What mass of magnesium oxide is formed during the complete thermal decomposition of magnesium nitrate?

Given:
m (Mg (NO3) 2) = 20 g

To find:
m (MgO) -?

Decision:
1) Write the reaction equation:
2Mg (NO3) 2 => 2MgO + 4NO2 ↑ + O2 ↑;
2) Calculate the molar masses of Mg (NO3) 2 and MgO:
M (Mg (NO3) 2) = Mr (Mg (NO3) 2) = Ar (Mg) + Ar (N) * 2 + Ar (O) * 6 = 24 + 14 * 2 + 16 * 6 = 148 g / mole;
M (MgO) = Mr (MgO) = Ar (Mg) + Ar (O) = 24 + 16 = 40 g / mol;
3) Calculate the amount of Mg (NO3) 2 substance:
n (Mg (NO3) 2) = m (Mg (NO3) 2) / M (Mg (NO3) 2) = 20/148 = 0.1 mol;
4) Determine the amount of MgO substance:
n (MgO) = n (Mg (NO3) 2) = 0.1 mol;
5) Calculate the mass of MgO:
m (MgO) = n (MgO) * M (MgO) = 0.1 * 40 = 4 g.

Answer: The mass of MgO is 4 g.