Question: how many water molecules are present in the sample?
Answer: Now by definition, in a mole of stuff there are 6.022×1023 individual items of that stuff. So, in the given quantity of water, there are 6.022×1023⋅mol−1×2.50⋅mol = 15.04×1023 water molecules . How many oxygen atoms in such a quantity; how many hydrogen atoms?