Chemistry a level quetsion help!!!!

Explain how Oxygen is delivered to body cells via Haemoglobin.

There is a buffer system in the blood which maintains the pH between the healthy range of 7.35 and 7.45.

This is done by the carbonic acid equilibrium:

H2CO3 ⇌ HCO3- + H+

Ka for this equilibrium is 4.27x10-3

A person has a ratio of [HCO3-]:[H2CO3] of 8.5:1

Determine whether this person has healthy blood

Reply 1

Nitrotoluene

8

Original post by Saya260

Explain how Oxygen is delivered to body cells via Haemoglobin.
There is a buffer system in the blood which maintains the pH between the healthy range of 7.35 and 7.45.
This is done by the carbonic acid equilibrium:
H2CO3 ⇌ HCO3- + H+
Ka for this equilibrium is 4.27x10-3
A person has a ratio of [HCO3-]:[H2CO3] of 8.5:1
Determine whether this person has healthy blood

Hello Saya260!
Oxygen is delivered through hemoglobin, which binds to oxygen molecules in red blood cells. The oxygenated blood travels through the circulatory system, releasing oxygen from hemoglobin and circulating it into cells for use in respiration.
To determine if the person's blood is healthy, you have to use the Henderson-Hasselbalch equation:
pH = pKa + log([HCO3^-] / [H2CO3])
pKa = -log(Ka) = -log(4.27 x 10^-3) = 2.37
[HCO3-]:[H2CO3] ratio = 8.5:1
Now you can calculate pH:
pH = 2.37 + log(8.5/1) = 2.37 + 0.93 = 3.30 Note: log = log to base 10.
The calculated pH of 3.30 is below the healthy range of 7.35-7.45.
Therefore, the person's blood is extremely acidic and not healthy.
Ciao
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Sandro

My signature: "Regardless of where you may be, expressing gratitude is a universally cherished gesture."

Reply 2

Saya260

OP

10

Original post by Nitrotoluene

Hello Saya260!
Oxygen is delivered through hemoglobin, which binds to oxygen molecules in red blood cells. The oxygenated blood travels through the circulatory system, releasing oxygen from hemoglobin and circulating it into cells for use in respiration.
To determine if the person's blood is healthy, you have to use the Henderson-Hasselbalch equation:
pH = pKa + log([HCO3^-] / [H2CO3])
pKa = -log(Ka) = -log(4.27 x 10^-3) = 2.37
[HCO3-]:[H2CO3] ratio = 8.5:1
Now you can calculate pH:
pH = 2.37 + log(8.5/1) = 2.37 + 0.93 = 3.30 Note: log = log to base 10.
The calculated pH of 3.30 is below the healthy range of 7.35-7.45.
Therefore, the person's blood is extremely acidic and not healthy.
Ciao
The flag of Italy.png

Sandro

My signature: "Regardless of where you may be, expressing gratitude is a universally cherished gesture."

Yes true but everyone is saying 7.3

Reply 3

Nitrotoluene

8

Original post by Saya260

Yes true but everyone is saying 7.3

You are right.

I did some research online and found that the Ka value for the bicarbonate-carbonic acid equilibrium in human blood at physiological conditions (37°C) is about 6.3×10^-7.

There is therefore an error in the Ka in the text of the problem.

pH = pKa + log([HCO3^-]/[H2CO3])
pKa = -log(Ka) = -log (6.3×10^-7) = 6.2
[HCO3^-]:[H2CO3] ratio = 8.5:1
Now you can calculate pH:
pH = 6.2 + log(8.5/1) = 6.2+ 0.93 = 7.13 Note: log = log to base 10.
The difference between pH = 7.13 and pH = 7.30 is due to a possible Ka different from the one I found.

(edited 2 months ago)

Reply 4

TypicalNerd

18

Original post by Saya260

Explain how Oxygen is delivered to body cells via Haemoglobin.

There is a buffer system in the blood which maintains the pH between the healthy range of 7.35 and 7.45.

This is done by the carbonic acid equilibrium:

H2CO3 ⇌ HCO3- + H+

Ka for this equilibrium is 4.27x10-3

A person has a ratio of [HCO3-]:[H2CO3] of 8.5:1

Determine whether this person has healthy blood

The explain how oxygen is delivered to body cells via haemoglobin bit can be explained in a little more depth. Oxygen molecules can exchange with water ligands bound to the iron(II) centres in the haemoglobin molecule. This is reversible, so once the oxygen reaches somewhere it is needed, it can be exchanged with water and the oxygen can be used in respiration or whatever.

I think the Ka should read as 4.27 x 10^-7 mol dm^-3, since carbonic acid has a pKa of 6.35 ish at 25 C and this corresponds to a Ka of the order 10^-7.

The standard A level method of approaching this question type is not to use the Henderson-Hasselbalch equation as suggested by Nitrotoluene, but it does work. The A level way is as follows:

Ka = [H^+][HCO3^-]/[H2CO3] = [H^+] x [HCO3^-]/[H2CO3]

But since the ratio [HCO3^-] : [H2CO3] = 8.5 : 1, [HCO3^-]/[H2CO3] = 8.5/1 = 8.5

So Ka = 8.5[H^+] and so [H^+] = 5.023... x 10^-8 mol dm^-3 ==> pH = 7.30 (2 dp)

If the person's blood was 3.30 as calculated with the questionable Ka value, then they would be very dead indeed.

Edit for clarification: I have taken the Ka to be 4.27 x 10^-7 mol dm^-3 in the above calculations, rather than the dubious 4.27 x 10^-3 mol dm^-3 given in the question.

(edited 2 months ago)

Reply 5

TypicalNerd

18

Original post by Nitrotoluene

You are right.

I did some research online and found that the Ka value for the bicarbonate-carbonic acid equilibrium in human blood at physiological conditions (37°C) is about 6.3×10^-7.

There is therefore an error in the Ka in the text of the problem.

pH = pKa + log([HCO3^-]/[H2CO3])
pKa = -log(Ka) = -log (6.3×10^-7) = 6.2
[HCO3^-]:[H2CO3] ratio = 8.5:1
Now you can calculate pH:
pH = 6.2 + log(8.5/1) = 6.2+ 0.93 = 7.13 Note: log = log to base 10.
The difference between pH = 7.13 and pH = 7.30 is due to a possible Ka different from the one I found.

I think the Ka given in the question is supposed to correspond to a temperature of 25 C (since if you calculate a pKa from it, it gives you 6.37 ish), so basically whoever wrote the question neglected the fact that physiological conditions should apply.

Chemistry a level quetsion help!!!!

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