How many atoms of hydrogen are present in 8.29 g of water?

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We have heard of Avogadro’s number (6.022 * 10^23). This is the number of atoms or molecules in 1 mol of something.

8.29g of water is not 1mol of water — the mass of 1 mol of water would be its molar mass (M) which is equal to 18g (2Ar(H)+Ar(O)).

To find how many mols of water we have, we can set a proportion:

1mol 18g
x mol 8.29g

x=8.29g*1mol / 18g
x = 0.46mol

So we have 0.46mol of water. 0.46mol can be written as 4.6*10^-1 mol, which will be useful since we have to work with exponents in the next step.

We remember that Avogadro’s number works if there is 1 mol, so we can set another proportion:

6.022*10^23 atoms 1mol
x atoms 4.6*10^-1 mol

x=6.022*10^23 * 4.6*10^-1

x=2.77*10^22 atoms

That should be your answer but do check it in case I’m wrong. If you don’t understand what I did, please say so in the comments :-)