Sciencemadness Discussion Board

weak acids

copperastic - 18-3-2014 at 12:27

Hi, could somebody please explian to me the way of calculating the pH and pOH of weak acids and alkalis? I know that with strong acids and alkalis its pH (or pOH if its a alkali)= -Log10of molarity of H3O ions.
Thanks

[Edited on 18-3-2014 by copperastic]

HgDinis25 - 18-3-2014 at 12:45

In weak acids you do it the exact same way. The difference between strong and weak acids is that strong acids will completly ionize in water. For instance, if you have 1 mol of HCl, it will produce 1 mol of H3O ions.

Now with weak acids, that's a different story.
For instance, let's think about acetic acid, wich is a weak acid. When it ionizes in water, it establishes an equilibrium:
CH3COOH(aq) + H2O(l) --> CH3COO(aq) + H3O(aq)
Now, because this is a weak acid (and therefore establishes an equilibrium), the following reaction is also present:
CH3COO-(aq) + H3O+(aq) --> CH3COOH(aq) + H2O(l)

So, how do you calculate de pH? You need the acidity constant. The acidity constant of a weak acid, is simply the equilibrium constant of the reaction of said acid with water.

In the case of Acetic Acid, the acidity constant is 4.76. Now, you simply calculate the hydronium concentration like in any other equilibrium:
Kc = ([H3O+] x [CH3COO-]) : ([CH3COOH])

[Edited on 18-3-2014 by HgDinis25]

DraconicAcid - 18-3-2014 at 12:46

The pH of any aqueous solution is the negative log of the hydronium ion concentration(*). The trick is to determine that concentration. For a strong monoprotic acid, this concentration is the concentration of the acid. For a weak acid, you have to set up an ICE table to find the equilibrium concentration of the ions involved.


*- ignoring activities, ionic strength, etc.

copperastic - 18-3-2014 at 13:17

Ok thanks

blogfast25 - 19-3-2014 at 05:45

Quote: Originally posted by DraconicAcid  
For a weak acid, you have to set up an ICE table to find the equilibrium concentration of the ions involved.




Boh. ICE for something that simple? Although it boils down to the same thing, simply set up mass balances, the equilibrium constant equation, the water constant and a neutrality equation. Solve that system for [H<sub>3</sub>O<sup>+</sup>]. In the case of a simple, monoprotic weak acid that leads to a quadratic equation with two real roots.

Even simpler:

For HA(aq) + H2O(l) < === > H3O+(aq) + A-(aq), set up the equilibrium constant:

Ka = ([H3O+] x [A-]) / [HA]

Now assume that dissociation of the acid is small, but suppresses water dissociation which means in practical terms that [HA] ≈ Ca (Ca is the nominal molarity of acid) and [A-] ≈ [H3O+].

Insert into the equilibrium equation and rework to:

[H3O+] = SQRT(Ka x Ca) or pH = 1/2 pKa - 1/2 logCa

Adequate for most dilute solutions.


[Edited on 19-3-2014 by blogfast25]

learningChem - 20-3-2014 at 13:23

What's the macroscopic evidence of strong acids (and bases) being fully ionized in water?

HgDinis25 - 20-3-2014 at 13:28

Quote: Originally posted by learningChem  
What's the macroscopic evidence of strong acids (and bases) being fully ionized in water?


The pH. If you add 1 mol of a strong acid it will produce 1 mol of hydronium ions (this for monoprotic acids).
To check if a acid is strong you can calculate any number of moles of the acid and add them to water. Then take the amount of moles of acid you added and divide it by the volume of solution (you're calculating the concentration of Hydronium ions).
Then:
pH = -log[Hydronium]

After calculating the theorethical pH of the final solution just measure the pH of your solution and compare.

The same goes with bases, but this time, 1 mol of a strong base will generate one 1 mol of hydroxide ion.
And, at 25ºC, pH + pOH = 14. Use this to calculate the theorethical pH and measure it and compare.

blogfast25 - 20-3-2014 at 13:47

Quote: Originally posted by learningChem  
What's the macroscopic evidence of strong acids (and bases) being fully ionized in water?


Also, the dramatic effect of small additions of strong acid or base to pure water on its conductivity. Very pure water is a very poor conductor of electricity because it contains almost no ions.

rcfunker - 23-3-2014 at 16:40

What about those machines that calculate Ph, do they actually contain any acid that would rise the pressure and result in increased number on the machine in high Ph cases or...?
Quote: Originally posted by HgDinis25  
In weak acids you do it the exact same way. The difference between strong and weak acids is that strong acids will completly ionize in water. For instance, if you have 1 mol of HCl, it will produce 1 mol of H3O ions.

Now with weak acids, that's a different story.
For instance, let's think about acetic acid, wich is a weak acid. When it ionizes in water, it establishes an equilibrium:
CH3COOH(aq) + H2O(l) --> CH3COO(aq) + H3O(aq)
Now, because this is a weak acid (and therefore establishes an equilibrium), the following reaction is also present:
CH3COO-(aq) + H3O+(aq) --> CH3COOH(aq) + H2O(l)

So, how do you calculate de pH? You need the acidity constant. The acidity constant of a weak acid, is simply the equilibrium constant of the reaction of said acid with water.

In the case of Acetic Acid, the acidity constant is 4.76. Now, you simply calculate the hydronium concentration like in any other equilibrium:
Kc = ([H3O+] x [CH3COO-]) : ([CH3COOH])

[Edited on 18-3-2014 by HgDinis25]

HgDinis25 - 23-3-2014 at 17:14

What machines are you talking about? If you're talking about digital pH meters, they operate using 2 electrodes. One is sensible to H3O concentrations, the other isn't. A voltage is set between the two depending on how much H3O is in the solution and then it calculates the pH from there.