Includes the "Assumption" teachers tell you to make. How do you find the pH of a weak acid solution? This video goes through the whole calculation. Just fill in your numbers! New website! www.chemistnate.com
If my professor were as entertaining as you while he teaches, maybe I would be doing better in Chem! Thank you sir. You're videos are much appreciated.
Every teacher has some different rule. I hate that. When I teach, I use 100. When I'm doing real chemistry, I just always assume I can make the assumption, and then once I'm done if the value of "x" that I get is too close to the initial concentration, I redo the calculation with quadratic formula.
You'll only get the same answer if 1) There's no A- or H+ in the solution before you dissolve your acid ("common ion effect") 2) Your acid has only one H+ on it (ie it works for HCN but not for H2C2O4 or H3PO4)
Here's a simpler method we were taught in school. We use this formula: √(Ka X C) So, √(Ka X C) = √(1.8 X 10^-5 X 0.1) = √(0.0000018) = 0.001342 Find -log. So, -log 0.001342 = 2.87. Thus pH of a 0.1M solution of Acetic acid with Ka of 1.8 X 10^-5 = 2.87. Finito! :)
My teacher advised us to only spend time trying the assumption rule if the exponent on the equilibrium constant is 10^-5 or smaller bc those numbers are more likely to be insignificant enough to omit the x in the denominator. :)
You do it a lot different to how we do it in England We assume [H⁺] = [A⁻] when water dissociated So we can rewrite the equation as Kw = [H⁺]² / [HA] Then if you rearrange you get [H⁺] = SQRT(Kw*[HA]) Seems a lot easier and it skips out steps like the 100 rule and use of ICE tables Am I missing something? You still get the same answer, is there any cases where this method will not work?
Yes its good. But can somebody tell me that how is Acetic acid desolvable in water being a covalent compound? Well I know that acids do that, but the fact is that covalent compounds dont.
The oxygen atoms in the carbonyl and the hydroxy group are more electronegative than the methyl group attached to the central carbon. This creates a dipole moment. Anything with a dipole moment is polar and will be dissolved in water because water is polar. Remember, like dissolves like!
You could, but once you practice the assumption method enough, taking the extra few seconds to do it and see if it'll work will be much quicker than the time it takes to type the quadratic into your calculator. It really can make a difference, and it's good to show flexibility and cognitive understanding of multiple methods in chemistry (as well as other fields) anyway.
