Molecular Weight Calculator Online Free

Molecular Weight Calculator

Calculate molecular weight (molar mass) from chemical formulas. Analyzes elemental composition and mass percentages automatically.

Real-time Calculations

Visual Analysis

118 Elements

Formula Input

Enter a chemical formula (e.g., H2O, C6H12O6, Ca(OH)2)

Chemical Formula

Use element symbols (H, C, O, etc.) and numbers for subscripts

Common Chemicals

Choose from 158 pre-loaded chemicals (67 organic, 91 inorganic)

Formula Guidelines

• Element symbols are case-sensitive (H, not h)
• Numbers go after element (H2, not 2H)
• Use parentheses for groups: Ca(OH)2
• No spaces in formula

Enter a Chemical Formula

Type a chemical formula or select a common molecule to calculate its molecular weight and analyze elemental composition.

Understanding Molecular Weight

Master the fundamentals of molecular weight calculations and their applications in chemistry

What is Molecular Weight?

Molecular weight, also known as molecular mass or molar mass, represents the mass of one mole of a substance. It's calculated by summing the atomic weights of all atoms in a molecular formula, expressed in grams per mole (g/mol) or atomic mass units (amu).

The Calculation Process

MW = Σ(n × atomic weight)

Sum of (count × atomic weight) for each element

MW = Molecular Weight (g/mol)

n = Number of atoms of each element

atomic weight = From periodic table

Key Characteristics:

Additive property: Sum of individual atomic weights in the molecule
Stoichiometry foundation: Essential for chemical equation balancing
Unit conversion bridge: Connects mass to number of molecules
Universal standard: Used globally in chemistry and related sciences

Practical Calculation Examples

Example 1: Water (H₂O)

Calculate the molecular weight of water.

H: 2 atoms × 1.008 = 2.016 g/mol

O: 1 atom × 15.999 = 15.999 g/mol

Total MW = 18.015 g/mol

Example 2: Glucose (C₆H₁₂O₆)

A common sugar molecule with multiple elements.

C: 6 atoms × 12.011 = 72.066 g/mol

H: 12 atoms × 1.008 = 12.096 g/mol

O: 6 atoms × 15.999 = 95.994 g/mol

Total MW = 180.156 g/mol

Example 3: Calcium Hydroxide Ca(OH)₂

Compound with parentheses - multiply contents by subscript.

Ca: 1 atom × 40.078 = 40.078 g/mol

O: 2 atoms × 15.999 = 31.998 g/mol

H: 2 atoms × 1.008 = 2.016 g/mol

Total MW = 74.092 g/mol

Key Concepts & Terminology

Molecular Weight vs Molar Mass

Often used interchangeably, but technically molar mass is the mass per mole in g/mol, while molecular weight is dimensionless (ratio to 1/12 of C-12).

Atomic Mass Unit (amu)

One amu equals 1/12 the mass of a carbon-12 atom (1.66054 × 10⁻²⁴ g). Numerically equal to g/mol for practical purposes.

Empirical Formula

The simplest whole-number ratio of atoms. For example, glucose (C₆H₁₂O₆) has empirical formula CH₂O.

Avogadro's Number

6.022 × 10²³ particles per mole. Links molecular weight to actual mass of individual molecules and macroscopic quantities.

Formula Weight

Used for ionic compounds that don't form discrete molecules. Calculated the same way but represents formula unit mass.

Mass Percent Composition

Percentage of each element's mass in the compound. Calculated as (element mass / total MW) × 100%.

Practical Applications

Laboratory & Research Uses

Stoichiometric Calculations

Converting between mass and moles in chemical reactions. Essential for determining reactant amounts and predicting product yields in synthesis.

Solution Preparation

Calculating mass needed to prepare solutions of specific molarity. Critical for analytical chemistry, biochemistry, and pharmaceutical applications.

Analytical Chemistry

Identifying unknown compounds through mass spectrometry, determining empirical formulas from elemental analysis data.

Quality Control

Verifying purity of compounds, detecting contaminants, and ensuring product specifications in manufacturing processes.

Drug Development

Determining dosage forms, calculating active ingredient concentrations, and optimizing drug formulations for efficacy.

Real-World Examples

Pharmaceutical Dosing

Aspirin (C₉H₈O₄, MW = 180.16 g/mol) tablets contain 325 mg. This equals 0.00180 moles, or about 1.09 × 10²¹ molecules per dose.

Nutritional Science

Glucose (C₆H₁₂O₆, MW = 180.16 g/mol) blood levels are measured in mg/dL. Normal fasting glucose of 90 mg/dL equals 5 mM concentration.

Environmental Monitoring

CO₂ (MW = 44.01 g/mol) atmospheric levels of 420 ppm help calculate total carbon mass in the atmosphere for climate models.

Industrial Chemistry

Sulfuric acid production (H₂SO₄, MW = 98.08 g/mol) requires precise molecular weight calculations for reactor design and efficiency optimization.

Polymer Science

Polyethylene molecular weight affects properties. Chains with MW of 100,000 g/mol contain about 3,571 ethylene repeat units (C₂H₄).

Common Mistakes to Avoid

Calculation Errors

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Miscounting Atoms in Parentheses

In Ca(OH)₂, the subscript 2 applies to both O and H. It's 1 Ca + 2 O + 2 H, not 1 Ca + 1 O + 2 H.

✗

Using Outdated Atomic Weights

Atomic weights are periodically refined by IUPAC. Always use current values for precise work.

✗

Confusing Empirical and Molecular Formulas

Glucose (C₆H₁₂O₆) has empirical formula CH₂O. Use molecular formula for molecular weight calculations.

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Rounding Too Early

Keep full precision during calculations, round only at the final answer to avoid accumulated errors.

Formula Interpretation Errors

✗

Case Sensitivity Mistakes

CO (carbon monoxide) is different from Co (cobalt). Always use proper capitalization for element symbols.

✗

Forgetting Hydration Water

CuSO₄·5H₂O includes 5 water molecules. MW is 249.68 g/mol, not 159.61 g/mol (anhydrous).

✗

Nested Parentheses Confusion

In complex formulas like [Cu(NH₃)₄]SO₄, carefully multiply through each level of brackets.

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Isotope Considerations

Standard atomic weights are averages. For isotope-specific work, use exact isotopic masses.

Best Practice: Always double-check your formula interpretation before calculating. Write out the atom count for each element separately, then sum. Use this calculator to verify hand calculations and catch errors early.

Key Takeaways & Best Practices

Essential Principles

Precision matters: Use atomic weights to at least 3 decimal places for accurate calculations. Small errors compound in stoichiometric calculations.

Systematic approach: List each element, count atoms (including those in parentheses), multiply by atomic weight, then sum all contributions.

Understand the application: Different fields may require different precision levels. Match your calculation precision to your needs.

Verify complex formulas: For compounds with multiple parentheses or large subscripts, break down the calculation into manageable steps.

Your Next Steps

Practice with common chemicals from our dropdown menu to build familiarity with different molecular structures.

Create a reference table of molecular weights for chemicals you use frequently in your work or studies.

Learn to quickly identify and calculate empirical formulas from molecular formulas using the calculator's results.

Apply molecular weight calculations to real problems: preparing solutions, analyzing reactions, or calculating yields.

Master Molecular Weight Calculations

Understanding molecular weight is fundamental to all quantitative chemistry. Whether you're balancing equations, preparing solutions, analyzing unknown compounds, or designing synthesis routes, accurate molecular weight calculations ensure reliable results. This calculator provides instant, precise calculations with visual analysis of elemental composition. Use it regularly to verify your work, explore different compounds, and deepen your understanding of molecular structure. Remember that mastery comes from practice—calculate the molecular weight of compounds you encounter daily to build intuition and confidence in your chemistry skills.

Modern Periodic Table of Elements

The modern periodic table, first developed by Russian chemist Dmitri Mendeleev in 1869, organizes all known chemical elements based on their atomic number, electron configuration, and recurring chemical properties. This systematic arrangement reveals periodic trends in element behavior and allows scientists to predict properties of undiscovered elements. The table follows the periodic law, which states that element properties are periodic functions of their atomic numbers. Today's periodic table contains 118 confirmed elements, with elements 1-94 occurring naturally and elements 95-118 being synthetically produced in laboratories through nuclear reactions.

Atomic #	Symbol	Element Name	Atomic Weight (u)	Density (g/cm³)	Phase (STP)
1	H	Hydrogen	1.008	0.00008988	Gas
2	He	Helium	4.0026	0.0001785	Gas
3	Li	Lithium	6.94	0.534	Solid
4	Be	Beryllium	9.0122	1.85	Solid
5	B	Boron	10.81	2.34	Solid
6	C	Carbon	12.011	2.267	Solid
7	N	Nitrogen	14.007	0.0012506	Gas
8	O	Oxygen	15.999	0.001429	Gas
9	F	Fluorine	18.998	0.001696	Gas
10	Ne	Neon	20.18	0.0008999	Gas
11	Na	Sodium	22.99	0.971	Solid
12	Mg	Magnesium	24.305	1.738	Solid
13	Al	Aluminum	26.982	2.698	Solid
14	Si	Silicon	28.085	2.3296	Solid
15	P	Phosphorus	30.974	1.82	Solid
16	S	Sulfur	32.06	2.067	Solid
17	Cl	Chlorine	35.45	0.003214	Gas
18	Ar	Argon	39.948	0.0017837	Gas
19	K	Potassium	39.098	0.862	Solid
20	Ca	Calcium	40.078	1.54	Solid
21	Sc	Scandium	44.956	2.989	Solid
22	Ti	Titanium	47.867	4.54	Solid
23	V	Vanadium	50.942	6.11	Solid
24	Cr	Chromium	51.996	7.15	Solid
25	Mn	Manganese	54.938	7.44	Solid
26	Fe	Iron	55.845	7.874	Solid
27	Co	Cobalt	58.933	8.86	Solid
28	Ni	Nickel	59.693	8.912	Solid
29	Cu	Copper	63.546	8.96	Solid
30	Zn	Zinc	65.38	7.134	Solid
31	Ga	Gallium	69.723	5.907	Solid
32	Ge	Germanium	72.63	5.323	Solid
33	As	Arsenic	74.922	5.776	Solid
34	Se	Selenium	78.971	4.809	Solid
35	Br	Bromine	79.904	3.122	Liquid
36	Kr	Krypton	83.798	0.003733	Gas
37	Rb	Rubidium	85.468	1.532	Solid
38	Sr	Strontium	87.62	2.64	Solid
39	Y	Yttrium	88.906	4.469	Solid
40	Zr	Zirconium	91.224	6.506	Solid
41	Nb	Niobium	92.906	8.57	Solid
42	Mo	Molybdenum	95.95	10.22	Solid
43	Tc	Technetium	98	11.5	Solid
44	Ru	Ruthenium	101.07	12.37	Solid
45	Rh	Rhodium	102.91	12.41	Solid
46	Pd	Palladium	106.42	12.02	Solid
47	Ag	Silver	107.87	10.501	Solid
48	Cd	Cadmium	112.41	8.69	Solid
49	In	Indium	114.82	7.31	Solid
50	Sn	Tin	118.71	7.287	Solid
51	Sb	Antimony	121.76	6.685	Solid
52	Te	Tellurium	127.6	6.232	Solid
53	I	Iodine	126.9	4.93	Solid
54	Xe	Xenon	131.29	0.005887	Gas
55	Cs	Cesium	132.91	1.873	Solid
56	Ba	Barium	137.33	3.594	Solid
57	La	Lanthanum	138.91	6.145	Solid
58	Ce	Cerium	140.12	6.77	Solid
59	Pr	Praseodymium	140.91	6.773	Solid
60	Nd	Neodymium	144.24	7.007	Solid
61	Pm	Promethium	145	7.26	Solid
62	Sm	Samarium	150.36	7.52	Solid
63	Eu	Europium	151.96	5.243	Solid
64	Gd	Gadolinium	157.25	7.895	Solid
65	Tb	Terbium	158.93	8.229	Solid
66	Dy	Dysprosium	162.5	8.55	Solid
67	Ho	Holmium	164.93	8.795	Solid
68	Er	Erbium	167.26	9.066	Solid
69	Tm	Thulium	168.93	9.321	Solid
70	Yb	Ytterbium	173.05	6.965	Solid
71	Lu	Lutetium	174.97	9.84	Solid
72	Hf	Hafnium	178.49	13.31	Solid
73	Ta	Tantalum	180.95	16.654	Solid
74	W	Tungsten	183.84	19.25	Solid
75	Re	Rhenium	186.21	21.02	Solid
76	Os	Osmium	190.23	22.61	Solid
77	Ir	Iridium	192.22	22.56	Solid
78	Pt	Platinum	195.08	21.46	Solid
79	Au	Gold	196.97	19.282	Solid
80	Hg	Mercury	200.59	13.5336	Liquid
81	Tl	Thallium	204.38	11.85	Solid
82	Pb	Lead	207.2	11.342	Solid
83	Bi	Bismuth	208.98	9.807	Solid
84	Po	Polonium	209	9.32	Solid
85	At	Astatine	210	7	Solid
86	Rn	Radon	222	0.00973	Gas
87	Fr	Francium	223	1.87	Solid
88	Ra	Radium	226	5.5	Solid
89	Ac	Actinium	227	10.07	Solid
90	Th	Thorium	232.04	11.72	Solid
91	Pa	Protactinium	231.04	15.37	Solid
92	U	Uranium	238.03	18.95	Solid
93	Np	Neptunium	237	20.45	Solid
94	Pu	Plutonium	244	19.84	Solid
95	Am	Americium	243	13.69	Solid
96	Cm	Curium	247	13.51	Solid
97	Bk	Berkelium	247	14.79	Solid
98	Cf	Californium	251	15.1	Solid
99	Es	Einsteinium	252	8.84	Solid
100	Fm	Fermium	257	N/A	Solid
101	Md	Mendelevium	258	N/A	Solid
102	No	Nobelium	259	N/A	Solid
103	Lr	Lawrencium	266	N/A	Solid
104	Rf	Rutherfordium	267	N/A	Solid
105	Db	Dubnium	268	N/A	Solid
106	Sg	Seaborgium	269	N/A	Solid
107	Bh	Bohrium	270	N/A	Solid
108	Hs	Hassium	277	N/A	Solid
109	Mt	Meitnerium	278	N/A	Solid
110	Ds	Darmstadtium	281	N/A	Solid
111	Rg	Roentgenium	282	N/A	Solid
112	Cn	Copernicium	285	N/A	Gas
113	Nh	Nihonium	286	N/A	Solid
114	Fl	Flerovium	289	N/A	Solid
115	Mc	Moscovium	290	N/A	Solid
116	Lv	Livermorium	293	N/A	Solid
117	Ts	Tennessine	294	N/A	Solid
118	Og	Oganesson	294	N/A	Gas

Atomic Number: Number of protons in the nucleus, defining the element's identity and position in the periodic table.

Atomic Weight (u): Average mass of an element's atoms in unified atomic mass units, accounting for isotope distribution.

Density (g/cm³): Mass per unit volume at standard temperature and pressure (STP: 0°C, 1 atm).

Phase (STP): Physical state of the element at standard conditions—solid, liquid, or gas.

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