Q4. Using the continuum model with water’s relative permittivity taken as 80, the attraction between two ions is how many times weaker than in a vacuum at the same distance? If the force between two singly charged ions 0.50 nm apart in a vacuum is 9.2 × 10−10 N, what is it in water at the same distance?
The force is divided by the dielectric constant, 80 for water: 9.2 × 10−10/80 ≈ 1.2 × 10−11 N, 80 times weaker in this simplified bulk-medium model. At a 0.50 nm separation, molecular hydration means this is an estimate rather than an exact microscopic force.
Q5. Ammonia dissolves very well in water, yet its solution is a poor conductor. Explain, writing the ionic equilibrium.
Ammonia is a weak electrolyte: NH3(aq) + H2O(l) ⇌ NH4+(aq) + OH−(aq) lies far to the left, so most of the dissolved ammonia stays as NH3 molecules and only a few ions carry the current.
Q6. Why is the bare proton, H+, never found free in water?
It is extremely small (radius about 10−15 m) with a very intense electric field, so it binds at once to a lone pair on a water molecule, giving H3O+, which is further hydrated to ions such as H9O4+.
Q7. A 0.20 M solution of a weak base BOH is 1.0% ionised. Find [OH−], [B+] and [BOH] at equilibrium.
Ionised: 0.010 × 0.20 = 2.0 × 10−3 M, so [OH−] = [B+] = 2.0 × 10−3 M and [BOH] = 0.20 − 0.002 = 0.198 M (≈ 0.20 M).
Q8. In what sense is a saturated solution of a sparingly soluble salt, such as silver chloride in contact with the solid, an ionic equilibrium?
Solid AgCl dissolves and its ions recombine at equal rates: AgCl(s) ⇌ Ag+(aq) + Cl−(aq). This equilibrium between the solid and its ions in solution is treated later in the unit through the solubility product. AgCl is still a strong electrolyte: the little that dissolves is completely dissociated. It is the solubility that is small, not the ionisation.