Consider the following data on some weak acids and weak bases acid base formula formula methylamine CH2NH hydroxylamine HONH name name hypochlorous acid HCIO 3.0x 108 hydrofluoric acidHF 6.8 x 10 | 1,1 × 10 Use this data to rank the following solutions in order of increasing pH. In other words, select a ‘1’ next to the solution that will have the lowest pH, a ‘2’ next to the solution that will have the next lowest pH, and so on solution pH 0.1 M KF choose one 0.1 M NaCIO choose one 0.1 M HONH3Br choose one 0.1 M KCI choose one
The Correct Answer and Explanation is :
To rank the solutions in order of increasing pH, we analyze the effect of each salt on the pH of the solution.
Step 1: Understanding Each Salt
Each salt will either be neutral, acidic, or basic based on the nature of its conjugate acid/base.
- 0.1 M KF (Potassium Fluoride)
- KF is the salt of a weak acid (HF) and a strong base (KOH).
- Fluoride (F⁻) is the conjugate base of hydrofluoric acid (HF), which has a Ka = 6.8 × 10⁻⁴.
- Since HF is a weak acid, its conjugate base (F⁻) hydrolyzes in water, increasing the pH.
- The solution is basic.
- 0.1 M NaClO (Sodium Hypochlorite)
- NaClO is the salt of hypochlorous acid (HClO) and a strong base (NaOH).
- Hypochlorite (ClO⁻) is the conjugate base of hypochlorous acid (HClO), which has a Ka = 3.0 × 10⁻⁸.
- Since HClO is a very weak acid, ClO⁻ strongly hydrolyzes, making the solution basic (more basic than KF).
- 0.1 M HONH₃Br (Hydroxylammonium Bromide)
- HONH₃Br comes from hydroxylamine (HONH₂), a weak base (Kb = 1.1 × 10⁻⁸).
- HONH₃⁺ is the conjugate acid of HONH₂ and hydrolyzes in water, releasing H⁺, making the solution acidic.
- 0.1 M KCl (Potassium Chloride)
- KCl is a salt of a strong acid (HCl) and a strong base (KOH).
- Neither K⁺ nor Cl⁻ hydrolyzes in water, so the solution is neutral (pH = 7).
Step 2: Ranking in Increasing pH Order
- 0.1 M HONH₃Br (Acidic, lowest pH) → Rank 1
- 0.1 M KCl (Neutral, pH ≈ 7) → Rank 2
- 0.1 M KF (Weakly basic, slightly above 7) → Rank 3
- 0.1 M NaClO (More basic due to weak acid conjugate, highest pH) → Rank 4
Final Ranking:
- 0.1 M HONH₃Br → 1 (lowest pH, acidic)
- 0.1 M KCl → 2 (neutral)
- 0.1 M KF → 3 (slightly basic)
- 0.1 M NaClO → 4 (most basic, highest pH)
Thus, the correct order from lowest to highest pH is:
HONH₃Br < KCl < KF < NaClO.