Answer:
Aluminum
Step-by-step explanation:

Mass of
= 1.68 g
Molar mass of
= 143.32 g/mol
Thus,

From the reaction below:

3 moles of
are produced when 1 mole of
undergoes reaction.
So,
1 mole of
are produced when
mole of
undergoes reaction.
0.01172 mole of
are produced when
mole of
undergoes reaction.
Thus, moles of
= 0.0039 moles
Let the atomic mass of X = x g/mol
atomic mass of chlorine = 35.5 g/mol
Thus, Molar mass of
= x + 3(35.5) g/mol = x + 106.5 g/mol
Moles = 0.0039 moles
Mass = 0.521 g
Thus, molar mass = Given mass/ Moles = 0.521 / 0.0039 = 133.5897 g/mol
So,
x + 106.5 = 133.5897
x = 27.0897 g/mol
This Atomic weight corresponds to Aluminum. Hence, X is aluminum.