Formulas and Equations
Formulas and Equations
This domain tests writing correct chemical formulas, balancing equations, including state symbols, and constructing ionic equations. Accuracy in formulas and charges is essential because every later calculation depends on the equation.
Core rules
- A balanced equation has the same number and type of atoms on both sides; charge must also balance in ionic equations.
- Write formulas from names using ion charges: group 1 +1, group 2 +2, aluminium +3, halides -1, oxide -2, hydroxide -1, nitrate -1, sulfate -2, carbonate -2, phosphate -3.
- Use subscripts only to define a formula; never change a subscript to balance an equation. Change coefficients instead.
- Include state symbols: (s), (l), (g), (aq).
- Net ionic equations omit spectator ions, which appear unchanged on both sides.
Worked example
Balance propane combustion: C3H8 + O2 → CO2 + H2O. Carbon needs 3CO2; hydrogen needs 4H2O. The right side now has 3×2 + 4×1 = 10 oxygen atoms, so use 5O2. Balanced: C3H8(g) + 5O2(g) → 3CO2(g) + 4H2O(l). Check: C 3=3, H 8=8, O 10=10.
Traps
Changing a subscript changes the substance: H2O is not H2O2. Do not omit charges on ions. In precipitation equations, include only the ions that form the solid; for AgNO3(aq) + NaCl(aq), the net ionic equation is Ag+(aq) + Cl−(aq) → AgCl(s).