Answer:
:
Definition: 2 Marks
Numerical: 3 Marks
Onemoleis defined as the amount of substance containing as many elementary entities (atoms, molecules, ions, electrons, radicals, etc.) as there are atoms in 12 grams of carbon-12. The number of particles that is atoms, ions and molecules present in one mole of a substance is fixed with a value of $6.02\times {10}^{23}$ molecules.
Mole of X element is ;
$nx=(9.03\times {10}^{23}\times (6.02\times {10}^{23})=0.15mol$
Atomic mass of X;
$A_x=\frac{8.4}{0.15}=56g/mol$
Molar mass of $X_2{Y}_3$ compound;
$M_{X_2{Y}_3}=\frac{16}{0.1}=160g/mol$
We find atomic mass of Y by;
2.X + 3Y =160
2.(56) + 3(Y)=160
Y=16g/mol is atomic mass of Y.