[Jules18]

The test I'm doing tells me this statement is true:

Higher concentration of carbonic acid (H₂CO₃) leads to a decrease in the concentration of carbonate ions (CO₃^-2). 

But I thought that when H₂CO₃ dissociated, it would create more carbonate ions.

H₂CO₃ -->  2 H⁺ +  CO₃^-2

My understanding of Le Chatelier's Principle was that an increase of reactants shifted the equilibrium to the right, increasing the concentration of products. 

Was I wrong?   

[AWK]

For reasonable concentrations of carbonic acid the concentration of carbonate anion is approximately constant (~K₂). But the equilibrium for carbonic acid is much more complex since it depends on pressure of CO₂ over solution.

[Borek]

Exact values, calculated with BATE:

Canalytical	pH	[CO32-]
1e-6	6.31	5.40e-11
1e-5	5.73	5.64e-11
1e-4	5.20	5.69e-11
1e-3	4.69	5.74e-11
1e-2	4.19	5.84e-11
1e-1	3.69	6.01e-11
1	3.19	6.33e-11

So the statement is false.

For what AWK wrote, see

http://www.chembuddy.com/?left=pH-calculation&right=pH-polyprotic-simplified

(scroll to the very bottom of the page). Note, that all concentrations of CO₃^2- listed are very similar - that's it.