How many molecules are in 7.5 mol of h2o

A mole refers to 6.022⋅ 1023 items. H2 O has 2 hydrogen atoms and 1 oxygen atom ... so a total of 3 atoms in each H2 O.

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⋅ 10 24 for one mole of h2o. multiply that by 7 and your answer is 13.55 * 1024