Question

What is the number of molecules in 9.0 g of steam?

Answer 1

Steam is water (H2O)

The relative molecular mass of water has to be calculated:

Refer to periodic table for molar mass/relative atomic mass for every element

Hydrogen has molar mass of 1g but as it is H2 there are 2 moles so : 2*1g =2g

Oxygen has molar mass of 16g

So relative molecular mass of water equals 18g

To find the number of moles of water in 9g divide mass (9g) by relative molecular mass of water (18g) u get 0.5 moles .

1 mole = 6.03*10^23 particles (in this case molecules)Avogadro's number

So 0.5 *(6.03*10^23)= 3.015*10^23 molecules

9 gram of steam contain 3.015*10^23 H2O molecules.

Note: 9 gram of water whether as ice or liquid water or steam always contains 3.015*10^23 water molecules.

9 grams of ice or liquid water or steam have different volumes but same number of molecules . Probably u need a lot balloons filled with water vapour to get 9 grams whereas u need a small cube of ice to get these 9 grams .Ice is more dense than water vapour.