Which one of the following statements is NOT correct? (a) A scale measuring hydrogen ion concentration in a solution is called pH scale (b) The higher the hydrogen ion concentration in a solution, higher is its pH (c) We can measure pH generally from 0 to 14 on a pH scale (d) The 'p' in pH stands for 'potenz' in German meaning power
- (a)A scale measuring hydrogen ion concentration in a solution is called pH scale
- (b)The higher the hydrogen ion concentration in a solution, higher is its pH
- (c)We can measure pH generally from 0 to 14 on a pH scale
- (d)The 'p' in pH stands for 'potenz' in German meaning power
Correct — B, the statement 'the higher the hydrogen ion concentration, the higher its pH' is the one that is NOT correct. pH is defined as pH = -log10[H+], so it moves opposite to the hydrogen-ion concentration. When [H+] goes up the solution is more acidic and its pH goes DOWN, not up. The inverse relationship is the whole point of the logarithmic scale, so statement (b) has it backwards.
- (a)A scale measuring hydrogen ion concentration in a solution is called pH scale — This is correct, so it cannot be the answer to a 'NOT correct' question. The pH scale is precisely a measure of the hydrogen-ion concentration (activity) of a solution.
- (c)We can measure pH generally from 0 to 14 on a pH scale — This is correct. The everyday pH scale runs 0 to 14, with 7 neutral, below 7 acidic and above 7 basic, so it is a true statement and not the exception.
- (d)The 'p' in pH stands for 'potenz' in German meaning power — This is correct. The 'p' comes from the German 'potenz' (power/exponent), reflecting that pH is the negative power of ten of the hydrogen-ion concentration, so the statement is true.
pH is a logarithmic measure of the hydrogen-ion concentration of a solution, pH = -log10[H+]. Because of the minus sign and the logarithm, pH falls as [H+] rises. Each whole-number drop in pH means ten times more hydrogen ions.
The question mixes three true facts with one reversed one. The safe method is to test each statement against pH = -log10[H+]. Statement (b) claims pH rises with [H+]; the formula says the opposite, so (b) is the incorrect statement being sought.
- pH = -log10[H+], so pH and hydrogen-ion concentration change in opposite directions.
- Lower pH means more hydrogen ions and stronger acidity; higher pH means fewer and more basic.
- The common scale runs 0 to 14, with 7 neutral at 25 degrees C.
- A change of one pH unit is a tenfold change in hydrogen-ion concentration.
- Reading pH as directly proportional to acidity rather than inversely.
- Forgetting each pH unit is a tenfold change in [H+].
- Assuming pH cannot go below 0 or above 14 (it can, for very concentrated solutions).
NDA/UPSC list several pH statements and ask which is NOT correct — the reversed [H+]-versus-pH relationship is the classic planted error.
What is the pH level of blood of a normal person?
- (a) 4.5 - 4.6
- (b) 6.45 - 6.55
- (c) 7.35 - 7.45
- (d) 8.25 - 8.35
Answer(c) 7.35 - 7.45
Same pH concept applied to a real fluid. Recognising that human blood is held in the narrow 7.35-7.45 band uses the same 0-14 pH scale (7 neutral, above 7 slightly basic) that this NDA statement question is built on.
Human body works in the pH range of:
- (a) 6.8 – 7.2
- (b) 7.0 – 7.8
- (c) 6.5 – 7.5
- (d) 7.5 – 8.0
Answer(b) 7.0 – 7.8
An earlier NDA GAT item on the same pH scale, here asking for the body's working pH band — testing that the student reads pH as a measure of acidity/basicity around neutral 7.
- practice — not a real PYQ
If the hydrogen-ion concentration of a solution increases ten times, its pH
- (a)increases by 1
- (b)decreases by 1
- (c)stays the same
- (d)doubles
Answer(b) decreases by 1 — since pH = -log[H+], a tenfold rise in [H+] lowers pH by one unit.
- practice — not a real PYQ
A solution of pH 3 compared with a solution of pH 5 is
- (a)10 times more acidic
- (b)100 times more acidic
- (c)2 times more acidic
- (d)less acidic
Answer(b) 100 times more acidic — two pH units lower means 10 x 10 = 100 times more hydrogen ions.