. Microalgae are being considered as an alternative feedstock for biofuel produc

Question

. Microalgae are being considered as an alternative feedstock for biofuel production in the United States and elsewhere. Microalgae are photosynthetic organisms that utilize sunlight for energy and carbon dioxide as their carbon source which they convert to algal biomass. An important consideration is the partitioning of carbon dioxide into an aqueous solution as a function of temperature a. Estimate the concentration of CO-(aq) if the system is in equilibrium with ambient air (with a CO2 (g) concentration of 389 ppm,) Estimate the CO (aq) concentration at a temperature of 15°C, 25°C and 40°C. Assume that atmospheric pressure (Potal) is 1 atm. CO2 (aq)

Explanation / Answer

For the part (a):
The equilibrium equation is: KH= [CO2](g)/[CO2](ac) (have in count that the reaction is unimolar)

Note that the equilibrium constant in the table ,is give in atm.L/ mol , so you have to obtain the concentration of [CO2](g) in atm, and [CO2](ac) in mol/L or M.

*Obtaining [CO2](g) : Remember that ppm is equivalent 1e-6 parts, so the concentration of CO2 in the air is 0,389 g/m3 . Taking the molar mass of CO2 as 44, the concentration of [CO2](g) is : 8,84e-6 mol/L

Now, for convert mol/L to atm , you have to use the ideal gas equation: Pco2=(n/V).R.T , with n: mol; V: the volume; R: the universal gas constant (0,08205746 L.atm/K.mol) ; and T: temperature in K. Remember that n/V is equal to concentration in M. ----> Pco2=M.R.T

For each temperature you will have a Pco2 value. Inserting the rights values in the equation, you will obtain:

Pco2@ 15C= 2, 09 e-4 atm / Pco2@ 25C= 2,16e-4 atm / Pco2@ 40C= 2,74e-4 atm

*Obtining the [CO2](ac) : With the values for Pco2 you can now obtain [CO2](ac) using the equilibrium equation (remember, for this case at Pt= 1atm, Pco2@ T = [CO2](g)@ T). Inserting the rights values in the equilibrium equation you will obtain:

[CO2](aq)@ 15C= 9,78e-6mol/L / [CO2](aq)@ 25C= 7,32e-6mol/L / [CO2](aq)@ 40C= 6,19e-6mol/L

For the part (b):

You can write the equilibrium equation as:

Ka1= [H2CO3*] / [HCO3-].[H+] and   Ka2= [HCO3-] / [CO3-].[H+] with pH =5 ------> [H+] = 0,00001

Solving a equation system 2x2 you will obtain the concentration for [HCO3-]

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