Question

Which one has greater number of atoms among the following? (a) 72 g water. (b)7 mol hydrogen gas . (c)90 g iron. (d) 33 g oxygen gas.

Answer 1

Note: According to Avogadro's the Gram Molecular Weight of a substance or 1 mole of a substance (G.M.W)  contains 6.02*10^23 electrons.

a) The gram molecular weight of water = 18 g
Therefore 18 g water contains 6.02*10^23
1 g water contains = 6.02*10^23/18
72 g water contains = 6.02*10^23*72/18
                                  = 24.08*10^23
                                  = 2.408*10^24 electrons

So, 72 g of water contains 2.408810^23 electrons

b) 1-mole hydrogen gas contains = 6.02*10^23 electrons
7 moles hydrogen gas contains = 6.02*10^23 * 7
                                                      =  42.14*10^23 electrons
                                                      = 4.214*10^24 electrons

So, 7 moles hydrogen gas contains 4.214*10^23 electrons

C) Gram molecular mass of iron = 56 g
Therefore 56 g of iron contains = 6.02*10^23 electrons
1 g iron contains = 6.02*10^23/56
90 g of iron contains = 6.02*10^23/56 * 90
                                     = 9.675*10^23 electrons

So, 90 g of iron contains = 9.675*10^23 electrons

d) Gram molecular mass of oxygen gas = 32 g
Therefore 32 g of oxygen gas contains = 6.02*10^23 electrons
1 g oxygen gas contains = 6.02*10^23 / 32
33 g of oxygen gas contains = 6.02*10^23*33/32
                                                  = 6.208 * 10^23 electrons

So, 33 g oxygen contains 6.208*10^23 electrons

On comparing a), b), c) and d) 
b) has more number of electrons i.e 7 moles of hydrogen gas contains more number of electrons

The answer is 7 moles of hydrogen gas.

Hope this helps you.