
Interpretation:
The equation 7.49 is to be derived.
Concept introduction:
When a non-volatile solute is added to a solvent, the boiling point of the solution gets raised in comparison to the pure solvent. This phenomenon is known as elevation in boiling point. Elevation in boiling point is a colligative property.

Answer to Problem 7.84E
The equation 7.41 can be represented for the vaporization process as shown below.
lnxsolvent=−ΔvapHR(1T−1TBP)
Where,
• xsolvent is the mole fraction of the solvent in the solution.
• ΔvapH is the enthalpy change for vaporization process.
• TBP is the boiling point of solvent.
The xsolvent can be written in the form of solute as xsolvent=(1−xsolute). Substitute the value of xsolvent with (1−xsolute) as shown below,
ln(1−xsolute)=−ΔvapHR(1T−1TBP)
Applying the Taylor-series expansion on ln(1−x)≈−x and substitute the ln(1−xsolute) with −xsolute as shown below.
xsolute≈ΔvapHR(1T−1TBP)
Rearrange the temperature terms as shown below.
xsolute≈ΔvapHR(TBP−TT.TBP)
The term T.TBP can be approximated to TBP2 as dilute solution are considered in this case and the temperature at equilibrium is not much different from boiling point. Also TBP−T can be rewritten as ΔTBP as shown below.
xsolute≈ΔvapHR(ΔTBPTBP2)
The relationship between molality and mole fraction is,
msolute=1000⋅xsolutexsolvent⋅Msolvent
Where,
• msolute is molality of solute.
• xsoluteand xsolvent is mole fraction of solute and solvent.
• Msolvent is the molecular weight of solvent.
Substitute the value of xsolute in the given equation and rearrange for the value of ΔTBP as shown below.
ΔTBP≈(MsolventRTBP2ΔvapH⋅1000)msolute
Explanation of Solution
The equation 7.41 can be represented for the vaporization process as shown below.
lnxsolvent=−ΔvapHR(1T−1TBP)
Where,
• xsolvent is the mole fraction of the solvent in the solution.
• ΔvapH is the enthalpy change for vaporization process.
• TBP is the boiling point of solvent.
The xsolvent can be written in the form of solute as xsolvent=(1−xsolute). Substitute the value of xsolvent with (1−xsolute) as shown below,
ln(1−xsolute)=−ΔvapHR(1T−1TBP)
Applying the Taylor-series expansion on ln(1−x)≈−x and substitute the ln(1−xsolute) with −xsolute as shown below.
xsolute≈ΔvapHR(1T−1TBP)
Rearrange the temperature terms as shown below.
xsolute≈ΔvapHR(TBP−TT.TBP)
The term T.TBP can be approximated to TBP2 as dilute solution are considered in this case and the temperature at equilibrium is not much different from boiling point. Also TBP−T can be rewritten as ΔTBP as shown below.
xsolute≈ΔvapHR(ΔTBPTBP2)
The relationship between molality and mole fraction is,
msolute=1000⋅xsolutexsolvent⋅Msolvent
Where,
• msolute is molality of solute.
• xsoluteand xsolvent is mole fraction of solute and solvent.
• Msolvent is the molecular weight of solvent.
Substitute the value of xsolute in the given equation and rearrange for the value of ΔTBP as shown below.
ΔTBP≈(MsolventRTBP2ΔvapH⋅1000)msolute
The derivation of equation of 7.49 has been rightfully stated.
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Chapter 7 Solutions
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