How many grams of H are there in 23.5 g of `H_2O`?

One way to approach this problem is to determine the percent composition of hydrogen in water.

You know that one water molecule contains

• one oxygen atom
• two hydrogen atoms

This means that every mole of water molecules will contain 2 moles of hydrogen and 1 mole of oxygen atoms.

To find the percent composition of hydrogen in water, use the molar mass of water and that of hydrogen

`(2 xx 1.00794color(red)(cancel(color(black)("g/mol"))))/(18.0153color(red)(cancel(color(black)("g/mol")))) xx 100 = 11.19%`

This means that every `"100 g"` of water will contain a total of `"11.19 g"` of hydrogen. Since your sample has a mass of `"23.5 g"`, it follows that it will contain

`23.5color(red)(cancel(color(black)("g water"))) * "11.19 g H"/(100color(red)(cancel(color(black)("g water")))) = color(green)("2.63 g H"`

Alternatively, you can first find the number of moles of water in get in that `"23.5 g"` sample

`23.5color(red)(cancel(color(black)("g"))) * ("1 mole H"_2"O")/(18.0153color(red)(cancel(color(black)("g")))) = "1.3044 moles H"_2"O"`

SInce one molecule of water contais 2 atoms of hydrogen, you will get

`1.3044color(red)(cancel(color(black)("moles water"))) * "2 moles H"/(1color(red)(cancel(color(black)("mole water")))) = "2.6088 moles H"`

Finally, use hydrogen's molar mass to find how many grams would contain this many moles

`2.6088color(red)(cancel(color(black)("moles"))) * "1.00794 g H"/(1color(red)(cancel(color(black)("mole")))) = color(green)("2.63 g H")`