# 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.

Versione interattiva: https://www.calcopenly.com/it/science/molar-mass-calculator
Argomento: Calcolatori scientifici

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.

## Dati

- **Chemical formula**: Case matters: Co is cobalt, CO is carbon monoxide. Hydrates: CuSO4·5H2O, CuSO4.5H2O or CuSO4*5H2O

## Risultati

- Massa molare (g/mol) — risultato principale
- Hill formula
- Mass percent
- Atoms per formula unit
- Different elements
- Mass of one formula unit (g)

## Formula

$$
M = \sum_i n_i\,A_r(i)\ \ \mathrm{g/mol},\qquad w_i = \frac{n_i A_r(i)}{M}
$$

## Esempi svolti

### Water

- Chemical formula: H2O
- **Massa molare: 18.015 g/mol**
- **Mass percent: H 11.19%, O 88.81%**
- **Atoms per formula unit: 3**
- Fonte di verifica: Python 3.8 decimal with IUPAC 2021 abridged weights: 2 × 1.0080 + 15.999 = 18.015 (PubChem 18.015)

### Glucose

- Chemical formula: C6H12O6
- **Massa molare: 180.156 g/mol**
- **Hill formula: C6H12O6**
- Fonte di verifica: 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
- **Massa molare: 249.677 g/mol**
- **Hill formula: CuH10O9S**
- Fonte di verifica: 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
- **Massa molare: 249.677 g/mol**
- Fonte di verifica: Same compound as above; '.' is read as the hydrate dot

### Calcium phosphate (brackets)

- Chemical formula: Ca3(PO4)2
- **Massa molare: 310.174 g/mol**
- **Atoms per formula unit: 13**
- Fonte di verifica: Python 3.8 decimal: 3 × 40.078 + 2 × 30.974 + 8 × 15.999 = 310.174

### Potassium ferrocyanide (nested brackets)

- Chemical formula: K4[Fe(CN)6]
- **Massa molare: 368.345 g/mol**
- **Different elements: 4**
- Fonte di verifica: Python 3.8 decimal: 4 × 39.098 + 55.845 + 6 × (12.011 + 14.007) = 368.345

## Domande

### 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.

### Quanto è preciso «Molar mass calculator»?

La precisione dipende dai dati inseriti e dalle ipotesi del metodo. Il calcolo decimale usa 50 cifre significative, ma stime, metodi numerici e dati di origine possono essere meno precisi; l’arrotondamento visualizzato non elimina questi limiti. Esempi svolti verificati con fonti indipendenti: 6. Per esempio, «Water» viene verificato con Python 3.8 decimal with IUPAC 2021 abridged weights: 2 × 1.0080 + 15.999 = 18.015 (PubChem 18.015).

### Da dove proviene il metodo?

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).

## Fonti

- [IUPAC CIAAW — Standard atomic weights of the elements 2021 (Prohaska et al., Pure Appl. Chem. 94, 573, 2022), abridged values](https://www.ciaaw.org/atomic-weights.htm)
- [PubChem compound pages (molecular weight cross-check)](https://pubchem.ncbi.nlm.nih.gov/)