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

• ${\log }_{10}x$: 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

  $$2.0\text{ yd}\times \frac{36\text{ in}}{1\text{ yd}}=72\text{ in}$$

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:

$$\text{Molecular formula}=(\text{Empirical formula})\times n$$

Where $n=\frac{\text{molar mass}}{\text{empirical formula mass}}$

7. Percent Composition

$$\mathrm{\%}\text{ by mass}=\frac{\text{mass of element}}{\text{molar mass of compound}}\times 100$$

8. Average Atomic Mass

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

📌 Example (Silicon):

$$(27.9769\times 0.9223)+(28.9765\times 0.0467)+(29.9738\times 0.0310)\approx 28.09$$

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:

$$\text{Percent Yield}=\left(\frac{\text{Actual Yield}}{\text{Theoretical Yield}}\right)\times 100$$

Stoichiometry In a nutshell

• Balance equations.

• Convert with moles.

• Use ratios from coefficients.

• Calculate mass, volume, or particles.