What is Stoichiometry?
Stoichiometry is the branch of chemistry that deals with the quantitative relationships between reactants and products in a chemical reaction. It allows chemists to predict how much product can be formed and how much reactant is needed.
Stoichiometric calculations are based on a balanced chemical equation.
Example:
\[ 2H_2 + O_2 \rightarrow 2H_2O \]
This equation shows that 2 moles of hydrogen react with 1 mole of oxygen to produce 2 moles of water.
Important Stoichiometric Quantities
| Quantity | Description |
|---|---|
| Mole | \(6.022 \times 10^{23}\) particles |
| Molar Mass | Mass of one mole of a substance (g/mol) |
| Avogadro's Number | \(6.022 \times 10^{23}\) |
| Mass | Measured in grams |
| Mole Ratio | Relationship from coefficients in a balanced equation |
The Mole Road Map
Many stoichiometry problems involve converting between:
- Mass ↔ Moles
- Moles ↔ Particles
- Moles ↔ Moles
Mass to moles:
\[ \text{Moles}= \frac{\text{Mass (g)}}{\text{Molar Mass (g/mol)}} \]
Moles to particles:
\[ \text{Particles} = \text{Moles} \times 6.022 \times 10^{23} \]
Example Stoichiometry Problem
Given:
\[ N_2 + 3H_2 \rightarrow 2NH_3 \]
If 4 moles of \(N_2\) react completely, how many moles of \(NH_3\) are produced?
Mole ratio:
\[ \frac{2\ mol\ NH_3}{1\ mol\ N_2} \]
Calculation:
\[ 4 \times \frac{2}{1}=8 \]
Answer: 8 moles of \(NH_3\)
Practice Problems
How many moles are in 36 g of water (\(H_2O\))?
Using \[ 2H_2 + O_2 \rightarrow 2H_2O \] How many moles of water are produced from 5 moles of oxygen?
How many molecules are in 3 moles of \(CO_2\)?
Calculate the molar mass of \(CO_2\).
How many moles are in 88 g of \(CO_2\)?
Challenge Problems
- Given: \[ 2Al + 3Cl_2 \rightarrow 2AlCl_3 \] How many moles of \(AlCl_3\) can be produced from 6 moles of aluminum?
- Calculate the mass of 3 moles of \(NaCl\).
- How many molecules are contained in 0.75 moles of methane (\(CH_4\))?
- Using \[ C + O_2 \rightarrow CO_2 \] determine how many moles of \(CO_2\) are produced from 12 moles of carbon.