How is PH of a solution determined?

[scilearner]

Hello everyone,

 

I'm really confused.

 

Here is a equation

 

CO₂ + H₂0 <----> H₂CO_{3 <----->} H⁺ +HCO₃^-

 

^{* Now if I increase carbon dioxide I know both both Hydrogen and bicarbonate acid increases}

 

^{1. Why does this make it more acidic. The thing is what I'm thinking is if a solution has an equal concentration of an acid and base it must be neutral. In this case both hydrogen and bicarbonate have increased in same amounts, shouldn't it be neutral}

2. Why is it that in formula, PH is only determined by H+ concentration, Can't we find PH using OH- concentration.

3. The definition of base says it must produce OH ions in solution, bicarbonate is not producing OH ions in solution why is it considered a base? Is carbon dioxide a base or acid?

4. I really don't get what is a weak acid, weak base etc. What is the difference between a weak acid and a base. Isn't a weak acid a base.

 

Thanks

[John Cuthber]

"The thing is what I'm thinking is if a solution has an equal concentration of an acid and base it must be neutral."

Oddly, no.

 

"Why is it that in formula, PH is only determined by H+ concentration, Can't we find PH using OH- concentration."

You generally can.

pH + pOH = 14

 

"The definition of base says it must produce OH ions in solution, bicarbonate is not producing OH ions in solution why is it considered a base?"

It does, but nobody wrote down that reaction

HCO₃^- +H₂O <--> OH^- + H₂CO₃

But bicarbonate is a weak base, so that reaction doesn't work very well.

It doesn't produce much OH-

 

I think you need to understand the idea of equilibrium

http://en.wikipedia.org/wiki/Chemical_equilibrium

Not all reactions go completely to products (in fact, strictly speaking, no reaction does- though some get extremely close)

[scilearner]

"The thing is what I'm thinking is if a solution has an equal concentration of an acid and base it must be neutral."

Oddly, no.

 

"Why is it that in formula, PH is only determined by H+ concentration, Can't we find PH using OH- concentration."

You generally can.

pH + pOH = 14

 

"The definition of base says it must produce OH ions in solution, bicarbonate is not producing OH ions in solution why is it considered a base?"

It does, but nobody wrote down that reaction

HCO₃^- +H₂O <--> OH^- + H₂CO₃

But bicarbonate is a weak base, so that reaction doesn't work very well.

It doesn't produce much OH-

 

I think you need to understand the idea of equilibrium

http://en.wikipedia.org/wiki/Chemical_equilibrium

Not all reactions go completely to products (in fact, strictly speaking, no reaction does- though some get extremely close)

 

 

Thanks a lot for your help John It did clear somethings up. I think the water reaction is what confused me. Since water reaction is

 

H20 <---> H+ + OH-

 

Since this reaction has equal concentration of hydrogen ions and hydroxide ions and it is neutral, I thought if a solution has an equal concentration of acid and base it is neutral. Using the same logic I thought since this reaction H₂CO_{3 <----->} H⁺ +HCO₃^- has equal concentration of acid (hydrogen ions) and base (bicarbonate) it would be neutral. But after reading your answer I think even though the concentrations are same bicarbonate is a weak base so it only produces little hydroxide ions so there is more acid (H+) in solution, so the solution is acidic. Hence sulfuric acid is a strong acid.

 

So is this concept right

 

1. If there are equal concentration of acid and base in a solution, it is not neutral

but if there are equal concentrations of hydrogen ions and hydroxide ions it is neutral.

2. How is this formula pH + pOH = 14, derived?

3. What property of bicarbonate makes it a weak base. Is there any way I can work it out, or do I simply have to memorise the weak acids and bases. Simply I'm asking is there a way to work out weak acids and bases, or do I have to memorise them. I think the strong acid and strong bases have H+ and OH- in their name, eg HCL, NaOH. Weak acid and bases, seem to need to react with some other molecule such as water to produce bases and acids.

 

Thanks a lot

Edited February 14, 2014 by scilearner

[scilearner]

Another question

Ok if we consider solution with an equal concetration of H+ and OH- ions as neutral.

What if there is a solution like this

X<----> H+ + OH-

Hydrogen ion concentration= 10 ^-2
Hydroxide ion concentration= 10 ^-2


Then if I use PH equation = -log [10 ^-2] = I get an acidic PH.

But in this case both hydrogen ions and hydroxide ions concentration is same, but PH is acidic instead of neutral. How is that possible. I just made up this equation, is it impossible for something like this to exist?

Does this mean that the definition of neutral is not actually equal concentration of hydrogen ions and hydroxide ions, but simply the PH of water?

Thanks

Edited February 14, 2014 by scilearner