First, compute the number of each atom in As2O5:
As: 2, O: 5
Then, lookup atomic weights for each element in periodic table:
As: 74.9216, O: 15.9994
Now, compute the sum of products of number of atoms to the atomic weight:
Molar mass (As2O5) = ∑ Counti * Weighti =
Count(As) * Weight(As) + Count(O) * Weight(O) =
2 * 74.9216 + 5 * 15.9994 =
229.8402 g/mol
To calculate molar mass of a chemical compound enter its formula and click 'Compute'. In chemical formula you may use:
Any chemical element. Capitalize the first letter in chemical symbol and use lower case for the remaining letters: Ca, Fe, Mg, Mn, S, O, H, C, N, Na, K, Cl, Al.
Functional groups: D, T, Ph, Me, Et, Bu, AcAc, For, Tos, Bz, TMS, tBu, Bzl, Bn, Dmg
Molar mass calculator also displays common compound name, Hill formula, elemental composition, mass percent composition, atomic percent compositions and allows to convert from weight to number of moles and vice versa.
Computing molecular weight (molecular mass)
To calculate molecular weight of a chemical compound enter it's formula, specify its isotope mass number after each element in square brackets.
Examples of molecular weight computations:
C[14]O[16]2,
S[34]O[16]2.
Definitions
Molecular mass (molecular weight) is the mass of one molecule of a substance and is expressed in the unified atomic mass units (u). (1 u is equal to 1/12 the mass of one atom of carbon-12)
Molar mass (molar weight) is the mass of one mole of a substance and is expressed in g/mol.
Mole is a standard scientific unit for measuring large quantities of very small entities such as atoms and molecules. One mole contains exactly 6.022 ×1023 particles (Avogadro's number)
Steps to calculate molar mass
Identify the compound: write down the chemical formula of the compound. For example, water is H2O, meaning it contains two hydrogen atoms and one oxygen atom.
Find atomic masses: look up the atomic masses of each element present in the compound. The atomic mass is usually found on the periodic table and is given in atomic mass units (amu).
Calculate molar mass of each element: multiply the atomic mass of each element by the number of atoms of that element in the compound.
Add them together: add the results from step 3 to get the total molar mass of the compound.
Example: calculating molar mass
Let's calculate the molar mass of carbon dioxide (CO2):
Carbon (C) has an atomic mass of about 12.01 amu.
Oxygen (O) has an atomic mass of about 16.00 amu.
CO2 has one carbon atom and two oxygen atoms.
The molar mass of carbon dioxide is 12.01 + (2 × 16.00) = 44.01 g/mol.
Lesson on computing molar mass
Weights of atoms and isotopes are from NIST article.
