The interaction of aluminum oxide with sodium hydroxide formed a sodium aluminate

The interaction of aluminum oxide with sodium hydroxide formed a sodium aluminate salt with a mass of 21 g. Find the mass of aluminum oxide.

Given:
m ((NaAlO2) = 21 g
To find:
m (Al2O3) =?
Decision:
1) We compose the equation of the chemical reaction, let the mass of aluminum oxide be x g.
Al2O3 + 2NaOH = 2NaAlO2 + H2O
2) Find the molecular weights of aluminum oxide and sodium aluminate.
M (Al2O3) = 54 + 48 = 102 g / mol
M (NaAlO2) = 23 + 27 + 32 = 82 g / mol, since the coefficient is 2, then 164 g / mol
3) x g: 102 g / mol = 21 g: 164 g / mol
x = (102 x 21): 164 = 13.06 g
Answer: m (Al2O3) = 13.06 g.