A white substance was known to be either magnesium hydroxide, calcium hydroxide, or zinc oxide. A 1.91 g sample of the substance was mixed with 50.0 mL of water and 100.0 mL of 0.717 M HCl was added. The mixture was stirred, causing all of the sample to dissolve. The excess HCl in the solution required 38.4 mL of 0.164 M NaOH for complete neutralization in a titration. What was the molecular formula of the white substance?