Question

CLASS X ICSE :
Calculate the number of gram atoms in 4.6g of sodium (Na=23)
2)Calculate the percentage of water of crystallization in CUSO$.5H2O
(h=1, O=16, S=32,Cu=64]
3)A compound of X and Y has the empherical formula XY2.
Its vaopour density is equal to its empherical formula weight. Determine its molecular formula.

Answer 1

1)
Molar mass of sodium = 23 g
Number of atoms in 23 g of sodium = 6.0221023 atoms
Number of atoms in 4.6 g of sodium = 
Gram atom is defined as that mass of an element which is numerically equal to atomic mass of the element and is expressed in grams. One gram atom is equal to the gram atomic weight of an element. 
 1 gram atom of sodium = Gram atomic weight of sodium = 23 g

2) Molar mass of CuSO4.5H2O =250
mass of water of crystallization = 5 x 18 = 90
% = 90/250 x 100 = 36 %

Answer 2

1)
given :
mass of sodium=4.6g
Atomic mass of Na=23
number of gram atoms=?
Number of gram atoms of Na= Mass of Na/ Atomic mass of Na
=4.6g/23g=0.2g

ii)Given :
Atomic mass of H=1u
Atomic mass of O=16u
Atomic mass of S=32u
Atomic mass of Cu=64u
molar mass of CUSO4.5H20 =64+32+4x16 +5x18=250g
mass of water of cyrstallisation=90g
Percentage of water of crystallisation =90x100/250=36% water of crystallization.

III)Empirical Formula=XY2
Vapour density=Empirical formula weight
molecular formula=?
n(empirical formula weight)=2x V.D
Since vapour density=empirical formula weight.
n=molecular formula = 2( Empirical formula)=2(XY2)=X2Y2