Question from the 2003 HSC (pH) (1 Viewer)

[Sober]

(resolved) Question from the 2003 HSC (pH)

Resolved

This is question 8, multiple choice from the 2003 HSC:

A sulfuric acid solution has a concentration of 5 x 10^-4 mol L^-1.

What is the pH of this solution, assuming the acid is completely ionised?'

(A) 3.0
(B) 3.3
(C) 3.6
(D) 4.0

Entering - log (5x10^-4) into my calculator yields 3.301 so I expect B to be correct but the solutions say it should be A, what the hell?

 

[[Damo]]

It is assumed that the acid is completetly ionised. This tells us that the hydrogen ion concentration is equal to the concentration of tha cid (100% ionisation). So the pH is

pH= -log [H+]
= -log [5 x 10-4 mol L-1]
= 3.301029996
= 3.3

Hence, the solution is B

 

[[Damo]]

My mistake,
since Sulfuric acid is diprotic (2 hydrogens)
you MUST multiply the concentration by 2
so : pH = -log [2 x 5 x 10-4] =3

 

[Sober]

My mistake,
since Sulfuric acid is diprotic (2 hydrogens)
you MUST multiply the concentration by 2
so : pH = -log [2 x 5 x 10-4] =3

Thankyou, until that insight I was suspecting that they were expecting us to round the value to the same number of significant figures as the acid concentration was provided, but that would have been terribly unfair.
Cheers.