Question

Which of the following is least volatile?

• A. $HCN$
• B. $H-F$
• C. $H_{2}O$
• D. $C{S}_2$

Answer 1

The correct option is C $H_{2}O$

• A volatile liquid is one that evaporates relatively easily. Volatile liquids have large vapour pressure because the intermolecular forces that prevent evaporation are relatively weak.
  
• As the strength of the intermolecular forces increases, the rate of evaporation decreases.
  
• As temperature increases, the rate of evaporation increases.
  
• As the area increases, the rate of evaporation increases.
  
• In this case, $HF$ and $H_{2}O$ both are less volatile compared to $HCN$ and $C{S}_2$ since the former two possess hydrogen bonding (intermolecular forces), whereas it is absent in the latter two.
  

Among water and $HF$, the H-bonding is stronger in water compared to $HF$. Hence, the least volatile liquid among the four is water.