Quick question on pH and buffers

[SephirothXR]

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If I mix 25mL of 1M HCl and 25mL of 1M NaOH, the pH is 7 and there is an equivalent amount of moles of acid neutralized by moles of base. If I do the same thing except use 25 mL of 1M HF (a weak acid) and the same amount of NaOH, the pH will be above 7 but are they still both completely neutralized?

 

[Axon hillock]

If I mix 25mL of 1M HCl and 25mL of 1M NaOH, the pH is 7 and there is an equivalent amount of moles of acid neutralized by moles of base. If I do the same thing except use 25 mL of 1M HF (a weak acid) and the same amount of NaOH, the pH will be above 7 but are they still both completely neutralized?

Well basically it has to be neutralized. Assuming that you've added enough base to the solution then an equivalence point will be reach. The fact that the base dissociates more than the acid means the equivalence point will be above 7.

 

[rick ross]

If I mix 25mL of 1M HCl and 25mL of 1M NaOH, the pH is 7 and there is an equivalent amount of moles of acid neutralized by moles of base. If I do the same thing except use 25 mL of 1M HF (a weak acid) and the same amount of NaOH, the pH will be above 7 but are they still both completely neutralized?

Wouldn't it depend on what the pH of the HF is? The HF would have to take the NaOH down to a neutral pH. If not you would have to change the amount of HF in the mixture.

 

[Axon hillock]

Wouldn't it depend on what the pH of the HF is? The HF would have to take the NaOH down to a neutral pH. If not you would have to change the amount of HF in the mixture.

I'm not quite following what your asking. If we add a surplus of NaOH to a 1M solution of HF it will be neutralized completely assuming we've added enough NaOH.

 

[rick ross]

I'm not quite following what your asking. If we add a surplus of NaOH to a 1M solution of HF it will be neutralized completely assuming we've added enough NaOH.

Okay, well if the pH of the mixture was over 7 how could it be neutralized? That's what I'm trying to say.

 

[Axon hillock]

Okay, well if the pH of the mixture was over 7 how could it be neutralized? That's what I'm trying to say.

You need to add more acid then because you have too much OH- ions.

 

[salim271]

Its not neutralized, just think about it. A strong acid will be neutralized by a strong base (and vice versa) if they have the same molar concentrations because strong acids lose protons easily and in the most basic (hah, a pun,) sense a strong base accepts protons easily. HF is a weak acid, it will not give up its protons easily, so you need more NaOH to reach the equivalence point.

At the equivalence point, the solution is NOT a buffer. Look at a titration curve, if you add any acid or base at the equivalence point, what happens? The pH skyrockets either up or down. The strongest buffer solution that can be formed is in the area right before the equivalent point, that is where the slope is closest to 0 and the acid/base that is being titrated is resisting pH changes best.

 

[Liquidice07]

Its not neutralized, just think about it. A strong acid will be neutralized by a strong base (and vice versa) if they have the same molar concentrations because strong acids lose protons easily and in the most basic (hah, a pun,) sense a strong base accepts protons easily. HF is a weak acid, it will not give up its protons easily, so you need more NaOH to reach the equivalence point.

At the equivalence point, the solution is NOT a buffer. Look at a titration curve, if you add any acid or base at the equivalence point, what happens? The pH skyrockets either up or down. The strongest buffer solution that can be formed is in the area right before the equivalent point, that is where the slope is closest to 0 and the acid/base that is being titrated is resisting pH changes best.

I agree with you, but I think what he is getting it and now I am also wondering about is
Why is the pH above 7 at the equivalence point in this reaction? Theoretically, the # of moles of weak acid and strong base are equal, then why would the equivalence point be located at a pH above 7?

 

[BerkReviewTeach]

There are several misconceptions being perpetuated with this thread. I think much of it stems from the use of the word neutralize in general chemistry.

To neutralize something does not mean to make it pH = 7. To neutralize a base (strong or weak), you have to add an equal number of moles of strong acid. Likewise, to neutralize an acid (strong or weak), you have to add an equal number of moles of strong base. So neutralize refers to the quantity of the titrant, not the strength.

Second, if we mix equal mole quantities of strong acid and strong base, the pH is 7 given that they perfectly cancel one another and give water and an inert salt. NaOH + HCl leads to NaCl in H2O. But, if we mix a weak acid (such as HF) with an equal mole quantity of strong base, the pH is greater than 7 given that the base is stronger than the acid which forms water and a basic salt. NaOH + HF leads to NaF in H2O. This is why the pH at equivalence point is greater than 7 when a weak acid is titrated by a strong base. Likewise, the pH at equivalence point is less than 7 when a weak base is titrated by a strong acid.

To understand the topic discussed so far in this thread it is critical that you work from the definitions of the words (1) neutralized, (2) equivalence point, and (3) weak/strong.

 

[Axon hillock]

There are several misconceptions being perpetuated with this thread. I think much of it stems from the use of the word neutralize in general chemistry.

To neutralize something does not mean to make it pH = 7. To neutralize a base (strong or weak), you have to add an equal number of moles of strong acid. Likewise, to neutralize an acid (strong or weak), you have to add an equal number of moles of strong base. So neutralize refers to the quantity of the titrant, not the strength.

Second, if we mix equal mole quantities of strong acid and strong base, the pH is 7 given that they perfectly cancel one another and give water and an inert salt. NaOH + HCl leads to NaCl in H2O. But, if we mix a weak acid (such as HF) with an equal mole quantity of strong base, the pH is greater than 7 given that the base is stronger than the acid which forms water and a basic salt. NaOH + HF leads to NaF in H2O. This is why the pH at equivalence point is greater than 7 when a weak acid is titrated by a strong base. Likewise, the pH at equivalence point is less than 7 when a weak base is titrated by a strong acid.

To understand the topic discussed so far in this thread it is critical that you work from the definitions of the words (1) neutralized, (2) equivalence point, and (3) weak/strong.

I don't agree on your statement "To neutralize something does not mean to make it pH = 7." Neutralize should mean to make a solution neutral, right? This occurs at a pH of 7. I agree with everything else, including the reason for all the confusion.

 

[Axon hillock]

Its not neutralized, just think about it. A strong acid will be neutralized by a strong base (and vice versa) if they have the same molar concentrations because strong acids lose protons easily and in the most basic (hah, a pun,) sense a strong base accepts protons easily. HF is a weak acid, it will not give up its protons easily, so you need more NaOH to reach the equivalence point.

At the equivalence point, the solution is NOT a buffer. Look at a titration curve, if you add any acid or base at the equivalence point, what happens? The pH skyrockets either up or down. The strongest buffer solution that can be formed is in the area right before the equivalent point, that is where the slope is closest to 0 and the acid/base that is being titrated is resisting pH changes best.

The half equivalence point is when the solution is considered buffered. This is when the solution can best resist pH changes.

 

[salim271]

I don't agree on your statement "To neutralize something does not mean to make it pH = 7." Neutralize should mean to make a solution neutral, right? This occurs at a pH of 7. I agree with everything else, including the reason for all the confusion.

'Neutralize' refers to the analyte/what is being titrated, not the pH of the solution.

The pH at the equivalence point of a weak monoprotic acid/base titrated with a strong acid/base is determined by the equilibrium of the conjugate. At the equiv point all of the base has been used to neutralize the analyte, all that is left is the conjugate which equilibrates like so:

A + H2O <=> HA + OH

Hence there is OH ions in the solution when a weak acid is titrated with a strong base to the equivalence, even though the strong base and weak acid technically 'cancelled out' at the equiv point. The reverse is true for a weak base and strong acid titration.

 

[Axon hillock]

'Neutralize' refers to the analyte/what is being titrated, not the pH of the solution.

The pH at the equivalence point of a weak monoprotic acid/base titrated with a strong acid/base is determined by the equilibrium of the conjugate. At the equiv point all of the base has been used to neutralize the analyte, all that is left is the conjugate which equilibrates like so:

A + H2O <=> HA + OH

Hence there is OH ions in the solution when a weak acid is titrated with a strong base to the equivalence, even though the strong base and weak acid technically 'cancelled out' at the equiv point. The reverse is true for a weak base and strong acid titration.

Wow. I'm such an idiot for not seeing what was wrong with my logic.

 

[salim271]

Wow. I'm such an idiot for not seeing what was wrong with my logic.

Considering neutral is the root of neutralize, its not a hard mistake to make. I only recently gained a new understanding of titrations because I'm taking analytical chem again due to it not transferring when I moved to a new uni.

 

[BerkReviewTeach]

I don't agree on your statement "To neutralize something does not mean to make it pH = 7." Neutralize should mean to make a solution neutral, right? This occurs at a pH of 7.

This is an easy mistake to make and one of the most common mistakes in this section. It's natural to think that neutralize means to make neutral in terms of pH, but the general chemists of the world decided to use the world more like a military term where a target is completely cancelled out (or neutralized). A neutral solution has a pH of 7, but a neutralized mixture does not necessarily have a pH of 7.

Wow. I'm such an idiot for not seeing what was wrong with my logic.

NOT an idiot at all. Trust me, after tutoring for nearly ten years, I've seen this thought many times. The good news is that once people realize the difference, they become masters of titration.

 

[bambam02]

If you neutralize a weak acid like HF, all the H's are going to be neutralized by the strong base, but that leaves the F- species in solution which has basic properties, therefore making the solution basic.