Oct 28 2008, 11:00 AM
QUOTE(work_tgr @ Oct 27 2008, 09:10 AM)
Not necessary.
There is no obvious statement to tell SPM students about what exactly makes the value of pH. Of course, it must be related with the content of OH- and H+.
So, week acid such as ethanoic acid of 0.1moldm^-3 could be higher than 1 because it doesn't dissociate completely.
But, if the molarity increase to 1moldm^-3 or higher, it could be pH1.
Difference between weak acids and strong acids.There is no obvious statement to tell SPM students about what exactly makes the value of pH. Of course, it must be related with the content of OH- and H+.
So, week acid such as ethanoic acid of 0.1moldm^-3 could be higher than 1 because it doesn't dissociate completely.
But, if the molarity increase to 1moldm^-3 or higher, it could be pH1.
Strong acids: 100% dissociation of hydrogen ions
Weak acids: less than 1% dissociation of hydrogen ions
To measure pH, we need to know the concentration of hydrogen ions in its solution, not the concentration of acid molecules/anions.
In strong monoprotic acids, [hydrogen ions] = [anion]. (ratio = 1:1)
In strong diprotic acids, [hydrogen ions] = 2 x [anion]. (ratio = 2:1)
In weak acids (whether mono-, di-, or triprotic), [hydrogen ions] << [acid molecules]. (ratio = less than 0.01 :1)
The larger the concentration of hydrogen ions, the smaller the pH value is. Its converse is also true.
But,
let weak acid X of 5 mol dm^-3 has 2% dissociation of hydrogen ions.
[hydrogen ions present in the solution] = 5 x 2% = 0.1 mol dm^-3 <---- pH=1
If I had compared X with a strong acid of 5 mol dm^-3, the pH of the strong acid would have been much lower than 1.
There's no need to worry about weak acids with pH=1. This has been excluded by SPM syllabus. This is useful for foundation in science/engineering, though.
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