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edited Mar 1, 2022 at 9:48

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I'm studying for chem olympiad and have a question about a problem from a past year's test:

A pure sample of a monoprotic acid is dissolved in water. The sample is titrated with sodium hydroxide solution. At the point where $20.0$ mL of the NaOH solution has been added, the pH is $4.15$. The phenolphthalein endpoint of the titration is observed when $50.0$ mL of NaOH have been added. What is the pKap$K_{\mathrm{a}}$ of the acid?

Here are my thoughts:

Since pH = $4.15$, pOH = $9.85$ $\implies$ $[OH^-] = 10^{-9.85}M$$\pu{[OH^-] = 10^{-9.85}M}$, which I think must also be the molarity of NaOH (this might be wrong).

I don't understand otherwise how to solve this question. I have $K_a = \frac{ [H^+]^2}{[HA]}$$K_a = \frac{ \mathrm{[H^+]^2}}{\mathrm{[HA]}}$ as concentration of protons equals concentration of conjugate base. I guess I have $[H^+] = 10^{-4.15}$$\pu{[H^+] = 10^{-4.15}}$ from the pH but I'm not sure if that's right since we added $20$ mL of NaOH first, which would have changed the pH from what it originally was.

How would I solve the question?

I'm studying for chem olympiad and have a question about a problem from a past year's test:

A pure sample of a monoprotic acid is dissolved in water. The sample is titrated with sodium hydroxide solution. At the point where $20.0$ mL of the NaOH solution has been added, the pH is $4.15$. The phenolphthalein endpoint of the titration is observed when $50.0$ mL of NaOH have been added. What is the pKa of the acid?

Here are my thoughts:

Since pH = $4.15$, pOH = $9.85$ $\implies$ $[OH^-] = 10^{-9.85}M$, which I think must also be the molarity of NaOH (this might be wrong).

I don't understand otherwise how to solve this question. I have $K_a = \frac{ [H^+]^2}{[HA]}$ as concentration of protons equals concentration of conjugate base. I guess I have $[H^+] = 10^{-4.15}$ from the pH but I'm not sure if that's right since we added $20$ mL of NaOH first, which would have changed the pH from what it originally was.

How would I solve the question?

I'm studying for chem olympiad and have a question about a problem from a past year's test:

A pure sample of a monoprotic acid is dissolved in water. The sample is titrated with sodium hydroxide solution. At the point where $20.0$ mL of the NaOH solution has been added, the pH is $4.15$. The phenolphthalein endpoint of the titration is observed when $50.0$ mL of NaOH have been added. What is the p$K_{\mathrm{a}}$ of the acid?

Here are my thoughts:

Since pH = $4.15$, pOH = $9.85$ $\implies$ $\pu{[OH^-] = 10^{-9.85}M}$, which I think must also be the molarity of NaOH (this might be wrong).

I don't understand otherwise how to solve this question. I have $K_a = \frac{ \mathrm{[H^+]^2}}{\mathrm{[HA]}}$ as concentration of protons equals concentration of conjugate base. I guess I have $\pu{[H^+] = 10^{-4.15}}$ from the pH but I'm not sure if that's right since we added $20$ mL of NaOH first, which would have changed the pH from what it originally was.

How would I solve the question?

added 14 characters in body

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edited Mar 1, 2022 at 9:42

Maurice

30k
3
32
64

I'm studying for chem olympiad and have a question about a problem from a past year's test:

A pure sample of a monoprotic acid is dissolved in water. The sample is titrated with sodium hydroxide solution. At the point where 20.0$20.0$ mL of the NaOH solution has been added, the pH is 4.15$4.15$. The phenolphthalein endpoint of the titration is observed when 50.0$50.0$ mL of NaOH have been added. What is the pKa of the acid?

Here are my thoughts:

Since pH = 4.15$4.15$, pOH = 9.85 $9.85$ $\implies$ $[OH-] = 10^{-9.85}M$$[OH^-] = 10^{-9.85}M$, which I think must also be the molarity of NaOH (this might be wrong).

I don't understand otherwise how to solve this question. I have $K_a = \frac{ [H^+]^2}{[HA]}$ as concentration of protons equals concentration of conjugate base. I guess I have $[H^+] = 10^{-4.15}$ from the pH but I'm not sure if that's right since we added 20Ml$20$ mL of NaOH first, which would have changed the pH from what it originally was.

How would I solve the question?

I'm studying for chem olympiad and have a question about a problem from a past year's test:

A pure sample of a monoprotic acid is dissolved in water. The sample is titrated with sodium hydroxide solution. At the point where 20.0 mL of the NaOH solution has been added, the pH is 4.15. The phenolphthalein endpoint of the titration is observed when 50.0 mL of NaOH have been added. What is the pKa of the acid?

Here are my thoughts:

Since pH = 4.15, pOH = 9.85 $\implies$ $[OH-] = 10^{-9.85}M$, which I think must also be the molarity of NaOH (this might be wrong).

I don't understand otherwise how to solve this question. I have $K_a = \frac{ [H^+]^2}{[HA]}$ as concentration of protons equals concentration of conjugate base. I guess I have $[H^+] = 10^{-4.15}$ from the pH but I'm not sure if that's right since we added 20Ml of NaOH first, which would have changed the pH from what it originally was.

How would I solve the question?

I'm studying for chem olympiad and have a question about a problem from a past year's test:

A pure sample of a monoprotic acid is dissolved in water. The sample is titrated with sodium hydroxide solution. At the point where $20.0$ mL of the NaOH solution has been added, the pH is $4.15$. The phenolphthalein endpoint of the titration is observed when $50.0$ mL of NaOH have been added. What is the pKa of the acid?

Here are my thoughts:

Since pH = $4.15$, pOH = $9.85$ $\implies$ $[OH^-] = 10^{-9.85}M$, which I think must also be the molarity of NaOH (this might be wrong).

I don't understand otherwise how to solve this question. I have $K_a = \frac{ [H^+]^2}{[HA]}$ as concentration of protons equals concentration of conjugate base. I guess I have $[H^+] = 10^{-4.15}$ from the pH but I'm not sure if that's right since we added $20$ mL of NaOH first, which would have changed the pH from what it originally was.

How would I solve the question?