# Equilibrium and Highest [A]

[SOLVED] Equilibrium and Highest [A]

Relevant equation
Consider the following equilibria in 0.10 moldm−3 carbonic acid.
(In attachment)
Which species is present in the highest concentration?
A. H2CO3 (aq)
B. H+(aq)
C. H(CO3)− (aq)
D. (CO3)2− (aq)
The attempt at a solution
The right answer is A but I would think that B would be the right answer because as carbonic acid includes 2 H+ cations so doubling their concentration. Please, say your comment on this problem, why only A is right.

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symbolipoint
Homework Helper
Gold Member
Understand that carbonic acid is a weak acid, Ka = about 1.8*10^(-5). Based on this, what percent of the carbonic acid will disassociate?

Understand that carbonic acid is a weak acid, Ka = about 1.8*10^(-5). Based on this, what percent of the carbonic acid will disassociate?
Good point. I got for [H+]= 10^-3, as this is much smaller than 0.1 so H2CO3 is in greatest concentration.

Is it always with weak acids that the concentration of initial acid is highest?

weak acids tend to remain as molecules in solutions, they ionise to a small extent.

Borek
Mentor
Is it always with weak acids that the concentration of initial acid is highest?
Depends on their strength and concentration, there is no one, general rule. However, for obvious reasons (stoichiometry) once undissociated form fails below concentration of dissociated form, it is H+ that starts to dominate the solution :)