H+ conc shortcut

[embrojam]

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Can we only use the shortcut [H+]=sqr root(Ka x [Ha]) when it is a monoprotic weak acid? What about a polyprotic weak acid?

 

[DAT Destroyer]

Can we only use the shortcut [H+]=sqr root(Ka x [Ha]) when it is a monoprotic weak acid? What about a polyprotic weak acid?

I do not recommend any short cuts. Understand what you are doing. I have set up these problems for you with the easiest possible solution........especially if there is variation on a problem. Often times, a silly formula can get you into trouble. For the DAT, the only polyprotic acids I would worry about is H2SO4... ( weak acids that are polyprotic would usually need the quadratic equation ) ...approach it by simply using the pH = - log H+......just remember that it is diprotic,,,,,,hence if they gave you 0.1M......plug it in as - log 0.2.

Hope this helps.

Dr. Romano

 

[molarmom]

Can we only use the shortcut [H+]=sqr root(Ka x [Ha]) when it is a monoprotic weak acid? What about a polyprotic weak acid?

I know what you're referring to - the shortcut that Chad talks about. Yes, this shortcut only works with monoprotic acids, not diprotic.

 

[sookstall]

Couldn't you use 2x for [HA] and still have the shortcut work?

 

[Dr. Dan]

Yes, Chad's shortcut will still work if you double the concentration of HA in the case of H2SO4. Best thing to do would be to find a diprotic acid problem and apply both strategies and see if you come out with the same answer. Also, understanding why and how it works is the most important thing.