James McGinn

2018-04-07 18:49:02 UTC

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So, why do you think they distinguish between "vapor" pressure and gas pressure?

Often vapor pressures are used as terms for the special partial pressures of gases that are in equilibrium with another phase, whereas in the generic term of "partial pressure" of a gas, it can be anything (no other phase or equilibration is implied). So the term "vapor" implies a history, and also another phase that drives it, as an equilibrium partial pressure from phase change.Thus the partial pressure of water gas above liquid at 96.7 C is 0.9 atm. We call this special partial pressure a "vapor pressure" because it's in equilibrium with water and has (once been) liquid water. BUT IT IS A GAS NOW. Of course you can have partial pressure of H2O gas as anything LESS than this, without condensation. That's not properly a vapor pressure, but just a partial pressure.

Were you, by chance, in a drug induced state when you formulated all of this?So not all partial pressures are vapor pressures, but all vapor pressures are partial pressures. They are equilibrium partial pressures. One is a subset of the other. However, they are both taking about the pressure of gas molecules.

Now you calculate the density of water GAS in equilibrium above liquid at 90 C. Congrats, that IS the vapor pressure. It's a special gas partial pressure from another phase.

Go ahead-- show us your math. Or look it up in a table. Any vapor pressure implies a DENSITY (moles per volume) which then can easily be converted to a molecular weight. Use some vapor pressures (I don't care where you get them) to calculate some molecular weights. Are these the weights of gas molecules or something larger? Show me your numbers.

For what?Now you calculate the density of water GAS in equilibrium above liquid at 90 C. Congrats, that IS the vapor pressure. It's a special gas partial pressure from another phase.

Go ahead-- show us your math. Or look it up in a table. Any vapor pressure implies a DENSITY (moles per volume) which then can easily be converted to a molecular weight. Use some vapor pressures (I don't care where you get them) to calculate some molecular weights. Are these the weights of gas molecules or something larger? Show me your numbers.