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 density(ratio between mass and volume as m/v) decreases from top to bottom in periodic table group as the size increases and nuclear charge decreases. but in periods density increases from left to right as atom size decreases and nuclear charge increases.

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