The ratio by the number of atoms for water will always be H : O = 2 : 1. Thus, 18 g of water contains 2 g of hydrogen and 16 g of oxygen.

i can understand that the ratio of h = 2 so  2 g of hydrogen contained in the water but why we have taken the oxygen as 16 g but it is given that the  ratio of o = 1 pls explain i need you help experts pls.  

Please note that here we are referring to the ratio of number of atoms and not the ratio of masses of atoms. In a molecule of water, hydrogen and oxygen are present in the ratio H : O = 2 : 1. That is the ratio of number of atoms of hydrogen to oxygen is 2 : 1. 

Now the mass of one mole of hydrogen is 1 grams and the mass of one mole of oxygen is 16 grams. As in water there are two atoms of hydrogen and one atom of oxygen, therefore one mole of water

= 2 moles of hydrogen atom + one mole of oxygen atom

= 2 x (1) g + (16) g

= 18 g

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The ratio of hydrogen and oxygen in one molecule of water is 2:1.

But the atomic weight ratio of hydrogen and oxygen 1:8, so if 2g of hydrogen is present in water then 16g of oxygen will be present in water. As the atomic weight of hydrogen is 1 and ayomic weight of oxygen is 16. And as 2 atoms of hydrogen and 1 atom of oxygen is present in one molecule of water the ratio will be 2:16 or simply 1:8.

  • 3

 The atomic ratio of a molecule of water is hydrogen:water is 2:1 but the atomic mass ratio of a water molecule is hydrogen:water i.e. 1:8. So, 18 gram of water which is the mass of water will have 2 gram of hydrogen and 16 gram of  oxygen.

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