mol concentration graph vs ph scale

[cmnh097]

hi guys im a fresh man student in civil engineering

i've been trying for hours but i just cant get grasp of this question.
i would really appreciate a little help!
i know what molarity concentration is but i dnt know anything more than that....

Use the MS excel or MATLAB to draw a pH-Concentration diagram for the three carbonate species in a closed system, hydrogen ion (H+) and hydroxyl ion (OH-). Set pH as the x-axis ranging from 1 to 14, and the molar concentration as Y-axis ranging from 10-1 to 10-14 M, respectively. Assume the total CO2 concentration, [CO2]T = [CO2(aq)]+[HCO3-]+[CO32-]=10-4 M.

 

[cmnh097]

10-1 and 10-14 and 10-4 actually means 10^-1 10^-14 10^-4

 

[bimr]

See the graph on the link:

http://www.wetnewf.org/pdfs/measuring-alkalinity.html

 

[EmmanuelTop]

If you are unfamiliar with acid-base chemistry it is hard to make a crash course in a single post/thread, but here are some basic rules which might help. It all boils down to the chart provided by bimr.

In every CO2-water system there is equilibrium between gaseous CO2, dissolved CO2, and dissociated CO2:

CO2(gas)<--> CO2(aq)
CO2(aq) + H2O <--> H2CO3(aq)
H2CO3(aq)<--> H+(aq) + HCO3-(aq)
HCO3-(aq)<--> H+(aq) + CO3--(aq)

Each of these equilibrium reactions is characterized by the equilibrium reaction constant (K), defined as:

http://home.pcisys.net/~bestwork.1/CO2Water/WaterCa_HTML.html

Following two basic rules (conservation of mass and electro-neutrality of the solution), it can be concluded that the initial concentration of dissolved CO2 will be distributed between H2CO3, HCO3-, and CO3-- species. Now, changing pH (which means changing concentration of H+ ions) will affect final concentrations of H2CO3, HCO3-, and CO3-- because the reaction constant K remains unchanged for all reactions, so the concentrations of other contributor(s) in the equation must change. This practically means if you reduce concentration of H+ ions, then the concentration of HCO3- and CO3-- ions must increase. You can see this illustrated on the chart provided by bimr.

pH is negative logarithm of H+ ion concentration (e.g. if H+ conc. is 10E-04 mol/dm3, pH = -log[sub]10[/sub](10E-04) = 4). If you look at the equations and pH to concentration conversion, now you have 3 equations with 3 unknowns and from there should be easy to start.
You should obtain a chart that looks like

http://ion.chem.usu.edu/~sbialkow/Classes/3650/CO2 Solubility/DissolvedCO2.html

Dejan IVANOVIC
Process Engineer, MSChE