To calculate the mass of one mole of barium nitrate, we need to add the atomic masses of each atom in the compound. Barium nitrate has the chemical formula Ba(NO3)2, which means it contains one barium atom, two nitrogen atoms, and six oxygen atoms.

- The atomic mass of barium (Ba) is 137.33 g/mol.

- The atomic mass of nitrogen (N) is 14.01 g/mol.

- The atomic mass of oxygen (O) is 16.00 g/mol.

So, the mass of one mole of barium nitrate can be calculated as follows:

Mass of Ba(NO3)2 = (1 × 137.33 g/mol) + (2 × 14.01 g/mol) + (6 × 16.00 g/mol)

= 137.33 g/mol + 28.02 g/mol + 96.00 g/mol

= 261.35 g/mol

Therefore, the mass of one mole of barium nitrate is 261.35 grams.