Stoichiometry

Rucete ✏ Chemistry In a Nutshell

1. Significant Figures

• Nonzero digits are always significant.
• Leading zeros are never significant.
• Trailing zeros are significant only if there is a decimal.

🔢 Examples:

• 8.802 → 4 sig figs
• 0.00050800 → 6 sig figs
• 1600. → 4 sig figs (decimal makes zeros significant)

2. Scientific Notation and Logs

• log₁₀x: the power to which 10 must be raised to get x
• ln x: natural logarithm (log base e)

3. Mole Conversions & Avogadro’s Number

• 1 mole = 6.02 × 10²³ particles (atoms, molecules, etc.)
• 1 mole = molar mass in grams
• 1 mole of gas at STP = 22.4 L

4. Factor-Label Method (Dimensional Analysis)

• Use conversion factors to switch units step by step.
• Ex: Convert yards to inches

  2.0 yd × (36 in / 1 yd) = 72 in

5. Balancing Chemical Equations

• Same number of atoms for each element on both sides.
• Balance using coefficients, not subscripts.

6. Empirical vs. Molecular Formulas

• Empirical Formula: Simplest whole-number ratio of atoms.
• Molecular Formula: Actual number of atoms in the compound.

🔎 To find molecular formula:

Molecular formula = (Empirical formula) × n

Where n = molar mass / empirical formula mass

7. Percent Composition

% by mass = (mass of element / molar mass of compound) × 100

8. Average Atomic Mass

• Multiply each isotope’s mass by its percent abundance, then sum.

📌 Example (Silicon):

(27.9769 × 0.9223) + (28.9765 × 0.0467) + (29.9738 × 0.0310) ≈ 28.09

9. Limiting Reactant

• The reactant that runs out first, limiting the amount of product formed.

10. Theoretical Yield, Actual Yield, and Percent Yield

• Theoretical Yield: Amount predicted from stoichiometry.
• Actual Yield: Amount actually obtained in the experiment.
• Percent Yield Formula:

Percent Yield = (Actual Yield / Theoretical Yield) × 100

Stoichiometry In a nutshell

• Balance equations.
• Convert with moles.
• Use ratios from coefficients.
• Calculate mass, volume, or particles.