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Confused about pH and ions..please help

by christian0710
Tags: confused, ionsplease
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Nov8-12, 01:14 PM
P: 210
1. If the statement ph=-log([H+] is true, then how come a solution can be acidic when carbonate is present? I have been told that even if carbonate does not accept H+ the solution will still be basic just by the presence of CO3(2-)? The confusion is this: ph only depends on [H+] then how come the presence of negative ions change the acidity level, even if they donít actually accept hydrons?

I thought acidity levels change ONLY because CO3(-2) ----> H2CO3 H+ is taken away.
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Nov8-12, 02:46 PM
HW Helper
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Carbonate anion is a base, but it can accept hydrogen ion and be protonated. If protonated, it is an acid, H2CO3, carbonic acid.

A solution of carbonic acid contains CO3-2, HCO3-1, H2CO3, and solvated H+ .

How does that work?
A solution of carbonic acid will give this incomplete reaction causing an equilibrated concentration of each reactant and product:
H2CO3 <------> H+ + HCO3-
HCO3- <-------> H+ + CO3-2

Carbonic acid is a weak acid, so the presence of reactant is favored in contrast to the products.
Nov8-12, 04:06 PM
P: 210
I see so HCO3(-) is preferred and therefore CO3(-2) takes away H+ from the solution, BUT is it tru that the presence of a acid or base in its acid or base form (with out reacting) would cause a solution to change acidity? And is it true that negatively charged ions can make a solution more basic without sucking up H+ ?? Or is it ONLY the presence of H+ and OH- that determines how acidic or basic a solution is?

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