Exercise 3 solution

Unbalanced skeleton: Ca(OH)2 + HCl -> CaCl2 + H2O

Step 1. Count atoms (the Chapter 6 method). The product CaCl2 has 2 Cl, so HCl needs the coefficient 2.
        Hydrogen: left side 2 (from OH) + 2 (from 2HCl) = 4. Right side needs 4 H, and 2H2O gives 4 H.
Step 2. Check every element:
        Ca: 1 left, 1 right
        O:  2 left, 2 right
        H:  2 + 2 = 4 left, 4 right
        Cl: 2 left, 2 right

Balanced equation: Ca(OH)2 + 2HCl -> CaCl2 + 2H2O

Type: an acid-base (neutralisation) reaction; it also fits the double displacement pattern of Chapter 6.

The salt is CaCl2, calcium chloride: Ca2+ and Cl- ions in the ratio 1 : 2 so the charges cancel (Chapter 4).
The other product is water.

Mole ratio: 1 mol Ca(OH)2 reacts with 2 mol HCl (Chapter 7). So 0.010 mol Ca(OH)2 needs 0.020 mol HCl.

Note: this answer is my own working, checked by atom counting (check).