To calculate the formula mass of ammonium ion (NH4+), we sum the atomic masses of each constituent atom:

- Nitrogen (N): 1 atom × 14.01 atomic mass units (amu) = 14.01 amu

- Hydrogen (H): 4 atoms × 1.008 amu = 4.032 amu

Formula mass of ammonium ion (NH4+) = 14.01 amu + 4.032 amu = 18.042 amu.