Acidity in solution

[silveryhair]

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Which of the following would produce an acidic solution in water?
NH3, CaCO3, CO2, CH3OH

I'm guessing its CO2.

 

[chemtex2008]

CO2 when dissolved in water produced carbonic acid (H2CO3)

 

[doc3232]

CO2 when dissolved in water produced carbonic acid (H2CO3)

yep, it is in our blood, in dissociated form that is.

 

[Tina324]

won't all of these form acids?

 

[doc3232]

won't all of these form acids?

Where can you get a hydrogen from CaCO3. This is a strong base.
NH3 is almost as strong as OH- (about pKa of 12).
Finally CH3OH or was it CH3CH2OH is an alcohol and is about as acidic as water, that isn't n acid either.

 

[Tina324]

sorry, but i don't follow.

if u put CaCO3 in water won't it produce HCO3...isn't that acidic?

 

[Thundercatz]

Where can you get a hydrogen from CaCO3. This is a strong base.
NH3 is almost as strong as OH- (about pKa of 12).
Finally CH3OH or was it CH3CH2OH is an alcohol and is about as acidic as water, that isn't n acid either.

I am going to disagree. The answer should be NH3, which is a weak base. Its conjugate acid, NH4 is more acidic than water, even though it is a weak acid.

CaCO3 will form a base in water. Ca is a group 2 element, meaning it will be a spectator ion when dissolved in water. So, you have CO3 + H20 -> OH + HCO3.

CO3 came from HCO3, which is a weak acid, so its conjugate base is CO3; therefore, it is a base!

 

[Thundercatz]

CO2 when dissolved in water produced carbonic acid (H2CO3)

This is wrong. When oxygen binds to RBC, it dissociates the H+ ion on the RBC. The H+ ion then binds to bicarbonate ion forming carbonic acid. Carbonic anhydrase then converts carbonic acid to CO2 and water.

 

[doc3232]

I am going to disagree. The answer should be NH3, which is a weak base. Its conjugate acid, NH4 is more acidic than water, even though it is a weak acid.

CaCO3 will form a base in water. Ca is a group 2 element, meaning it will be a spectator ion when dissolved in water. So, you have CO3 + H20 -> OH + HCO3.

CO3 came from HCO3, which is a weak acid, so its conjugate base is CO3; therefore, it is a base!

I agree NH4 is somewhat acidic, but they didn't ask about NH4, they asked about NH3.

 

[chemtex2008]

This is wrong. When oxygen binds to RBC, it dissociates the H+ ion on the RBC. The H+ ion then binds to bicarbonate ion forming carbonic acid. Carbonic anhydrase then converts carbonic acid to CO2 and water.

Who said anything about red blood cells? This question is asking about a solution, not the pH of biological systems.

Thundercatz, don't confuse the question... You are correct that the carbonate/bicarb buffer system creates CO2, which because of it's high vapor pressure is easily expelled from the body.

I swear this is the right answer... test it. Get a cup of pure water and boil it to expell all gases. Then check the pH. it should be 7. Let it sit for a few hours and check it again... it will be close to 6.5.

To answer the other issues, CaCO3 dissociates into Ca 2+ and CO3 2-.
The CO3 takes on protons, but at the expense of making OH-. OH- is a strong base. This is the logic that will generally get you to the right answer.

Same goes for NH3... Ammonia is actually NOT a strong base, but rather a weak base. It forms ammonium, but at the expense of making OH-.

Keep with this logic, based on Arrhenius acid base theory, and you will be okay on the DAT.

With regard to the alcohol, although alcohols are technically considered amphoteric, this is phenomena can be neglected unless it is in the presence of either an acid or a base.