Login

Search the forums now!

Welcome, Guest. Please login or register.

November 01, 2025, 01:03:44 pm
Pages: [1]   Go Down

Author Topic: Weak acid assumptions  (Read 947 times)  Share 

0 Members and 1 Guest are viewing this topic.

NE2000

  • Victorian
  • Part of the furniture
  • *****
  • Posts: 1221
  • living an alternate reality
  • Respect: +4
Weak acid assumptions
« on: November 11, 2009, 06:24:03 pm »
0
Is the [H3O+] = [A-] a weak acid assumption? Because I always had thought it was just because of the equation and would apply to any acid  :-\
Logged
2009: English, Specialist Math, Mathematical Methods, Chemistry, Physics

homghomg1

  • Victorian
  • Forum Regular
  • **
  • Posts: 70
  • Respect: +1
Re: Weak acid assumptions
« Reply #1 on: November 11, 2009, 06:30:35 pm »
0
hmm I don't see why it wouldn't apply to strong acids. Unless you have a situation where you're adding HCl so a NaCl solution or something like that, in that case [Cl-] would be greater.
Logged

Mao

  • CH41RMN
  • Honorary Moderator
  • Great Wonder of ATAR Notes
  • *******
  • Posts: 9181
  • Respect: +390
  • School: Kambrya College
  • School Grad Year: 2008
Re: Weak acid assumptions
« Reply #2 on: November 11, 2009, 06:55:06 pm »
0
The weak acid assumption is where [HA]equilibrium = [HA]initial (i.e. not much has ionised)

The [H3O+] = [A-] assumption is used for many calculations, basically stating H+ from acid is much greater than self-ionisation (10-7M).
Logged
Editor for ATARNotes Chemistry study guides.

VCE 2008 | Monash BSc (Chem., Appl. Math.) 2009-2011 | UoM BScHon (Chem.) 2012 | UoM PhD (Chem.) 2013-2015

kimtywong

  • Victorian
  • Adventurer
  • *
  • Posts: 21
  • Respect: +1
Re: Weak acid assumptions
« Reply #3 on: November 11, 2009, 07:08:51 pm »
0
yea, it's actually better to write what u're assuming when calculating ph of a weak acid.
that's what I've been doing since learning the stuff. Better be safe, in case they've decided to mark it harsh.

assuming: 1) [H3O+] = [A-]
              2) [HA] initial = [HA] equilibrium
Logged
Pages: [1]   Go Up