Question

why does vapour pressure of a liquid decrease when a nonvolatile solute is added to it

Answer 1

if you added a non volatile solute,you decrease the tendency for water molecules to evaporate into the gas phase. As a result fewer molecules change from liquid to gas phase,thus reducing the vapour pressure

Answer 2

$\huge\star\sf\pink{\underline{\underline{ Answer}}}$

Here is your answer in the above pic.