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VCE Stuff => VCE Science => VCE Mathematics/Science/Technology => VCE Subjects + Help => VCE Chemistry => Topic started by: Andiio on November 07, 2010, 04:14:47 pm

Title: Could someone please explain a few things?
Post by: Andiio on November 07, 2010, 04:14:47 pm

I've heard of the each pH 'step' is a dilution by a factor of 10.. but it's really 'fuzzy'; could someone please explain this and expand on it further?

Also,

just wondering about the difference between strength and concentration (also for dilute/weak) for an acid/base. Kinda forgot about all the ionisation stuff >_>

Thanks in advance!

Title: Re: Could someone please explain a few things?
Post by: scocliffe09 on November 07, 2010, 04:27:54 pm

pH=-log[H+]
What it doesn't say there ^, is that it's log to the base 10, which is why each step is a dilution by factor 10.
e.g. 1M HCl --> [H+] =1M --> pH=-log1=0 .... pH=0
0.1M HCl (dilution by factor 10) --> [H+]=0.1M --> pH=-log0.1=--1 ... pH = 1

so as you go from pH 0 to pH 1, you dilute by a factor of 10.


Ok Strong acids ionise (effectively) to completion. That means if you put HCl in water, you get
HCl + H2O --> Cl- + H3O+
i.e. there are (almost) zero actual HCl molecules floating around because they have all dissociated / ionised.

Weak acids only partially ionise. (i.e. have low equilibrium constants)
this means that say for HOCl
HOCl + H2O <--> OCl- + H3O+
In this solution, there will be some HOCl molecules still floating around as well as some that have ionised into OCl- and you can use the acidity constant to work that out.
Hope that helps