.
Atomic Mass Distribution of Elements in a Compound
Element Mass Contribution Cumulative Mass Contribution
Q1: What is the difference between molecular weight and molar mass?
A: Molecular weight is the mass of a single molecule in atomic mass units (amu). 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 the same for a given compound (e.g., H₂O has a molecular weight of ~18.015 amu and a molar mass of ~18.015 g/mol).
Q2: Can this calculator handle ions (e.g., SO₄²⁻)?
A: This calculator is designed for neutral compounds based on their elemental composition. For ions, it will calculate the molecular weight of the neutral species (e.g., SO₄) based on the provided formula. It does not inherently calculate the mass of the ion including the charge contribution.
Q3: What if the chemical formula includes parentheses, like Ca(OH)₂?
A: The current version of the calculator expects a flattened formula (e.g., Ca(OH)₂ should be entered as Ca O2 H2). The input parsing is designed for simpler direct element-count entries. For formulas with parentheses, you need to distribute the subscript outside the parenthesis to each element inside.
Q4: How accurate are the atomic masses used?
A: The calculator uses standard, internationally accepted atomic masses from the periodic table, typically with several decimal places for accuracy. These are averages reflecting natural isotopic abundance.
Q5: Can I calculate the molecular weight of a mixture?
A: This calculator is for pure chemical compounds. To find the average molecular weight of a mixture, you would need to know the composition (mole fractions or mass fractions) of each component and calculate a weighted average.
Q6: What does "amu" stand for?
A: amu stands for atomic mass unit. It is a standard unit of mass used to express the mass of atoms and molecules. 1 amu is approximately 1.660539 x 10⁻²⁷ kilograms.
Q7: How do I find the atomic mass of an element?
A: The atomic mass of an element can be found on the periodic table, usually listed below the element's symbol. For most practical chemistry, the standard atomic weight is used.
Q8: What if I enter an element not found on the periodic table?
A: The calculator will attempt to find a value. If an unrecognized element symbol is entered, it might default to a value of 0 or trigger an error, depending on implementation. It's best to use correct, recognized element symbols.
Calculate properties of gases using the Ideal Gas Law (PV=nRT), where the number of moles (n) is directly related to mass and molecular weight.
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": 63.546, "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": 95.95, "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,
"Np": 237, "Pu": 244, "Am": 243, "Cm": 247, "Bk": 247, "Cf": 251, "Es": 252, "Fm": 257, "Md": 258, "No": 259, "Lr": 266, "Rf": 267, "Db": 268,
"Sg": 269, "Bh": 270, "Hs": 269, "Mt": 278, "Ds": 281, "Rg": 280, "Cn": 285, "Nh": 286, "Fl": 289, "Mc": 290, "Lv": 293, "Ts": 294, "Og": 294
};
var currentElements = {};
var chartInstance = null;
function parseChemicalFormula() {
var formulaInput = document.getElementById("chemicalFormula");
var formula = formulaInput.value.trim();
var errors = {
"chemicalFormula": ""
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currentElements = {};
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errors.chemicalFormula = "Chemical formula cannot be empty.";
document.getElementById("chemicalFormulaError").innerText = errors.chemicalFormula;
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// Regex to find elements and their counts. Handles elements like 'Ca', 'Cl', 'H2', 'O12'
// It captures an uppercase letter optionally followed by a lowercase letter (element symbol)
// and then optionally followed by digits (count).
var regex = /([A-Z][a-z]?)(\d*)/g;
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var countStr = match[2];
var count = countStr === "" ? 1 : parseInt(countStr, 10);
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errors.chemicalFormula = "Invalid count for element " + element + ". Counts must be positive integers.";
isValidFormula = false;
break;
}
if (!atomicMasses.hasOwnProperty(element)) {
errors.chemicalFormula = "Unknown element symbol: " + element + ". Please use standard periodic table symbols.";
isValidFormula = false;
break;
}
if (currentElements[element]) {
currentElements[element] += count;
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currentElements[element] = count;
}
}
// Check if the entire formula was parsed correctly
var parsedLength = 0;
for(var i = 0; i < formula.length; ) {
var elementMatch = regex.exec(formula.substring(i));
if (elementMatch && elementMatch.index === i) {
parsedLength += elementMatch[0].length;
i += elementMatch[0].length;
} else {
errors.chemicalFormula = "Formula contains unrecognized characters or format.";
isValidFormula = false;
break;
}
}
if (parsedLength !== formula.length) {
errors.chemicalFormula = "Could not fully parse the chemical formula. Check format.";
isValidFormula = false;
}
if (!isValidFormula) {
document.getElementById("chemicalFormulaError").innerText = errors.chemicalFormula;
return false;
}
// Create input fields for each unique element found
var sortedElements = Object.keys(currentElements).sort();
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div.className = "input-group";
var label = document.createElement("label");
label.htmlFor = "elementCount_" + element;
label.innerText = element + " (Atomic Mass: " + atomicMasses[element].toFixed(3) + " amu)";
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input.id = "elementCount_" + element;
input.value = currentElements[element];
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// Update original formula based on parsed elements if parsing was successful
if (isValidFormula) {
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sortedElements.forEach(function(el) {
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var errorElement = document.getElementById("elementCountError_" + elementId);
errorElement.innerText = "";
if (value === "") {
// Allow empty for calculation reset, but mark as error if not reset
return;
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var numValue = parseFloat(value);
if (isNaN(numValue)) {
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return false;
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errorElement.innerText = "Count cannot be negative.";
return false;
}
// Update internalElements count
currentElements[elementId] = numValue;
return true;
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var formulaInput = document.getElementById("chemicalFormula");
var formula = formulaInput.value.trim();
var formulaError = document.getElementById("chemicalFormulaError");
formulaError.innerText = "";
if (formula.length === 0) {
formulaError.innerText = "Please enter a chemical formula.";
return;
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if (!parseChemicalFormula()) {
// parseChemicalFormula already displays error
return;
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var totalWeight = 0;
var elementalCompositionHtml = "
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var totalAtoms = 0;
var massSum = 0;
for (var element in currentElements) {
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var count = parseFloat(document.getElementById("elementCount_" + element).value);
var atomicMass = atomicMasses[element];
if (isNaN(count) || count < 0) { // Recalculate validation if input was manually cleared after parse
document.getElementById("elementCountError_" + element).innerText = "Invalid count.";
return;
}
var elementWeight = count * atomicMass;
totalWeight += elementWeight;
elementalCompositionHtml += "