Question

Suppose the elements “X” and “Y” combine to form two compounds $X{Y_2}\,and\,{X_3}{Y_2}$. When 0.1 moles of $X{Y_2}$ weighs 10 g , and 0.05 moles of $\,{X_3}{Y_2}$ weighs $9\,g$ , the atomic weight of “X” is ___?

Answer 1

Atomic weight: The sum of masses of protons and neutrons in an atom is called its atomic weight
Molecular weight: Sum of masses of all the atoms present in a molecule multiplied by their coefficients in the formula is called molecular weight
Mole concept: Gram atomic weight or gram molecular weight of an atom/molecule contains ${N_0}$ atoms/molecules where $N_0 = 6.023\times {10}^{23}$

Complete step by step solution:
Let atomic weight of element X is x and that of element Y is y
0.1 mole of $X{Y_2}$ weighs $10\,g$
$\therefore$ 1 mole of $X{Y_2}$ weighs $\dfrac{10}{0.1}=100\,g$
$\Rightarrow x + 2y = \dfrac{{10}}{{0.1}} = 100$ --(I)
0.05 moles of $\,{X_3}{Y_2}$ weighs $9\, g$.
Molar mass of $\,{X_3}{Y_2}$= $\dfrac{9}{{0.05}} = 180$
$\Rightarrow 3x+2y=180$ --(II)
Solving (I) and (II) by substitution method.
$\Rightarrow x = 100 - 2y$
$\Rightarrow 3(100 - 2y) + 2y = 180$
$\Rightarrow 300-6y+2y=180 \Rightarrow y =30$

**Hence, x = $[100 - 2(30)] = 40$

Note: **
Molecule is the general term used to describe any atoms that are connected by chemical bonds. Every combination of atoms is a molecule. A compound is a molecule made of atoms from different elements. All compounds are molecules, but not all molecules are compounds
The combination of atoms occurs because they want to attain a stable configuration of octet
Oxygen atoms have six electrons in its valence shell. To attain stability it shares two electrons with another oxygen atom or two hydrogen atoms.