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Molar mass calculator

Molar mass in g/mol and mass percent composition of any chemical formula, with brackets and hydrates such as CuSO4·5H2O, from IUPAC 2021 atomic weights.

Actualizado Ejemplos verificados: 6

Case matters: Co is cobalt, CO is carbon monoxide. Hydrates: CuSO4·5H2O, CuSO4.5H2O or CuSO4*5H2O
Probar
Masa molar
g/mol
Masa molar: 180.156 g/mol
Decimales: 3; Al más cercano; empates alejándose de cero
Hill formula
C6H12O6
Mass percent
C 40.00%, H 6.71%, O 53.28%
Atoms per formula unit
24
Different elements
3
Mass of one formula unit
2.99156 × 10⁻²²g

One mole of C₆H₁₂O₆ weighs 180.156 g; oxygen contributes the most mass (53.28%).

Mass percent by element

180.156g/mol
C40.00%H6.71%O53.28%
Composition Filas: 3
ElementAtomsAtomic weightMass (g/mol)Mass %
C (Carbon)612.01172.06640.00%
H (Hydrogen)121.008012.0966.71%
O (Oxygen)615.99995.99453.28%
Cómo se calcula S

  1. Count the atoms

    C:6, H:12, O:6\mathrm{C}: 6,\ \mathrm{H}: 12,\ \mathrm{O}: 6

    Brackets multiply everything inside; a hydrate's leading number multiplies its whole part. Hill formula: C6H12O6.

  2. Add the atomic weights

    M=6×12.011+12×1.0080+6×15.999=180.156 g/molM = 6\times 12.011 + 12\times 1.0080 + 6\times 15.999 = 180.156\ \mathrm{g/mol}

    Shown to 3 decimals: the IUPAC abridged atomic weights carry about 5 significant figures, and some (Li, S, Cl, Pb…) fewer.

  3. Mass of one formula unit

    m=MNA=180.1566.02214076×1023=2.99156×10−22 gm = \frac{M}{N_A} = \frac{180.156}{6.02214076\times10^{23}} = 2.99156 \times 10^{-22}\ \mathrm{g}

Acerca de Molar mass calculator

Molar mass is the mass of one mole of a substance, in grams per mole. The calculator reads the formula, counts the atoms of each element (multiplying through brackets and hydrate dots) and adds count × standard atomic weight, using the IUPAC 2021 abridged values. Each element's share of the total is its mass percent.

Chemists need molar mass to convert between grams and moles when weighing reagents. Glucose, C6H12O6, comes to 180.156 g/mol, of which oxygen supplies 53.28 %; a single molecule weighs 2.99 × 10⁻²² g.

Results are shown to three decimals because the abridged atomic weights carry about five significant figures. For elements with no standard atomic weight, such as technetium, the mass number of a long-lived isotope stands in, and the page says so.

Ejemplos resueltos

Water

Chemical formula
H2O
Masa molar
18.015 g/mol
Mass percent
H 11.19%, O 88.81%
Atoms per formula unit
3

Fuente de comprobación: Python 3.8 decimal with IUPAC 2021 abridged weights: 2 × 1.0080 + 15.999 = 18.015 (PubChem 18.015)

Glucose

Chemical formula
C6H12O6
Masa molar
180.156 g/mol
Hill formula
C6H12O6

Fuente de comprobación: Python 3.8 decimal: 6 × 12.011 + 12 × 1.0080 + 6 × 15.999 = 180.156 (PubChem 180.16)

Copper sulfate pentahydrate

Chemical formula
CuSO4·5H2O
Masa molar
249.677 g/mol
Hill formula
CuH10O9S

Fuente de comprobación: Python 3.8 decimal: 63.546 + 32.06 + 9 × 15.999 + 10 × 1.0080 = 249.677 (PubChem 249.69 with older S = 32.065)

Hydrate written with a plain dot

Chemical formula
CuSO4.5H2O
Masa molar
249.677 g/mol

Fuente de comprobación: Same compound as above; '.' is read as the hydrate dot

Preguntas

Is molar mass the same as molecular weight?

Numerically, yes. Molar mass carries units of g/mol, while molecular weight (relative molecular mass, Mr) is the same number without units, measured against one twelfth of the mass of a carbon-12 atom. Water has a molar mass of 18.015 g/mol and a relative molecular mass of 18.015. For ionic solids such as NaCl, which have no discrete molecules, the usual term is formula mass.

How do you calculate molar mass by hand?

Multiply each element's atom count by its atomic weight and add the products. For water, H2O: 2 × 1.008 + 15.999 = 18.015 g/mol. Brackets multiply everything inside them, so Ca3(PO4)2 holds 2 phosphorus and 8 oxygen atoms. A hydrate's water counts in full: CuSO4·5H2O adds 10 hydrogen and 5 oxygen atoms to copper sulfate, for 249.677 g/mol.

Why does my textbook give a slightly different molar mass?

Atomic weights are revised as isotope measurements improve, and books round them differently. IUPAC's 2021 abridged table gives sulfur as 32.06 where older tables used 32.065, so copper sulfate pentahydrate comes to 249.677 g/mol here and 249.69 g/mol on PubChem. A difference in the second decimal place is normal; use whichever table your course or method specifies.

What does mass percent composition tell you?

It is the share of a compound's mass that each element contributes: the element's mass in one formula unit divided by the molar mass, times 100. Glucose is 40.00 % carbon, 6.71 % hydrogen and 53.28 % oxygen. Chemists compare these figures with percentages measured by combustion analysis to confirm a sample's identity or work out an empirical formula.

Why does capitalisation matter in a chemical formula?

Every element symbol starts with a capital letter and any second letter is lowercase, so case decides which atoms you mean. Co is cobalt, 58.933 g/mol, while CO is carbon monoxide, one carbon and one oxygen atom, 28.010 g/mol. Likewise NO is nitric oxide but No is nobelium. Type formulas with standard capitalisation to get the compound you intend.

¿Qué precisión tiene «Molar mass calculator»?

La precisión depende de tus datos y de los supuestos del método. El cálculo decimal usa 50 cifras significativas, pero las estimaciones, los métodos numéricos y los datos de origen pueden ser menos precisos; el redondeo mostrado no elimina esos límites. Ejemplos resueltos comprobados con fuentes independientes: 6. Por ejemplo, «Water» se comprueba con Python 3.8 decimal with IUPAC 2021 abridged weights: 2 × 1.0080 + 15.999 = 18.015 (PubChem 18.015).

¿De dónde procede el método?

IUPAC CIAAW — Standard atomic weights of the elements 2021 (Prohaska et al., Pure Appl. Chem. 94, 573, 2022), abridged values; PubChem compound pages (molecular weight cross-check).

Acerca de esta calculadora

M=∑ini Ar(i)  g/mol,wi=niAr(i)MM = \sum_i n_i\,A_r(i)\ \ \mathrm{g/mol},\qquad w_i = \frac{n_i A_r(i)}{M}

Fuentes

  1. IUPAC CIAAW — Standard atomic weights of the elements 2021 (Prohaska et al., Pure Appl. Chem. 94, 573, 2022), abridged values
  2. PubChem compound pages (molecular weight cross-check)

Verificado con las referencias

Esta calculadora incluye 6 ejemplos resueltos con respuestas de fuentes independientes. Forman parte del conjunto de pruebas y también puedes ejecutarlos aquí.

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