Stoichiometry Calculator | Moles & Grams
Convert between moles and grams using molar mass. Calculate mass from moles and vice versa.
Common compounds:
What Is the Stoichiometry Calculator | Moles & Grams?
The Stoichiometry Calculator operates in four modes: Moles → Grams (mass = n × M), Grams → Moles (n = mass / M), Molar Mass (parses a chemical formula like H2SO4 or C6H12O6 and computes total molar mass from atomic masses), and % Composition (the percentage by mass of each element). Fifteen common compound presets are available, and the formula parser handles both ASCII (H2O) and Unicode subscripts (H₂O).
- ›Molar mass (M) is the mass of one mole of a substance in g/mol
- ›One mole = 6.022×10²³ particles (Avogadro's number)
- ›Use H2O or H₂O, both formats accepted by the formula parser
- ›% composition: sum of all element percentages = 100%
- ›Step-by-step working shown for every calculation
Formula
Stoichiometry & Mole Formulas
Moles → Grams
mass = moles × M (g)
Grams → Moles
moles = mass / M (mol)
Molar mass
M = Σ(atomic mass × count)
Avogadro
N = n × 6.022×10²³
% Composition
% = (element mass / M) × 100
At STP
1 mol gas = 22.414 L
How to Use
- 1Select a mode: Moles → Grams, Grams → Moles, Molar Mass, or % Composition
- 2For mole/gram conversions: enter the amount and the molar mass (or click a compound preset)
- 3For Molar Mass or % Composition: type a chemical formula (e.g., H2O, NaCl, C6H12O6)
- 4Click Calculate, results appear with full step-by-step working
- 5For quick reference: click any Quick Examples button to load a common compound
Example Calculation
Molar mass of glucose (C₆H₁₂O₆):
H: 1.008 × 12 = 12.096 g/mol
O: 15.999 × 6 = 95.994 g/mol
Total M = 180.156 g/mol
Percent composition of water (H₂O):
%H = (2.016 / 18.015) × 100 = 11.19%
%O = (15.999 / 18.015) × 100 = 88.81%
Avogadro's Number in Practice
One mole of water (18 g) contains 6.022×10²³ molecules. If you could count one molecule per second, it would take 1.9×10¹⁶ years, more than a million times the age of the universe. This enormous scale explains why chemists work in moles rather than individual atoms.
Understanding Stoichiometry | Moles & Grams
Molar Masses of Common Compounds
| Compound | Formula | Molar Mass (g/mol) | Common Use |
|---|---|---|---|
| Water | H₂O | 18.015 | Universal solvent |
| Sodium chloride | NaCl | 58.44 | Table salt |
| Carbon dioxide | CO₂ | 44.01 | Combustion product |
| Ammonia | NH₃ | 17.031 | Fertiliser production |
| Sulfuric acid | H₂SO₄ | 98.079 | Industrial acid |
| Glucose | C₆H₁₂O₆ | 180.156 | Cellular respiration |
| Ethanol | C₂H₅OH | 46.069 | Beverages, fuel |
| Calcium carbonate | CaCO₃ | 100.087 | Limestone, chalk |
| Sodium hydroxide | NaOH | 39.997 | Caustic soda, bases |
| Iron(III) oxide | Fe₂O₃ | 159.688 | Rust, iron ore |
| Methane | CH₄ | 16.043 | Natural gas |
| Hydrochloric acid | HCl | 36.461 | Stomach acid, labs |
Frequently Asked Questions
What is a mole in chemistry?
The mole bridges the microscopic atomic world and the macroscopic laboratory world. It allows chemists to count atoms by weighing, a practical necessity since individual atoms are unmeasurably small.
- ›1 mol = 6.02214076×10²³ entities (Avogadro constant, exact by SI since 2019)
- ›1 mol H₂O = 18.015 g; 1 mol NaCl = 58.44 g; 1 mol Fe = 55.845 g
- ›At STP (0°C, 100 kPa): 1 mol of any ideal gas = 22.414 L
- ›Molarity (M) = moles of solute / litres of solution
How do I calculate molar mass from a chemical formula?
The formula parser in this calculator handles standard element symbols and subscripts automatically. Enter the formula in the Molar Mass tab.
- ›H₂O: M = 2(1.008) + 15.999 = 18.015 g/mol
- ›NaCl: M = 22.990 + 35.45 = 58.44 g/mol
- ›CO₂: M = 12.011 + 2(15.999) = 44.009 g/mol
- ›C₆H₁₂O₆ (glucose): M = 6(12.011) + 12(1.008) + 6(15.999) = 180.156 g/mol
What is percent composition and why is it useful?
- ›Water: 11.19% H, 88.81% O
- ›CO₂: 27.29% C, 72.71% O
- ›NaCl: 39.34% Na, 60.66% Cl
- ›Used in combustion analysis to find empirical formulas of organic compounds
- ›Nutritional labels report % composition by mass for fat, protein, carbohydrate
What are moles used for in chemical reactions?
The mole ratio is the core of stoichiometric calculations. Once you know moles of one substance, multiply by the mole ratio from the balanced equation to find moles of any other substance.
- ›2H₂ + O₂ → 2H₂O: mole ratio H₂:O₂:H₂O = 2:1:2
- ›4g H₂ = 2 mol H₂ → 1 mol O₂ needed (32g) → 2 mol H₂O produced (36g)
- ›Limiting reagent: the reactant that runs out first
- ›Excess reagent: leftover after the limiting reagent is consumed
How do I find the limiting reagent?
The limiting reagent determines the maximum theoretical yield. All other quantities of products and excess reagents can be calculated once the limiting reagent is identified.
- ›Step 1: convert all masses to moles using molar masses
- ›Step 2: divide moles by stoichiometric coefficients
- ›Step 3: smallest ratio = limiting reagent
- ›Step 4: use limiting reagent moles × product ratio for theoretical yield
How accurate is this molar mass calculator?
- ›Uses IUPAC 2021 standard atomic weights (e.g., H=1.008, C=12.011, O=15.999)
- ›Accurate to ±0.001 g/mol for most compounds
- ›Handles 30 common elements, sufficient for undergraduate chemistry
- ›Does not handle brackets/parentheses (e.g., Ca(OH)₂), expand manually: CaO2H2
Is this stoichiometry calculator free?
Yes, completely free with no registration required. All calculations run locally in your browser.