Interesting chemical engineering questions - 3: With the same molar concentration of non-volatile solutes, why is the water vapor partial pressure different? - Already...
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This post was last edited by yjqin1 on 2015-8-20 at 17:46. Assuming that the saturated vapor pressure of pure water at 30 degrees is 0.003 MPa, and each aqueous solution contains the following solutes at a concentration of 1 mole per liter, what is the partial pressure of water vapor in the air in equilibrium with these solutions? The solutes are glucose, sodium chloride, potassium chloride, hydrogen chloride, hydrogen bromide, sulfuric acid, and phosphoric acid. It’s quite tedious to do the calculations; you might as well sort the saturated partial pressures corresponding to these aqueous solutions in order of magnitude. This tests your chemistry knowledge from junior high to junior year of college.
It’s a great series; let’s make it a series along with what you just created, and publish them as a series in the Chemical Engineering Principles section
Not many people are interested in this question. In fact, there are more and more professionals working in the fields of evaporation, multi-effect evaporation, membrane vaporization (MVR), and membrane distillation. Some companies that specialize in MVR systems generate annual revenues of hundreds of millions, or even several hundred million, with their annual contract values doubling each year. These people need to have such basic knowledge.
This post was last edited by yjqin1 on 2015-8-15 20:25. The standard answer: The vapor pressure of water corresponding to 1M aqueous solutions, in descending order, is: glucose > phosphate > sodium chloride ≈ potassium chloride > hydrogen chloride ≈ hydrogen bromide ≈ sulfuric acid. Glucose is a single molecule; it does not dissociate or associate in any way ； Phosphoric acid is a medium-strength acid that undergoes a certain degree of hydrolysis ； Sodium chloride and potassium chloride completely hydrolyze into ions ； Hydrochloric acid (an aqueous solution of hydrogen chloride) and hydrobromic acid (an aqueous solution of hydrogen bromide) dissociate almost completely, but the hydrogen ions combine with water to form H3O+, thereby occupying another water molecule ； Although sulfuric acid is a strong acid and its first dissociation is complete, HSO4- is a moderately strong acid (with a dissociation constant pKa2 of 1.98); when the pH of a 1M sulfuric acid solution is less than 0, it hardly dissociates. So, this involves H3O- learned in junior high school chemistry, as well as Raoult’s law and the laws of chemical colligativity learned in physical chemistry.
I would like to ask the original poster: what would happen if there is HF in the solution? Will there be HF in the gas phase? If so, how can it be removed more economically? How to remove F- in the liquid phase? Thank you!
If HF is present in the aqueous solution, hydrogen bonds form between them, with the result that: 1) the acidity of hydrofluoric acid decreases； 2) HF becomes a poorly volatile component. 3) An azeotrope is formed at higher HF concentrations. Although the volatility of HF decreases, HF is still present in the gas phase, and there is also HF association in the gas phase. If it is to be removed from the gas phase, using water for absorption is definitely no problem. In the liquid phase, another method involves using calcium chloride or calcium hydroxide to react and produce calcium fluoride, which is insoluble in water.
HF in the gas phase refers to the gas phase after absorption. It has already been absorbed by the solution. For example, in flue gas from high-F coal, after SO2 is absorbed, HF does not fully enter the liquid phase; thus, HF remains present in the flue gas after absorption. It’s impossible to absorb it with water once again. How to handle it?