Molybdenum trioxide - MoO3
The sample, shown here, is not completely white. This is a sample from a ceramics and pottery supplier. This solid, however, dissolves easily in solutions of NaOH and forms a colorless and totally clear solution of a molybdate. The slight offset in color may be due to a minute oxygen deficiency in the solid: MoO3-x, instead of MoO3, with x very small.