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

Answer :- (b) 7 mol of hydrogen gas

Explanation :- higer the number of moles of atom in any molecule or compound , higher will be nimber of atoms in that molecule or compound .

Let's check all cases ,
(a) 72g of water
number of mole of water = given weight/molar weight
= 72/18 = 4 mol
Now, number of mole of atom = 3 × 4 = 12 mol [ because water contains 2 atoms of hydrogen and one atom of Oxygen ]

(b) 7 mol of hydrogen has
so, mole of hydrogen atom in gas = 2 × 7 = 14 mol [ hydrogen gas has two hydrogen atoms ]

(c) 90g of Fe
mole of Fe = 90/56 [ atomic mass of Fe = 56g/mol ]
so, number of mole of Fe atom = 90/56

(d) 33g of Oxygen gas
Mole of oxygen gas = 33/32
mole of oxygen atom = 2 × 33/32 = 66/32 = 2.06

Here , you can see that highest number of mole of atom in option b
Hence answr is option (b)

Answer 2

Correct Answer is b) 7 mol of Hydrogen gas

Explanation :

Let us see all the four cases :

case 1: 72g of water 
molecular mass of Water=18u
molar mass of water =18g

1 mole (18g) of water contain 6.022 x 10²³ atoms
72g of water ---------->?
=72x6.022 x 10²³/18
=24..088 x 10 ²³ atoms

Case II:7mol of Hydrogen gas
1 mole of hydrogen gas contains 2g 
7 mole--------->?
=7x2/1=14g of hydrogen gas.
so, 
1mol (2g) --------> 6.022 x 10²³ atoms
14g --------->?
=14x6.022x10²³/2
=42.154x 10 ²³ atoms

Case III: 90g of Fe.
56g of Fe ----------->6.022 x 10²³
90g----------> ?
=90x6.022 x 10²³/56
=9.6352 x10 ²³ atoms

Case IV: 33g of O2 gas
1 mole of O2 contains 32g 
32g--------->6.022x10²³
33g--------?
=33x6.022x10²³/33
=6.210 x 10²³