Answer:
The molarity of MgCl₂ in water is 0.088 mol/L.
Step-by-step explanation:
The molarity of MgCl₂ can be calculated as follows:

Where:
C: is the molarity =?
η: is the number of moles of MgCl₂ = m/M
V: is the volume of water = 500 mL = 0.500 L
m: is the mass of MgCl₂ = 4.18 g
M: is the molar mass of MgCl₂ = 95.211 g/mol
Hence, the molarity is:

Therefore, the molarity of MgCl₂ in water is 0.088 mol/L.
I hope it helps you!