Step 1) We must balance our equation.
[tex]Mg_3N_2+3K_2O\Rightarrow3\text{MgO}+2K_3N[/tex](we will work with this equation)
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Step 2) The limiting reactant = K2O
We need to find the mass of K3N, so we need its molar mass (use the periodic table):
Molar Mass K3N = 130 g/mol (approx.)
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Step 3) Use the stoichiometry
3 x 1 mole of K2O ----------- 2 x 130 g of K3N
14 moles of K2O----------- X
[tex]X\text{ = }\frac{14\text{ moles x 2x130 g}}{3\text{ x 1mole}}=\text{ 1213.13 g =1200 g (approx.) }[/tex]Answer: Mass of K3N = 1200 g (approx.)