Ammonium arsenite is a compound that doesn't exist in a stable form.

Here's why:

* Arsenite ion (AsO₃⁻³) is unstable in water solutions. It readily hydrolyzes, forming arsenate (AsO₄⁻³) and arsenious acid (H₃AsO₃).

* Ammonium ions (NH₄⁺) are also unstable in the presence of arsenite. The reaction between ammonium and arsenite would likely produce ammonia gas (NH₃) and an arsenite salt.

Therefore, you wouldn't find a specific formula for ammonium arsenite because it's not a stable compound.