Molecular Weight Calculator: Molar Mass from Chemical Formula

Table of Contents - Molecular Weight

How to Use This Calculator
The Core Principle: Summing Atomic Masses
How to Calculate Molecular Weight Manually
Real-World Applications
Scenarios People Actually Run Into
Trade-Offs and Decisions People Underestimate
Common Mistakes and How to Recover
Related Topics
How This Calculator Works
FAQs

How to Use This Calculator - Molecular Weight

Enter your Chemical Formula using standard notation:

Simple formulas: H2O, NaCl, CO2
Subscripts as numbers: H2SO4 (sulfuric acid)
Parentheses for groups: Ca(NO3)2 (calcium nitrate)
Hydrates with dot: CuSO4·5H2O (copper sulfate pentahydrate)

Case sensitivity matters: Co is cobalt, CO is carbon monoxide.

Click "Calculate" to see results. The output displays:

Total molecular weight in g/mol
Breakdown by element showing atom count, atomic mass, and contribution
Percent composition by mass for each element
Total atom count in the formula

The Core Principle: Summing Atomic Masses

Molecular weight (molar mass) is the sum of atomic weights of all atoms in a molecule. Each element has a characteristic atomic mass, measured in atomic mass units (amu) or equivalently g/mol.

The process is straightforward:

Identify each element in the formula
Count atoms of each element (including those in parentheses and hydrates)
Multiply each count by the element's atomic mass
Sum all contributions

For example, water (H₂O):

H: 2 atoms × 1.008 g/mol = 2.016 g/mol
O: 1 atom × 15.999 g/mol = 15.999 g/mol
Total: 18.015 g/mol

This value tells you: one mole of water molecules weighs 18.015 grams.

How to Calculate Molecular Weight Manually

Simple compound: NaCl (sodium chloride)

Na: 1 × 22.990 = 22.990
Cl: 1 × 35.453 = 35.453
Total: 58.443 g/mol

With subscripts: H₂SO₄ (sulfuric acid)

H: 2 × 1.008 = 2.016
S: 1 × 32.065 = 32.065
O: 4 × 15.999 = 63.996
Total: 98.077 g/mol

With parentheses: Ca(NO₃)₂ (calcium nitrate)

Ca: 1 × 40.078 = 40.078
N: 2 × 14.007 = 28.014 (2 because (NO₃)₂)
O: 6 × 15.999 = 95.994 (3 × 2 = 6)
Total: 164.086 g/mol

Hydrate: CuSO₄·5H₂O (copper sulfate pentahydrate) Anhydrous part:

Cu: 1 × 63.546 = 63.546
S: 1 × 32.065 = 32.065
O: 4 × 15.999 = 63.996

Hydrate part (5 × H₂O):

H: 10 × 1.008 = 10.08
O: 5 × 15.999 = 79.995

Total: 249.682 g/mol

Percent composition: % of element = (element mass contribution / total mass) × 100

For H₂O: % H = (2.016 / 18.015) × 100 = 11.19% % O = (15.999 / 18.015) × 100 = 88.81%

Real-World Applications

Solution preparation. To make 1L of 1M NaCl solution, weigh out 58.44 grams (1 mol) of NaCl and dissolve in water to 1L volume.

Stoichiometry calculations. Given 50g of reactant, how many moles? Divide by molecular weight. How many grams of product? Use molar ratios and molecular weights.

Empirical formula determination. Given percent composition, calculate moles of each element, find simplest ratio, and determine empirical formula.

Drug dosage. Pharmaceutical calculations often involve converting between mass and moles for precise dosing.

Chemical ordering. Suppliers sell by mass or moles. Knowing molecular weight converts between the two for accurate ordering.

Scenarios People Actually Run Into

The hydrate confusion. You weigh out 249.68g of "copper sulfate" expecting 1 mol, but the bottle contains the pentahydrate. You actually have 1 mol of the hydrate, not 1 mol of anhydrous CuSO₄.

The case sensitivity error. You type "CO" for cobalt, but the calculator reads it as carbon monoxide. Co (cobalt) = 58.93 g/mol; CO (carbon monoxide) = 28.01 g/mol.

The parenthesis parsing. Al₂(SO₄)₃ contains 2 Al, 3 S, and 12 O. Forgetting the subscript applies to everything inside parentheses throws off the calculation.

The isotope question. Standard atomic weights are averages based on natural isotope abundance. For isotope-specific work (like radioactive tracers), use exact isotope masses instead.

The percent composition check. You calculate 40% carbon in a compound, but your analysis shows 35%. Either the formula is wrong, the sample is impure, or there's a calculation error somewhere.

Trade-Offs and Decisions People Underestimate

Precision of atomic weights. Standard atomic weights are given to multiple decimal places. For most purposes, 2-3 significant figures suffice. For analytical work, more precision matters.

Hydrate versus anhydrous. Chemicals often exist in hydrated and anhydrous forms with very different molecular weights. Always check which form you have.

Natural abundance versus isotopic. Atomic weights reflect natural isotope mixtures. For enriched or depleted isotopes, standard weights don't apply.

Significant figures. Molecular weight precision is limited by the least precise atomic weight. Reporting too many digits implies false precision.

Notation variations. CuSO4.5H2O, CuSO₄·5H₂O, and copper sulfate pentahydrate all mean the same thing. Know your notation conventions.

Common Mistakes and How to Recover

Forgetting subscripts apply to parentheses contents. Ca(OH)₂ has 2 O and 2 H, not 1 of each. The subscript multiplies everything inside.

Confusing element symbols. Co (cobalt) vs CO (carbon monoxide). Pb (lead) vs P + B (phosphorus + boron). Attention to capitalization is critical.

Ignoring hydrate water. CuSO₄ and CuSO₄·5H₂O have very different molecular weights. Use the correct form.

Using atomic number instead of atomic mass. Carbon's atomic number is 6; its atomic mass is ~12.01. These are different values.

Rounding errors accumulating. Keep extra digits during intermediate calculations; round only the final answer.

How This Calculator Works

Formula parsing:

regex = /([A-Z][a-z]?)(\d*)/g

Matches element symbols (capital + optional lowercase) followed by optional count.

Parentheses handling: Recursively parse contents, multiply all atom counts by the subscript following the closing parenthesis.

Hydrate handling: Split on · or . character, parse each part separately, sum total masses.

Atomic weight lookup: Built-in table of IUPAC standard atomic weights for all elements.

Mass calculation:

for each element:
  contribution = atomCount × atomicWeight
  totalMass += contribution

Percent composition:

percentage = (elementContribution / totalMass) × 100

All calculations happen locally in your browser.

FAQs

How do I enter subscripts?

Just type the number after the element symbol: H2O, not H₂O. The calculator interprets numbers as subscripts.

How do I enter hydrates?

Use · or . between the anhydrous compound and water: CuSO4·5H2O or CuSO4.5H2O.

Why does case matter?

CO is carbon monoxide (C + O). Co is cobalt. Correct capitalization is essential for element identification.

What atomic weights does this use?

Standard IUPAC values based on natural isotope abundance. For isotope-specific calculations, use exact isotope masses.

Can I enter ionic compounds?

Yes, the calculator handles ionic compounds the same as molecular compounds. NaCl, CaCl2, Fe2O3 all work.

What about complex ions?

Parentheses handle polyatomic ions: Ca(NO3)2 for calcium nitrate, (NH4)2SO4 for ammonium sulfate.

How precise are the results?

Results use atomic weights to 3 decimal places, sufficient for most laboratory work. For high-precision analytical work, use tabulated values with more digits.

What if my formula is invalid?

The calculator will report an error for unrecognized element symbols or malformed formulas.

How do I calculate the mass of a specific amount of compound?

Mass = moles × molecular weight. For 0.5 mol of NaCl (MW = 58.44): 0.5 × 58.44 = 29.22 grams.

How do I calculate moles from a given mass?

Moles = mass / molecular weight. For 100g of H₂O (MW = 18.015): 100 / 18.015 = 5.55 moles.

What is percent composition used for?

Verifying compound identity, checking purity, determining empirical formulas from analytical data, and calculating the mass of a specific element in a sample.

How do I handle isotope-enriched compounds?

Standard atomic weights assume natural isotope abundance. For enriched compounds (like deuterated water D₂O), use exact isotope masses: D = 2.014 instead of H = 1.008.

What's the difference between molecular weight and formula weight?

Molecular weight applies to discrete molecules. Formula weight applies to ionic compounds (like NaCl) that don't exist as discrete molecules. Calculation is identical; terminology differs based on compound type.

How do I calculate dilution requirements?

First determine moles needed: M × V = moles. Then calculate mass: moles × MW = grams. For 500 mL of 0.1 M NaCl: 0.5 L × 0.1 mol/L = 0.05 mol; 0.05 × 58.44 = 2.92 grams.

Why do some elements have variable atomic weights?

Atomic weights are averages based on natural isotope distribution, which varies slightly by geographic source for some elements. IUPAC provides ranges for elements like lithium, boron, and sulfur. Standard weights work for most applications.

What tools complement molecular weight calculations?

Stoichiometry calculators for reaction quantities, molarity calculators for solution preparation, percent composition for analytical verification, and balancing calculators for reaction equations. Molecular weight is foundational to all quantitative chemistry.

How precise should my molecular weight calculations be?

For educational purposes and most lab work, 3-4 significant figures suffice. For analytical chemistry and research requiring high precision, use IUPAC values with full precision and consider isotope distribution effects.

What common compounds should I know the molecular weight of?

Water: 18.015 g/mol. Sodium chloride: 58.44 g/mol. Glucose: 180.16 g/mol. Carbon dioxide: 44.01 g/mol. Ethanol: 46.07 g/mol. These reference points help you sanity-check calculations and develop intuition for molecular sizes.

How does molecular weight relate to physical properties?

Generally, higher molecular weight correlates with higher boiling/melting points, lower vapor pressure, and different solubility characteristics. However, molecular structure (polarity, hydrogen bonding) often matters more than weight alone.

What is the difference between molecular weight and molar mass?

These terms are often used interchangeably. Technically, molecular weight is dimensionless (ratio to carbon-12), while molar mass has units of g/mol. For practical purposes, the numerical values are identical and both terms describe the same quantity.

How do I handle polymers and large molecules?

Polymers have variable molecular weights described by distributions. For proteins and other biological molecules, molecular weight can reach tens of thousands. This calculator works for any formula you can enter, but very large molecules are typically characterized by specialized analytical methods.

Additional Notes and Tips

This calculator processes all inputs locally in your browser, ensuring both privacy and instant results without data transmission. For specialized applications or complex planning scenarios, consider consulting professionals who can account for your specific circumstances and goals.

Practical Tips

Keep a reference sheet of commonly used molecular weights in your lab notebook. Verification through multiple calculation methods helps catch errors before they affect experimental results.

Molecular Weight Calculator — Chemical Formula Mass Calculator

Common Compounds