Oxide of element has 72.72% oxygen. Relative vapor density of chloride with respect to hydrogen is 77.
Calculate the atomic weight of that element.
Solution: Mass fraction of the oxygen in the oxide of the unknown element E can be calculated as:
 (O ) 
1
2  A( E )  y  A(O ) 2  A( E )
y  A(O)
y  A(O)


 1, and we can find

, then
 (O )
y  A(O )
y  A(O)
M ( E2O y ) 2  A( E )  y  A(O)
the equivalent weight of the element: A( E ) y 
 16  1
A(O)  1


 1   
 1  3.0 g/eq.
2   (O )  2  0.7272 
As you know, relative density of the gas with respect to hydrogen can be calculated as: Dx / H 2 
M ( x)
,
M (H 2 )
where M(x) is the molar mass of the gas, g/mol.
Then, M ( ECl y )  M ( H 2 )  DECl y / H 2  2  77  154 g/mol.
M ( ECl y )  A( E )  y  A(Cl )  y   A( E ) y  A(Cl ) , and then y 
A( E )  3  y  3  4  12 g/mol.
Answer: 12 g/mol.
M ( ECl y )
A( E ) y  A(Cl )

154
 4.
3  35.5