CASE STUDY CO H,0 N CO H,0 Consider equilibrium between CO2 and water in a conta

Question

CASE STUDY CO H,0 N CO H,0 Consider equilibrium between CO2 and water in a container. 1. If gas initially contained no water and its pressure were known, can you find the amount of water in the gas phase at equilibrium? What table values do you need in real and ideal cases? Assume gas to be pure co2 or N2. 2. What do you need to calculate concentration of CO2 in water if amount of Co2 in the gas phase is 1%? Calculate this value for a real solution case. 3. How much carbonic acid will be produced in the reaction with water? What do you need to calculate its concentration? Do you need to know volumes of gas/liquid and what approximations need to be made?

Explanation / Answer

Q1.

You will not be able to calculate water amount (humidity) in gas, if no Temperature is stated, recall that this is an equilibrium, according to the phase rule, there must be at least two variables set, pressure, and in this case temperature, in order to set a % humidity value.

Note that if CO2 and N2 content are stated, then we could calculate H2O content via:

x-CO2+ x-N2 + x-H2O = 1

x-H2O = 1- x-CO2+ x-N2

Q2.

In order to calculate CO2 in water... if CO2 in gas is 1%, we can use Henry Law (ideal case)

Molarity of CO2 = H-CO2* Partial Pressure of CO2

H-CO2 = Henry constant for CO2, 3.4*10^-2 M/atm

solve for Molarity of CO2

assume 1 atm, 25C;

M-CO2 = (3.4*10^-2 M/atm) ( 0.01 atm of CO2) = 0.00034 mol of CO2 per liter

for a real solution, we can still assume ideal value

since activity coefficient of CO2 will be almost near to 1, since this is dilute

Q3.

CO2 + H2O = H2CO3

basis: 1 liter

0.00034 mol of CO2 produces = 0.00034 mol of H2CO3

in order to calculate TOTAL amomunt, we DO need amount of liquid, since the basis is 1 liter, but we need to know total volume, since this is actually concentration ( mol per liter)

Q4.

In order to calculate pH we need H+ ions concentration

For this, we need the total coefficient values for Ka, aci dissociation

Ka1 = [H+][HCO3-]/[H2CO3]

Ka2 = [H+][CO3-2]/[HCO3-]

Overall K

K = [H+]^2[CO3-2]/[H2CO3]

we will need K values from tables,

circumstances which affect CO2(aq) , if pH is low, then this favours H2CO3 formation, so CO2(aq) increases in order to form even more H2CO3 (i.e. solubility increases)

for pH high, this will favour CO3-2 formation from carbonic acid

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