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In this case we note that ta ta boiling temperature

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Unformatted text preview: nce state of each component was the pure state (limiting Raoult’s law behavior). We can similarly take the reference state to be the infinite dilution state (limiting Henry’s law) and define activities and Marand’s Notes: Chapter 5 - The Properties of Simple Mixtures 175 activity coefficients referring to Henry’s limiting law. This is done in your book and will be looked at in more detail during your second semester of thermodynamics. Use of Raoult’s Law: Colligative Properties of Mixtures. Raoult’s law is extremely useful in many respects, but especially in one in particular situation. We will show now, how the use of this law allows for example the determination of the molar mass of an unknown substance. The experiments to be described below have been used during the late 19th century and most of the 20th century for such purpose. One of these experiments (osmotic pressure measurement is still used to this day for estimation of molecular weights of proteins and polymers in general). All we are going to discuss right now is based on the fact that as we mix A with B in the liquid phase, we decrease the chemical potential of A below the value that it would have if it were pure. Indeed, we know that µAL(T, PA) = µA*L(T, PA*) + RT ln(PA / PA*) and the partial pressure PA is always lower than PA*. Therefore, the activity of component A (aA = PA / PA*) is always less than unity and ln(PA / PA*) is always negative. Marand’s Notes: Chapter 5 - The Properties of Simple Mixtures 176 We can now look at how the presence of a solute (we call it B) affects 1) the boiling point of A an...
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