For a current of 15.0 A is passed through molten magnesium chloride for 15.0 hours, the amount of moles of magnesium metal is mathematically given as
M=4.2mole
Generally, the equation for the Chemical reaction is mathematically given as
Mg+2e---->Mg
Therefore
Charge=Currect*time
Q=15*5*4.0*10^4
Q=8.10*10^5C
In conclusion, the number of moles
8.10*10^5C*1M/(2*96500)
M=4.2mole
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