FE Chemical Practice Question Solution included after investigation
CASE E-064 · FE

The Liquid Mixture That Started Boiling at 64 kPa

A binary liquid did not boil when total pressure equaled either pure-component vapor pressure. Its bubble pressure was the composition-weighted sum.

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Case E-064 engineering failure visual
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3

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4

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5

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Case Brief

SAME PRINCIPLES. HIGHER STANDARDS.
PROBLEM STATEMENT

An ideal binary liquid at a fixed temperature contains 40 mol% component A and 60 mol% component B. At this temperature, the pure-component saturation pressures are 100 kPa for A and 40 kPa for B. A calculation averages the two values and predicts a 70 kPa bubble pressure. Determine the correct bubble pressure and the vapor-phase mole fraction of A at incipient boiling.

Thermodynamics Chemical Design Verification Root Cause Analysis Safe Correction
E-064
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CASE CLOSED · ENGINEERING VERDICT

What actually failed — and what should change.

What failed

Pure-component vapor pressures were averaged without composition weighting.

What was missed

Raoult's-law partial pressures.

Corrected model

Use P=sum(xiPsat,i) and yi=xiPsat,i/P.

QUICK ANSWER

For an ideal binary liquid, Pbubble=sum(xiPsat,i). With xA=0.40, Psat,A=100 kPa, xB=0.60, and Psat,B=40 kPa, Pbubble=64 kPa. The first vapor has yA=40/64=0.625.

ENGINEERING TAKEAWAY

For an ideal liquid mixture, use Raoult's law: yiP=xiPsat_i and Pbubble=sum(xiPsat_i).

CASE QUESTIONS

Questions this case should settle.

How do you calculate ideal-mixture bubble pressure?

Use the sum of liquid mole fraction times saturation pressure for each component.

What is yA at this bubble point?

0.625.

NEXT FE PRACTICE QUESTION

The Methane Flame That Released 890 kJ per Mol

Thermodynamics · 5–10 min

Continue Practice →
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