How many molecules are in 7.5 mol of h2o

A mole refers to 6.022⋅1023 items. H2O has 2 hydrogen atoms and 1 oxygen atom...so a total of 3 atoms in each H2O.

Therefore (6.022⋅1023)⋅ (3 atoms) =18.066⋅1023

Then we need to change the answer into proper scientific notation:
18.066⋅1023=1.8066⋅1024

Lastly, change to the proper number of significant figures. There are 4 significant figures in 6.022. Due to this we need to also have 4 significant figures in the final answer, so we would round 1.8066⋅1024→1.807⋅1024

So, your final answer is 1.807⋅1024 for one mole of h2o. multiply that by 7 and your answer is 13.55 * 1024 

the product of the pressure and volume is also constant