What is the normal boiling point of each compound? 800 700 600 Vapor pressure (mm Hg) 500 CS
400 300 CH
NO
200 100 0 -30 -10 10 30 50 70 90 110 Temperature (°C) CS
: °C CH
NO
: °C
The Correct Answer and Explanation is:
To determine the normal boiling point of each compound, we use the vapor pressure vs. temperature graph.
🔍 Definition
Normal boiling point is the temperature at which a compound’s vapor pressure equals standard atmospheric pressure (760 mm Hg).
So on a vapor pressure vs. temperature graph, find the temperature where the vapor pressure = 760 mm Hg.
📉 From the Graph (Based on the Description)
We are given vapor pressure data for two compounds:
- CS₂ (Carbon disulfide)
- CH₃NO₂ (Nitromethane)
We identify the temperature where each compound’s vapor pressure curve crosses 760 mm Hg.
🔸 For CS₂:
- The vapor pressure reaches 760 mm Hg around 46°C.
- So, normal boiling point of CS₂ = 46°C.
🔸 For CH₃NO₂:
- The vapor pressure reaches 760 mm Hg around 101°C.
- So, normal boiling point of CH₃NO₂ = 101°C.
✅ Final Answers
- CS₂: 46°C
- CH₃NO₂: 101°C
📘 Explanation
The boiling point of a liquid is the temperature at which its vapor pressure equals the external atmospheric pressure. Under standard atmospheric pressure (1 atm = 760 mm Hg), this is called the normal boiling point.
Vapor pressure increases with temperature because more molecules have enough energy to escape the liquid phase into the vapor phase. The relationship between temperature and vapor pressure is generally exponential, as described by the Clausius-Clapeyron equation.
To determine the normal boiling point from a vapor pressure vs. temperature graph, we locate the temperature at which the compound’s vapor pressure curve intersects 760 mm Hg.
In this case, we analyze two substances:
- For carbon disulfide (CS₂), the vapor pressure curve reaches 760 mm Hg at approximately 46°C, indicating its normal boiling point. This relatively low boiling point reflects CS₂’s high volatility and weak intermolecular forces (London dispersion forces).
- For nitromethane (CH₃NO₂), the vapor pressure reaches 760 mm Hg at around 101°C. This higher boiling point suggests stronger intermolecular forces, likely due to dipole-dipole interactions from the polar nitro group.
In summary, the normal boiling point is determined by the temperature at which the liquid’s vapor pressure equals 760 mm Hg. It provides important information about the volatility and intermolecular forces in the compound.
