Ammonium molybdate is a chemical compound with the formula (NH4)2MoO4. It is commonly encountered in two different forms:

1. Ammonium molybdate tetrahydrate ((NH4)2MoO4 · 4H2O): This is the most common form of ammonium molybdate. It is a white crystalline solid that contains four water molecules per formula unit. It is often used as a source of molybdenum in various applications, such as analytical chemistry, fertilizers, and catalysts.

2. Ammonium molybdate dihydrate ((NH4)2MoO4 · 2H2O): This form of ammonium molybdate is similar to the tetrahydrate but contains two water molecules per formula unit. It is also a white crystalline solid and finds applications in similar fields as the tetrahydrate.

Both forms of ammonium molybdate are widely used in laboratories and industries for their molybdenum content, which is an important element in various chemical processes and applications.