Question

Two liquids x and y boil at 110 degrees and 130 degrees Celsius respectively. Which one of them has higher vapour pressure at 50 degree Celsius? Why?

Answer 1

The normal boiling point is the temperature in which the vapour pressure of a liquid becomes equal to atmospheric pressure. So at 50 degree Celsius,if the vapour pressure of the liquid is close to atmospheric pressure, it will need only a little bit more temperature to boil. On the other case,if vapour pressure at 50 degree Celsius is very low,it will need alot more heat or higher temperature to boil. So we can understand that boiling point of a liquid is inversly related to vapour pressure of that liquid. So x has higher boiling point.
Another way to look at this is, both x and y have same vapour pressure ( atmospheric pressure) at their respective boiling points. So when we decrease the temperature to 50, temperature of x gets decreased by 60 while y gets reduced by 80. So y lose more vapour pressure compared to x. So x has higher vapour pressure compared to y at 50 degree Celsius.