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Molecular Weight Calculator

Enter element symbols with subscript numbers (e.g., H2O, Ca(OH)2)

Common Elements

H
1.008
C
12.011
N
14.007
O
15.999
Na
22.99
Mg
24.305
P
30.974
S
32.065
Cl
35.453
K
39.098
Ca
40.078
Fe
55.845

Common Compounds

Water (H2O)18.015 g/mol
CO244.01 g/mol
NaCl58.44 g/mol
Glucose (C6H12O6)180.16 g/mol

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Understanding Molecular Weight

What is Molecular Weight?

Molecular weight (MW), also called molar mass, is the sum of the atomic weights of all atoms in a molecule. It is expressed in grams per mole (g/mol) or atomic mass units (amu). One mole of any substance contains 6.022 x 10^23 particles (Avogadro is number).

Calculation Method

MW = Sum of (Atomic Weight x Number of Atoms) for each element
Example: H2O = (1.008 x 2) + (16.00 x 1) = 18.015 g/mol

Common Elements

ElementSymbolAtomic Weight (amu)
HydrogenH1.008
CarbonC12.011
NitrogenN14.007
OxygenO15.999
SodiumNa22.990
ChlorineCl35.453
IronFe55.845

Mole Calculations

Mass to Moles

n = m / MW

Where n = moles, m = mass (g), MW = molecular weight

Moles to Mass

m = n x MW

Where m = mass (g), n = moles, MW = molecular weight

Formula Notation

Simple Formulas

  • H2O - Water
  • NaCl - Sodium chloride
  • CO2 - Carbon dioxide
  • NH3 - Ammonia

Complex Formulas

  • C6H12O6 - Glucose
  • H2SO4 - Sulfuric acid
  • Ca(OH)2 - Calcium hydroxide
  • Fe2O3 - Iron(III) oxide

Avogadro is Number

One mole of any substance contains exactly 6.02214076 x 10^23 particles (atoms, molecules, or ions). This constant links the macroscopic world of grams to the microscopic world of atoms.

Frequently Asked Questions

How do I calculate molecular weight from a formula?

Add up the atomic weights of all atoms in the molecule. For H2O: (2 x 1.008) + (1 x 16.00) = 18.015 g/mol. For compounds with subscripts, multiply the atomic weight by the subscript number.

What is the difference between molecular weight and molar mass?

They're numerically the same but have different units. Molecular weight (MW) is measured in atomic mass units (amu) for single molecules. Molar mass is measured in g/mol and represents the mass of one mole (6.022x10^23) of molecules.

What is Avogadro's number and why does it matter?

Avogadro's number (6.022x10^23) is the number of particles in one mole. It connects the microscopic world of atoms to measurable quantities. One mole of any substance contains this many particles and has a mass equal to its molecular weight in grams.

How do I calculate moles from grams?

Divide the mass in grams by the molecular weight: moles = mass / MW. For example, 36 grams of water (MW = 18 g/mol) equals 36/18 = 2 moles of water.