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PCAT Chemical Processes – Boiling Point

Which of the following explains why water (H2O) boils at 100oC while hydrogen sulfide (H2S) boils at -60 oC?

A) Compounds with lower molecular weights boil at a higher temperature
B) Oxygen is more electronegative than sulfur
C) The stronger acidity of H2S makes it less volatile
D) Water contains both hydrogen bonds and London dispersion forces while hydrogen sulfide contains only London dispersion forces

Click for Explanation

This question is testing your understanding of factors that influence boiling point, namely molecular weight and intermolecular forces. In general, compounds with a higher molecular weight have a higher boiling point making choice A incorrect. Since boiling requires breaking all intermolecular bonds, compounds with stronger intermolecular forces boil at a higher temperature. Comparing the intermolecular forces of water to that of hydrogen sulfide, you can see that H2O is capable of forming hydrogen bonds while H2S is not. Therefore, the stronger intermolecular forces between water molecules results in a more significant increase in the boiling point than the lower molecular weight, as compared to the hydrogen sulfide. Thus, D is the correct answer.

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