Stoichiometry Reactant | Interactive Chemistry model

Reactant calculation

Choose a balanced reaction Known reactant Known mass (g) Calculate required reactant

1. Known mass—

→

2. Convert to moles—

→

3. Apply mole ratio—

→

4. Required mass—

Known reactant molar mass—

Known reactant moles—

Stoichiometric mole ratio—

Required reactant moles—

Required reactant molar mass—

Required reactant mass—

Reactant	Coefficient	Molar mass (g mol⁻¹)	Meaning in equation

Key idea: The balanced equation relates moles, not grams. Never use the coefficients directly as a mass ratio. First convert the known mass into moles, use the coefficient ratio, and only then convert the required number of moles back into grams.

Stoichiometry with Python

The Python model follows exactly the same pathway: grams → moles → mole ratio → moles → grams.

# Stoichiometry example
#
# 2H2 + O2 -> 2H2O
#
# Question:
# How much O2 is required to react
# completely with 10.0 g of H2?

mass_H2 = 10.0

# Approximate molar masses
molar_mass_H2 = 2.016
molar_mass_O2 = 31.998

# Balanced equation coefficients
coefficient_H2 = 2
coefficient_O2 = 1

# -------------------------------
# Step 1: grams H2 -> moles H2
# -------------------------------

moles_H2 = (
    mass_H2 / molar_mass_H2
)

# -------------------------------
# Step 2: apply mole ratio
#
# 2 mol H2 : 1 mol O2
# -------------------------------

moles_O2 = (
    moles_H2
    * coefficient_O2
    / coefficient_H2
)

# -------------------------------
# Step 3: moles O2 -> grams O2
# -------------------------------

mass_O2 = (
    moles_O2 * molar_mass_O2
)

print(
    "H2 mass:",
    round(mass_H2, 3),
    "g"
)

print(
    "H2 moles:",
    round(moles_H2, 4),
    "mol"
)

print(
    "O2 required:",
    round(moles_O2, 4),
    "mol"
)

print(
    "O2 mass required:",
    round(mass_O2, 3),
    "g"
)

# General stoichiometry formula:
#
# target mass =
# known mass / known molar mass
# x target coefficient
# / known coefficient
# x target molar mass

Why the Mole Ratio Matters

For 2H₂ + O₂ → 2H₂O, two moles of hydrogen require one mole of oxygen. This does not mean that 2 g of hydrogen reacts with 1 g of oxygen because hydrogen and oxygen have different molar masses.

Try switching the known and required reactants. The calculation reverses, but the balanced-equation ratio remains the foundation of the calculation.