General Chemistry course

1. 1.Moles and Molar Mass

   Atoms are far too small to count individually, so chemists count them in moles. One mole is $6.022 \times 10^{23}$ particles, Avogadro's number. The molar mass of a substance, in grams per mole, is the sum of the atomic masses on the periodic table, so one mole of water ($\text{H}_2\text{O}$) has a mass of about $2(1.008) + 16.00 = 18.02$ g.

   • moles $= \frac{\text{mass}}{\text{molar mass}}$; particles $=$ moles $\times 6.022 \times 10^{23}$.
   • Subscripts multiply the atomic mass of the element they follow.
   • Parentheses multiply everything inside, as in $\text{Ca(OH)}_2$.
   Quick check: Molar mass of $\text{NaCl}$ (Na 22.99, Cl 35.45), in g/mol.

   $58.44$

   Includes a worked example, 3 common mistakes and 3 practice questions with answers. Study moles and molar mass

2. 2.Stoichiometry

   Stoichiometry uses a balanced chemical equation as a recipe. The coefficients give mole ratios: in $2\text{H}_2 + \text{O}_2 \to 2\text{H}_2\text{O}$, two moles of hydrogen react with one mole of oxygen to make two moles of water. Grams do not appear in the ratio, so every calculation goes grams to moles, across the ratio, then back to grams.

   • Balance the equation first; never change subscripts to balance.
   • Path: grams A $\to$ moles A $\to$ moles B $\to$ grams B.
   • Limiting reactant: compute product from each reactant; the smaller amount wins.
   Quick check: In $\text{N}_2 + 3\text{H}_2 \to 2\text{NH}_3$, how many moles of $\text{NH}_3$ come from 6 mol $\text{H}_2$?

   $4$

   Includes a worked example, 3 common mistakes and 3 practice questions with answers. Study stoichiometry

3. 3.Gas Laws

   Gases are described by four variables: pressure $P$, volume $V$, temperature $T$ and amount $n$. The ideal gas law $PV = nRT$ ties them together, with $R = 0.08206$ L·atm/(mol·K). Boyle's, Charles's and Gay-Lussac's laws are special cases where some variables are held constant.

   • Combined gas law: $\frac{P_1 V_1}{T_1} = \frac{P_2 V_2}{T_2}$ for a fixed amount of gas.
   • Boyle: $P_1 V_1 = P_2 V_2$ at constant $T$.
   • Charles: $\frac{V_1}{T_1} = \frac{V_2}{T_2}$ at constant $P$.
   Quick check: Convert $27^\circ$C to kelvins.

   $300.15$ K

   Includes a worked example, 3 common mistakes and 3 practice questions with answers. Study gas laws

4. 4.Acids, Bases and pH

   Acids donate hydrogen ions (protons) and bases accept them, in the Brønsted-Lowry picture. Strength is about how completely a substance ionises in water, not how concentrated it is: hydrochloric acid ionises fully, while acetic acid in vinegar ionises only slightly.

   • $\text{pH} = -\log[\text{H}^+]$; $[\text{H}^+] = 10^{-\text{pH}}$.
   • For strong acids $[\text{H}^+]$ equals the acid concentration (per ionisable proton).
   • Neutralisation: acid + base $\to$ salt + water.
   Quick check: pH of a solution with $[\text{H}^+] = 10^{-5}$ M.

   $5$

   Includes a worked example, 3 common mistakes and 3 practice questions with answers. Study acids, bases and ph

These topics belong to the course outline. Full explainers for them have not been written yet, so they are summarised here rather than given pages of their own.

Solutions and Molarity

Concentration and dilution.

• Chemistry overview
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