Pure water never has a pH of 7 because of Henry’s law. This law (equilibrium) th

Question

Pure water never has a pH of 7 because of Henry’s law. This law (equilibrium) that is discussed in CHEM 2 and has been mentioned earlier in class dictates the pH of water. Henry’s law defines the equilibrium for a solute in the air vs. a liquid phase. I would be remiss not to have you do an example of a Henry’s law equilibrium calculation. Consider the following equilibrium: CO2(g) ? CO2 (aq) {all dissolved carbon dioxide} KH = 10-1.5 The current measured pressure of CO2 is roughly 10-3.4 atm in the atmosphere (Henry’s law uses pressure for the gas in units of atm and molarity for the liquid phase). For carbonic acid, Ka1 is really defined as: {All dissolved CO2 (CO2 (aq) + H2CO3)} ? HCO3- + H+ Ka1 = 4.46 × 10-7 Ka2 = 4.69 × 10-11 Noting that atmospheric pressure is 1 atm, calculate the ppm of CO2 in the atmosphere when its pressure is 10-3.4 atm. If you are still confused, 1 ppm would be 1 x 10-6 atm.

Explanation / Answer

Ans. Given-

            pCO­2 = 10-3.4 atm = 3.9811 x 10-4 atm

            1 ppm = 10-6 atm

            Or, 1 atm = 1 ppm / 10-6 = 106 ppm

# So, [CO2],ppm = 3.9811 x 10-4atm = 3.9811 x 10-4 x (106 ppm) = 398.11 ppm

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