The solution of which one of the following will have pH less than 7?
- (a)NaOH
- (b)KCl
- (c)FeCl₃
- (d)NaCl
Correct — C, FeCl₃. Ferric chloride is the salt of a strong acid, HCl, and a weak base, Fe(OH)₃. When it dissolves, the chloride ion is the leftover of a strong acid and does nothing, but the small, highly charged Fe³⁺ ion pulls water molecules in around itself and forces them to give up protons. That cationic hydrolysis releases H⁺ into the solution, so the concentration of hydrogen ions rises above that of pure water and the pH falls below 7. The same reasoning is why an aqueous solution of copper sulphate turns litmus red even though the crystals themselves are a neutral salt.
- (a)NaOH — Sodium hydroxide is a strong base. It dissociates completely to give OH⁻, so its solution is strongly alkaline and its pH lies far above 7, not below it.
- (b)KCl — Potassium chloride comes from a strong base, KOH, and a strong acid, HCl. Neither of its ions hydrolyses, so the solution stays neutral at a pH of about 7.
- (d)NaCl — Common salt is likewise the child of a strong acid and a strong base. Na⁺ and Cl⁻ simply stay hydrated in solution without exchanging protons with water, so the pH remains close to 7.
A salt need not be neutral in water. Its behaviour depends on the strength of the acid and the base it came from. Strong acid with strong base gives a neutral solution; strong acid with weak base gives an acidic one, because the cation hydrolyses and frees H⁺; weak acid with strong base gives a basic one, because the anion takes a proton from water and frees OH⁻; and weak acid with weak base depends on which of the two is the weaker.
The quickest route through the options is to split them into the odd one and the family. NaOH is not a salt at all and is plainly alkaline, so it cannot be the answer to a question asking for pH below 7. KCl and NaCl are the classic strong-acid, strong-base pair and are neutral, and they are placed together precisely so that a candidate who spots that they behave identically realises neither can be the unique answer. That leaves FeCl₃, and the reason it stands apart is that Fe(OH)₃ is a weak base while HCl is strong. A good rule of thumb is that salts of small, highly charged metal ions such as Fe³⁺, Al³⁺ and Cu²⁺ almost always give acidic solutions.
- The pH scale runs 0 to 14 at 25 °C, with 7 neutral, below 7 acidic and above 7 basic.
- FeCl₃ is a salt of the strong acid HCl and the weak base Fe(OH)₃, and hydrolyses to give an acidic solution.
- NaCl and KCl come from strong acid and strong base pairs, so neither ion hydrolyses and both solutions are neutral.
- Sodium carbonate is the mirror image of ferric chloride — a salt of a weak acid and a strong base, so its solution is alkaline.
Only the salt with a weak-base parent turns its solution acidic, which makes FeCl₃ the answer.
- Assuming every salt is neutral because the word 'salt' suggests neutralisation.
- Picking NaOH by reflex because it is the only obviously reactive substance on the list, without reading whether the question wants acidic or basic.
- Forgetting that two of the four options behave identically, which by itself rules both of them out in a single-answer question.
Asked as a one-from-four recall of salt hydrolysis, usually with one strong base planted among three salts.
An aqueous solution of copper sulphate is acidic in nature because the salt undergoes
- (a) dialysis
- (b) electrolysis
- (c) hydrolysis
- (d) photolysis
Answer(c) hydrolysis
Names the exact mechanism this question relies on. Copper sulphate, like ferric chloride, is a strong-acid, weak-base salt whose metal ion hydrolyses and releases hydrogen ions.
The pH value of Milk of Magnesia is approximately
- (a) Zero
- (b) 7
- (c) 10
- (d) 14
Answer(c) 10
Tests the same scale from the alkaline end. Milk of magnesia is a suspension of magnesium hydroxide, a weak base, which is why its pH sits near 10 rather than at the extreme of 14.
- practice — not a real PYQ
An aqueous solution of sodium carbonate is basic because
- (a)both its ions hydrolyse to release hydrogen ions
- (b)the carbonate ion takes a proton from water and releases hydroxide ions
- (c)sodium carbonate does not dissolve in water
- (d)the sodium ion hydrolyses to release hydroxide ions
Answer(b) the carbonate ion takes a proton from water and releases hydroxide ions — it is the anion of the weak acid H₂CO₃, so it hydrolyses and pushes the pH above 7.
- practice — not a real PYQ
Which one of the following salts will give a neutral aqueous solution?
- (a)Ammonium chloride
- (b)Sodium acetate
- (c)Potassium nitrate
- (d)Aluminium chloride
Answer(c) Potassium nitrate — it comes from the strong base KOH and the strong acid HNO₃, so neither ion hydrolyses; the other three each have one weak parent.