Answer is: the mass of oxygen is 3.31 grams.
m(NaHCO₃) = 5.8; mass of sodium bicarbonate.
n(NaHCO₃) = m(NaHCO₃) ÷ M(NaHCO₃).
n(NaHCO₃) = 5.8 g ÷ 84 g/mol.
n(NaHCO₃) = 0.07 mol; amount of sodium bicarbonate.
In one molecule of sodium bicarbonate there are three oxygen atoms:
n(NaHCO₃) : n(O) = 1 : 3.
n(O) = 3 · 0.07 mol.
n(O) = 0.21 mol.
m(O) = 0.21 mol · 16 g/mol.
m(O) = 3.31 g.
