Solubility of CO2 in water

[663697]

Which one of the following will decrease the solubility of CO2 in water?

A. Increasing the external pressure of CO2

B. Increasing the temperature of the water Correct Answer

C. Increasing the pH of the water

D. None of the above


Given the equilibrium CO2 + H2O <-> H2CO3 <-> HCO3- + H+, why wouldn't increasing the pH decrease the solubility of CO2? Wouldn't it cause the eq'm to shift right, therefore removing CO2? Or does that count as CO2 becoming more soluble since it forms HCO3-?

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[akimhaneul]

Which one of the following will decrease the solubility of CO2 in water?

A. Increasing the external pressure of CO2

B. Increasing the temperature of the water Correct Answer

C. Increasing the pH of the water

D. None of the above


Given the equilibrium CO2 + H2O <-> H2CO3 <-> HCO3- + H+, why wouldn't increasing the pH decrease the solubility of CO2? Wouldn't it cause the eq'm to shift right, therefore removing CO2? Or does that count as CO2 becoming more soluble since it forms HCO3-?

Since it becomes hco3- co2 becomes more soluble in blood. Therefore decreasing ph makes co2 more soluble.


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[babyoreo]

First, let's be clear that we are talking about solubility, not concentration. The solubility of a substance is its ability to dissolve in a solvent. The concentration of a substance is the amount of the solute dissolved in a solvent.

A) Henry's Law states that the concentration of a gas dissolved C = k P, where k is the solubility constant, and P is the external pressure of the gas. Increasing P would increase C, but not its solubility.
B) Generally, the solubility constant k of a substance increases with temperature. This is why sugar dissolves more easily in hot water.
C) You have listed the bicarbonate buffer equation, which maintains the pH of the blood. However, the question is talking about water. Additionally, your conclusion that the equilibrium would shift to the right is correct, but again, this is talking about the concentration of the CO2 dissolved, not its ability to be dissolved (solubility).

Where did you find this question?

 

[aldol16]

First, let's be clear that we are talking about solubility, not concentration. The solubility of a substance is its ability to dissolve in a solvent. The concentration of a substance is the amount of the solute dissolved in a solvent.

A) Henry's Law states that the concentration of a gas dissolved C = k P, where k is the solubility constant, and P is the external pressure of the gas. Increasing P would increase C, but not its solubility.

You are interchanging several concepts here that are not interchangeable. Yes, we are talking about solubility. Solubility is measured by the amount of some substance that is dissolved in some solvent at a specified temperature and pressure. Why is the italicized important? Because temperature and pressure do, in fact, affect solubility. Look up solubility tables. They will always specific pressure. Here's why. Solubility can be thought of as an equilibrium process. CO2 (g) <---> CO2 (aq). Solubility here is simply [CO2 (aq)] at a given temperature and pressure. One needs to specify pressure because increasing the pressure of CO2 gas means that [CO2 (g)] increases and therefore the equilibrium is affected. Specifically, the equilibrium above, that is the dissolution of CO2 (g), is shifted to the right. Therefore, we say the solubility has increased.

The other concept is solubility constant. That's the "k" you mention. You are correct here that k does not change with pressure. Why? What is k? Look at it closely. k is simply the equilibrium expression for the above process:

k = [CO2 (aq)]/[CO2 (g)]
[CO2 (aq)] = k*[CO2 (g)]
C = k*pCO2

Therefore, saying that k doesn't change with pressure is the same as saying that equilibrium constant doesn't change with changing product or reactant concentrations. But the equilibrium position, or solubility, does.

C) You have listed the bicarbonate buffer equation, which maintains the pH of the blood. However, the question is talking about water. Additionally, your conclusion that the equilibrium would shift to the right is correct, but again, this is talking about the concentration of the CO2 dissolved, not its ability to be dissolved (solubility).

The bicarbonate system is also relevant in water. When you dissolve CO2 in water, you do see formation of bicarbonate - it is a slow equilibrium but it is equilibrium nonetheless. CO2 reacts with water to form carbonic acid (very small equilibrium constant), which can deprotonate. But the equilibrium constant is too small for you to see any measurable change in pH of the water.

 

[babyoreo]

You are interchanging several concepts here that are not interchangeable.

It appears that we disagree on what "solubility" is. I am used to seeing it refer to k, the solubility constant, and referring to [X] as "concentration". From your definition of solubility as "the amount of some substance that is dissolved in some solvent at a specified temperature and pressure" and the rest of your post, you are using it to be synonymous as concentration. If that's the case, I agree with all of your points.

I think its strange to refer to [CO(aq)] as "solubility" and prescribe it units such as mol/L when [X] and molarity is widely used to refer to concentration. In my non-expert opinion, I think the distinction should be "The solubility k of N2 in the blood is 1 mol/L atm" or "The concentration C of N2 in the blood is 2 mol/L" vs "The solubility of CO2, aka [CO2], is 1 mol/L..."

After some quick Googling, I see that C is often referred to as "concentration" and/or "solubility", so it depends on who you talk to.

Because the... [CO2] in water is affected by A) pressure, B) temperature, and C) pH, I would now dismiss my initial assumption and believe the question is written to refer to [CO2] as solubility, as well explained by @aldol16. I'm still curious to where this question came from, OP.

 

[aldol16]

It appears that we disagree on what "solubility" is. I am used to seeing it refer to k, the solubility constant, and referring to [X] as "concentration". From your definition of solubility as "the amount of some substance that is dissolved in some solvent at a specified temperature and pressure" and the rest of your post, you are using it to be synonymous as concentration. If that's the case, I agree with all of your points.

I think its strange to refer to [CO(aq)] as "solubility" and prescribe it units such as mol/L when [X] and molarity is widely used to refer to concentration. In my non-expert opinion, I think the distinction should be "The solubility k of N2 in the blood is 1 mol/L atm" or "The concentration C of N2 in the blood is 2 mol/L" vs "The solubility of CO2, aka [CO2], is 1 mol/L..."

After some quick Googling, I see that C is often referred to as "concentration" and/or "solubility", so it depends on who you talk to.

Because the... [CO2] in water is affected by A) pressure, B) temperature, and C) pH, I would now dismiss my initial assumption and believe the question is written to refer to [CO2] as solubility, as well explained by @aldol16. I'm still curious to where this question came from, OP.

Hmm, interesting. I can't say I've ever seen k referred to as "solubility." A simple search yields the following definition of solubility as used in chemistry: "Solubility is defined as the maximum quantity of a substance that may be dissolved in another. It is the maximum amount of solute that may be dissolved in a solvent at equilibrium, which produces a saturated solution." It's measured in units of grams solute per L of solution. A similar concept, molar solubility, is measured in units of moles dissolved per liter of solution. The reason why solubility and concentration have the same units is because they are intricately linked. Solubility is the maximum concentration of solute that will dissolve in a given amount of solution. Concentration is any amount of solute dissolved in solution, up to a point. That is, when solubility = concentration of solute, then the solution is said to be saturated.

 

[663697]

It appears that we disagree on what "solubility" is. I am used to seeing it refer to k, the solubility constant, and referring to [X] as "concentration". From your definition of solubility as "the amount of some substance that is dissolved in some solvent at a specified temperature and pressure" and the rest of your post, you are using it to be synonymous as concentration. If that's the case, I agree with all of your points.

I think its strange to refer to [CO(aq)] as "solubility" and prescribe it units such as mol/L when [X] and molarity is widely used to refer to concentration. In my non-expert opinion, I think the distinction should be "The solubility k of N2 in the blood is 1 mol/L atm" or "The concentration C of N2 in the blood is 2 mol/L" vs "The solubility of CO2, aka [CO2], is 1 mol/L..."

After some quick Googling, I see that C is often referred to as "concentration" and/or "solubility", so it depends on who you talk to.

Because the... [CO2] in water is affected by A) pressure, B) temperature, and C) pH, I would now dismiss my initial assumption and believe the question is written to refer to [CO2] as solubility, as well explained by @aldol16. I'm still curious to where this question came from, OP.

The question came from a TPR practice passage online.