Find the total percentage of oxygen in magnesium nitrate crystals : Mg(NO₃)₂. 6H₂O

[O = 16, N = 14, H = 1, Mg = 24]

Molecular weight of magnesium nitrate Mg(NO₃)₂. 6H₂O

= 24 + 2[14 + 3(16)] + 6[2(1) + 16]
= 24 + 2[14 + 48] + 6(18)
= 24 + 2(62) + 6(18)
= 24 + 124 + 108
= 256 g

Mass of oxygen in Mg(NO₃)₂. 6H₂O
= 6(16) + 6(16)
= 96 + 96 = 192

256 g of magnesium nitrate crystals contains 192 g of oxygen

∴ 100 g of magnesium nitrate crystals contains = $\dfrac{192}{256}$ x 100 = 75%

Hence, total percentage of Oxygen in magnesium nitrate crystals is 75%