Answer: 17.796 g
Explanation: Molar masses of the given elements:
mMg= 24.3 g/mol
mN=14 g/mol
mO=16 g/mol
molar mass of Mg(NO3)2:
mMg(NO3)2= 24.3+(14+3*16)*2
mMg(NO3)2=148.3 g/mol
To calculate the mass of 0.12 mol of Mg(NO3)2 we multiply the molar mass by the number of moles:
mass=0.12mol*148.3 g/mol
mass=17.796 g