Question

PH curves - What is the relationship between endpoint, moles, and pH of the solution?

I have been studying pH curves, and have a few questions:

1. At the equivalence point, in the main 'vertical section' of the curve, is this the point where the moles of acid is equal to the moles of alkali in the flask?

2. At the equivalence point, is the pH always 7? If so, why, if not, why?

3. How do concentrations of the acid and alkali affect the position of the vertical section? Will a less concentrated alkali cause the vertical section to occur later because more alkali will need to be added before the equivalence point is reached?

4. Does a 0.5M HCl start at a higher pH on the Y-axis than a 1M HCl for example?

Thanks in advance. I'm quite a mathematical person, and would perhaps like to understand this topic in terms of molar quantities. Thank you!

Answer 1

1) Yes
2) Not always
3) Less concentrated causes a later vertical section
4) Technically yes but its weird

Explanation:

1) This is correct, when moles of acid are reacted with an equal amount of moles of a base that is the equivalence point

2) This is an interesting one. For strong acid strong base titrations the equivalence point will always be at 7. These reactions do so because the solutions completely dissociate. For example if HCl is placed in water, every molecule of HCl will completely split into `H^+` and `Cl^-` This is not always the case for every chemical, and as such these chemicals are known as "weak" acids or bases. With these chemicals they do not always split apart meaning that some atoms will hold together. This is expressed as a variable known as K.

In weak acid or base titrations a, a weak acid is typically titrated with a strong base, or a weak base is titrated with a strong acid. This is normally expressed through a simplified equation like this `HA + OH^(- ) -> A^- + H2O` with HA being the weak acid and OH being the strong base. Now lets say we have 1 mol of HA reacting with 1 mol of OH. This will create 1 mol of `A^-` and for now we don't care about water. From before we know that not all will separate since it is a weak acid, or some may separate and return back to HA. Because of this, the pH is shifted slightly up or down based on how likely the chemical is to shift back to an acid.

3) The pH graphs typically have the pH on the Y axis of the graph and volume of the base added on the X axis. The equivalence point occurs when moles of acid = moles of base. Because of this, if we had 1 mol of HCl and were titrating it with 0.1 M NaOH it would come out after quite a bit of volume added, as it would take 10 liters of solution to have 1 mol of NaOH added. On the other hand if we used 1 M NaOH it would take 1 liter to add in a mol.

4) Technically yes because `pH = -log(H^+)` meaning that pH is the negative log of the concentration of H+ ions in the solution. In theory you are completely right in that `-log (0.5) = 0.301` is higher than `-log(1) = 0` but the issue comes when we have concentrations much higher. For example `-log(6) = -0.778` Now the pH scale only goes from 0-14 so this can't exist, but it does. So higher concetrations have lower pH, but at much higher concentrations's the actual pH scale starts to fail, and other methods have to be used. If you want to learn more about negative pH here is more on the subject: https://www.thoughtco.com/is-a-negative-ph-possible-603653