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Chemistry Forums for Students => Physical Chemistry Forum => Topic started by: morten925 on September 23, 2015, 09:55:47 AM

Title: Partial quantity
Post by: morten925 on September 23, 2015, 09:55:47 AM

Hi,

The system at 20⁰C consists of a flask that holds a mixture of ethantol in water (20 % m/m) with a total density ρ = 968.7 kg/m³. The partial molar volume of ethanol in the solution is 52.3 cm³/mol. The objective is to calculate the partial molar volume of water in the mixture.

What I would like to do is to choose the total volume freely so that I can calculate the total mass of the solution and then the number of moles of each component in interest.

The total volume can be written as V_tot = V_EtOHn_EtOH + V_watern_water, and V_water is easily isolated from the other quantities.

What I am uncertain about is the background for the free choise of the total volume, is it not because both the density and the partial molar volume will have the same values regardless of the magnitude of this volume V_tot, i.e. they are intensive properties?

Title: Re: Partial quantity
Post by: cseil on September 23, 2015, 03:40:12 PM

You know the density and the mass composition, so every volume you choose (or every mass of the solution you choose) you know the composition and the moles for both of the components.

The quantity moles water / moles ethanol is constant.

If you try to do the calculations (the one you wrote) with different volumes you always get 18.1cm³ mol^-1 for water. That's how I'd do it.

Title: Re: Partial quantity
Post by: morten925 on September 23, 2015, 07:52:11 PM

Thank you, I got that too, however, would you agree on: "i.e. they are intensive properties"? My proffesor will punish me if I say something that is not correct, unfortunately.

Title: Re: Partial quantity
Post by: mjc123 on September 24, 2015, 07:29:52 AM

Yes, density and PMV are intensive properties.