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# Chapter 05: Formulas and Equations

Formulas and Equations • 1. Write the formulas of ions, starting with cation the anion.
• 2. Bring the number from the charge of the ions diagonally down to obtain ratio of cation to anion.
• Simplify when necessary.
• 3. Write formula according to the number of ions needed.
• If only 1 oin is needed, do not put the number 1 as subscript. What about ionic compounds containing polyatomic ions? Eg. Calcium phosphate. Calcium being a group II metal will lose 2 valence electrons to form Ca2+ ions and will experience electrostatic forces of attraction with phosphate (PO43-) ions. The common multiple of 2 and 3 is 6. Thus, 3 Ca2+ ions are needed to provide a total charge of 6+ to balance out the total charge of 2 PO43- ions which is 6-. Thus, the formula will be Ca3(PO4)2.

The diagram below shows how the cross method is done to find the formula of ionic compounds.

• 1. Write the formulas of ions, starting with cation the anion.
• 2. Bring the number from the charge of the ions diagonally down to obtain ratio of cation to anion.
• 3. Write formula according to the number of ions needed.
• NOTE: Bracket is needed if more than one polyatomic ion is needed in formula.  Definition

Acid: Substance that dissociates in water to produce hydrogen (H⁺) ions.
Base: Substance that reacts with acid to form salt and water.
Alkali: A soluble base that will produce hydroxide (OH⁻) ions when dissolved in water.
Salt: A salt is an ionic compound formed when a metallic ion or an ammonium ion (NH₄⁺) replaces one or more hydrogen ions of an acid.

State symbol

This is the state that the compounds or elements are in at room temperature and pressure.

4 state symbols:

• 1. Solid, denoted by (s)
• 2. Liquid or molten, denoted by (l)
• 3. Gas, denoted by (g)
• 4. Aqueous or dissolved in water, denoted by (aq)

3 acid reactions to remember:

• 1. Metal + Acid → Salt + Hydrogen gas
• Zn(s)   +   HSO₄(aq)   →    ZnSO₄(aq)   +   H₂(g)
• 2. Carbonate + Acid → Salt + Water + Carbon dioxide gas
• ZnCO₃(s) + 2HNO₃(aq) → Zn(NO₃)₂(aq) + HO(l) + CO₂(g)
• 3. Oxide or Hydroxide + Acid → Salt + Water
• ZnO(s)   +   2HCl(aq)   →   ZnCl₂(aq)   +   HO(l)

2 alkali reactions to remember:

• 1. Alkali + Acid → Salt + Water (same as acid reaction #3)
• 2. Alkali + Ammonium Salt → Salt + Water + Ammonia gas
• KOH(aq)   +   NHCl(aq)   →   KCl(aq)   +   H₂O(l)   +   NH₃(g)

Balancing chemical equations (including ionic equations) will have two requirements:

• 1. The mass of reactants and the mass of products must be the same OR the number of each type of atoms must be the same before and after the reaction.
• 2. The total charge of reactants must be the same as the total charge of products OR the charges before and after the reaction must be the same.  In order to construct ionic equations, we need to first know of the solubilities of the compounds by looking at the solubility table.

Salt Soluble Insoluble
Group I salt All are soluble NIL
Ammonium (NH₄⁺) salt All are soluble NIL
Nitrates (NO₃⁻) All nitrates are soluble NIL
Chlorides (Cl⁻) and Iodides (l⁻) Most are soluble AgCl, AgI
AgCl₂, PbI
Carbonates (CO₃²⁻) Group I Carbonates,
(NH₄)₂CO₃
Most carbonates are insoluble
Sulfates (SO₄²⁻) Most sulfates are soluble BaSO₄, CaSO₄, PbSO₄
Hydroxides (OH⁻) and Oxides (O²⁻) Group I Hyroxides,
NH₄OH,
Ca(OH)₂, Ba(OH)₂,
Sr(OH)₂
Most hydroxides and oxides are insoluble.
Salt
Group I salt
Ammonium (NH₄⁺) salt
Nitrates (NO₃⁻)
Chlorides (Cl⁻) and Iodides (l⁻)
Carbonates (CO₃²⁻)
Sulfates (SO₄²⁻)
Hydroxides (OH⁻) and Oxides (O²⁻)
Soluble
All are soluble
All are soluble
All nitrates are soluble
Most are soluble
Group I Carbonates,
(NH₄)₂CO₃
Most sulfates are soluble
Group I Hyroxides,
NH₄OH,
CA(OH)₂, BA(OH)₂,
SR(OH)₂
Insoluble
NIL
NIL
NIL
AgCl, AgI
AgCl₂, PbI
Most carbonates are insoluble
BaSO₄, CaSO₄, PbSO₄
Most hydroxides and oxides are insoluble.

To construct an ionic equation from a balanced chemical equation, refer to 5CConcentrate, Cut, Cancel, Copy, Check.

For example, using the reaction between ZnO and HCl, this is what you need to do after constructing the balanced chemical equation. For step 2, when separating ionic compounds into its individual ions,

• 1. The number in front of the compound will need to be applied to both cation and anion.
• 2. The number as subscript needs to only be applied to the ion before it.  