The solution of which one of the following will have pH less than 7?
- A.NaOH
- B.KCl
- C.FeCl₃
- D.NaCl
Correct answer
Explanation
The correct answer is C, FeCl₃. Ferric chloride is a salt of a weak base, iron(III) hydroxide, and a strong acid, hydrochloric acid, so its solution is acidic and has a pH below 7. When a salt dissolves, its ions may react with water; this is called hydrolysis. The Fe³⁺ ion is small and highly charged, so it pulls on water molecules and releases hydrogen ions, which makes the solution acidic. The general rule is simple: a salt of a strong acid and a strong base is neutral, a salt of a strong acid and a weak base is acidic, and a salt of a weak acid and a strong base, such as sodium carbonate, is basic. Ammonium chloride and copper sulphate solutions are acidic for the same reason. Option A is wrong because sodium hydroxide is a strong base, and its solution has a pH well above 7. Option B is wrong because potassium chloride comes from a strong acid and a strong base, so its solution is neutral, with pH 7. Option D is wrong because sodium chloride is also a neutral salt of HCl and NaOH. Exam tip: strong acid + weak base salt = acidic (FeCl₃, NH₄Cl); strong + strong = neutral.
Practice Questions
View allIn the joining of railway tracks, iron oxide is made to react with
- A.Aluminium
- B.Zinc
- C.Copper
- D.Tin
Show answer
Railway tracks are joined by the thermite process, in which aluminium powder reduces iron oxide to molten iron. The reaction is Fe₂O₃ + 2Al → 2Fe + Al₂O₃.
Read the full explanation →If atomic number of an element is 11, the correct name, symbol and valency are :
- A.Lithium, Li, 1
- B.Magnesium, Mg, 2
- C.Boron, B, 3
- D.Sodium, Na, 1
Show answer
The element with atomic number 11 is sodium, and with one electron in its outermost shell its valency is 1.
Read the full explanation →A car of mass 800 kg is travelling at a speed of 40 km/h on a straight road. A force of 1000 N is applied on it in the direction of its velocity for 12 s. Which one among the following is the final speed of the car?
- A.55 km/h
- B.60 km/h
- C.94 km/h
- D.104 km/h
Show answer
The force gives the car an extra 15 m/s, which is 54 km/h, and 40 + 54 = 94 km/h. By Newton's second law, acceleration a = F/m = 1000 N ÷ 800 kg = 1.25 m/s².
Read the full explanation →Consider the electromagnetic radiations having wavelengths 200 nm, 500 nm and 1000 nm. Which wavelength(s) of the following can make visual sensation to a human eye?
- A.200 nm and 500 nm
- B.500 nm and 1000 nm
- C.500 nm only
- D.200 nm and 1000 nm
Show answer
The human eye responds only to visible light, which lies roughly between 400 nm (violet) and 700 nm (red), and 500 nm is the only one of the three values in that range.
Read the full explanation →Bright light is found to emit from photographer's flashgun. This brightness is due to the presence of which one of the following noble gases?
- A.Argon
- B.Xenon
- C.Neon
- D.Helium
Show answer
A photographer's flash tube is filled with xenon, which gives an intense burst of white light when a high-voltage spark passes through it.
Read the full explanation →