Hi Ultima,
Thanks for taking the time.
You're right on the limitations of the approximation
approach.
So I did both the approximation (pH = 1.63) and quadratic method
(pH = 3.13) and posted only the approximation approach.
The question was actually from a JC worksheet, hence my doubts
was if it was deviating from the requirements of not needing
quadratic equations.
If I could squeeze in another chemical equilibrium question.
This was described to me verbally, and so I am not sure if it
contains sufficient info. Still stumped after thinking for
while.
Chemical
Equilibrium
Sulfur dioxide and oxygen reacts to form sulfur trioxide
2SO2 + O2 <--> 2SO3 Kp = 0.13 atm
Some SO3 was added to a container and equilibirum was
reached.
If P is the total pressure at any time, show
that intial pressure of SO3 is (P minus Partial
pressure of O2)
The flawed question both contradicts and proves itself. It
implies that at initial, only SO3 was present. Hence the
initial pressure of SO3 = P - 0 = P, since P is
(ridiculously) defined as total pressure at
any time, hence the
question proves itself and there's no need for any working.
I'd advise you to advise your students not to bother chasing the
dodgy and the dubious. Many JC prelim paper questions are
so problematic (every year, there will always be many JC
teachers who disagree with, openly sneer at and even
outright ridicule the prelim paper questions and/or mark schemes
from other JCs, and instruct their students to ignore the question
and/or mark scheme) that I routinely tell my
students to skip many of such questions entirely.
I prefer to set my own questions, and/or use more reliable
quality sources such as practice questions from various
University textbooks (which I use for teaching my
BedokFunland JC students).
Be discriminating when encountering dubious, dodgy questions, in
your (students') practice work.
