BR Gen Chem I Ch.1 Passage V

[EmpathMan]

Hey all, got a question here for #27 or the first question of passage 5.

Passage: "The empirical formula for a compound can be determined using the technique of elemental analysis. For hydrocarbons and carbohydrates, the process involves trapping and removing water vapor and carbon dioxide gas and then quantifying the amounts collected. The products can be trapped in many different ways. In this experiment, the trapping of the gases is accomplished by passing the product gas through a series of low-temperature gas traps. By lowering the temperature, the gas can be converted into solids, which cannot flow and thus are easily collected at the bases of their respective temperature traps. The system is evacuated completely and then flushed with nitrogen gas to remove any remaining air. THe vapor from the reaction vessel is then allowed to enter one trap at a time. The gas remains isolated in the region of each trap for a short interval, to allow any gases to form a solid or freeze into a liquid."

-Picture-

"It is important to allow the excess oxygen gas to flow out of the system. To accomplish this, the line is fitted with a one way oil bubbler. The oil bubbler maintains the closed system by not allowing air to flow into the system, while allowing the pressure to equillibrate with the environment through venting."

27. "At what temperature should the first trap be held to order to isolate water vapor?"
a) 25 degrees celsius (standard temp)
b) 0 degrees celsius (melting point of ice)
c) -33 degrees clesius (boiling point of Freon refridgerator)
d) -196 degrees celsius (boiling point of liquid nitrogen)

So I chose answer B, because I thought water froze / phase changed into ice at 0 degrees celsius. Does water actually freeze right below 0 degrees instead of directly at that temperature?

Thanks for any help!

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[theCLE]

tests idea of colligative property: freezing point depression. When water is in a solution, the freezing point is below that which it's normal freezing point is (0C).

 

[BerkReviewTeach]

Hey all, got a question here for #27 or the first question of passage 5.

Passage: "The empirical formula for a compound can be determined using the technique of elemental analysis. For hydrocarbons and carbohydrates, the process involves trapping and removing water vapor and carbon dioxide gas and then quantifying the amounts collected. The products can be trapped in many different ways. In this experiment, the trapping of the gases is accomplished by passing the product gas through a series of low-temperature gas traps. By lowering the temperature, the gas can be converted into solids, which cannot flow and thus are easily collected at the bases of their respective temperature traps. The system is evacuated completely and then flushed with nitrogen gas to remove any remaining air. THe vapor from the reaction vessel is then allowed to enter one trap at a time. The gas remains isolated in the region of each trap for a short interval, to allow any gases to form a solid or freeze into a liquid."

-Picture-

"It is important to allow the excess oxygen gas to flow out of the system. To accomplish this, the line is fitted with a one way oil bubbler. The oil bubbler maintains the closed system by not allowing air to flow into the system, while allowing the pressure to equillibrate with the environment through venting."

27. "At what temperature should the first trap be held to order to isolate water vapor?"
a) 25 degrees celsius (standard temp)
b) 0 degrees celsius (melting point of ice)
c) -33 degrees clesius (boiling point of Freon refridgerator)
d) -196 degrees celsius (boiling point of liquid nitrogen)

So I chose answer B, because I thought water froze / phase changed into ice at 0 degrees celsius. Does water actually freeze right below 0 degrees instead of directly at that temperature?

Thanks for any help!

At 0 degrees C, water exists in an equilibrium between two phases (and a very, very small amount of vapor as well). Consider a glass of icewater that has been mixed and allowed to equilibrate over time. It's at 0 degrees C and both liquid and ice are present. You can make it completely ice at 0 degrees C by removing heat from the system, but that's not necessarily happening in the question here. To ensure that it exists 100% as ice, you need a temperature lower than 0 degrees C, but not so low that everything in the system freezes (or deposits) out.

 

[ChemTutor]

CO2 melting point is -78C. so even though most of water will be trapped 0C it won't freeze due to CO2 admixture maybe as mentioned above.

-33C is a better choice because H2O will freeze for sure while CO2 still remains as gas.