If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

# Titration of a strong acid with a strong base (continued)

Calculating the pH at and after the equivalence point for titration of strong acid, hydrochloric acid, with strong base, NaOH.  Created by Jay.

## Want to join the conversation?

• Is it right to use the Hasselbach equation pH= pKa + log([conj. base]/[acid]) ?
• Nope! Since we are mixing a strong base with a strong acid, at no point do we have a buffer (which would require a weak acid plus its conjugate base, or a weak base plus its conjugate acid). Since we don't have a buffer we can't use the Henderson-Hasselbach equation.
• What is the pH level of soil from the Sahara desert?
• Why does the pH increase so much from only ,0002L of the hydroxide ion solution?
• Remember that pH is a log scale so the change will "appear" to happen fast because each unit is changing by a factor of 10. In a titration what you are basically doing is trying to find the number of either H+ or OH- in the solution by adding a known concentration of the opposite so at pH 7 the number of OH- and H+ are equal, the solution is neutral. Once you hit that point a small change in the amount of OH- will rapidly increase the pH because the solution you're adding is very concentrated and there is no H+ left to react with the OH-'s you're adding.
• Why does Dave insert the negative sign on the top of the O instead of at the end of OH?
• It doesn't really matter where you place the negative charge on the OH species. OH is so strongly held together that the negative charge belongs to the OH species and not to any individual atom (the O will be more polar)
• I was wondering why we couldnt use MV=MV in this situation? For the past videos I have been able to use MV=MV to find the concentration and use the pH equation to find pH. With this problem, I set it up as (20)(.5)=(x)(40) and got x=.25M
The -log(.25) is not 7. Can you please tell me why MV=MV would not work to get the answer for this question?
• Several problems:
The MV=MV formula measures the original, not the final values. You are being asked the final amount.
You are mixing and matching units. You must use liters, not mL for pH computations.
Since you are mixing together the two substances, the final volume will be the sum of the two volumes.
The chemicals are reacting, so you no longer have what you started with. The H⁺ and OH⁻ ions are combining with each other, so that they no longer exist.
You are not taking into account the amount of H⁺ that naturally exists in water.
• What does the equivilance pt. mean?
• The equivalence point is where the moles of acid, and the moles of base in a titration are equal to each other in the solution.
• Is there any way to calculate mathemetically that the pH becomes 7 when 20 ml of NaOH neutralizes 20 ml of HCl?
• When you have added exactly the same number of moles of NaOH as there are moles of HCl, then they will have reacted together to give water. So you have no excess acid, nor any excess base. All you have is the hydronium and hydroxide ions resulting from the autoionisation of water. Furthermore, these two concentrations will be equal to each other.

At 25 C, Kw = 1.0 x 10^-14. Kw = [H3O+][OH-] = [H3O+]^2 because the two ion concentrations are equal. Taking the square root, [H3O+] = 1.0 x 10^-7. Take the negative log of this and you find that the pH is 7.0.
• How do we know that all the hydrogen ions and hydroxide ions will react to form water? Shouldn't some of them dissociate back into hydrogen ions and hydroxide?
• There is an equilibrium between the (OH- & H+) and H2O. Some will continuously dissociate while others will reform water.
• I got pH=11.395773947 for problem d) which is slightly off. During my calculation, I didn't rounded up randomly. What caused this? If I write a slightly wrong answer like this on an exam, I am going to get it wrong.. right? How do I avoid slightly wrong answers like this?