To calculate the mass of an Sn atom in atomic mass units (AMU), you need to consider the number of protons and neutrons in its nucleus. Each proton contributes approximately 1 AMU, while each neutron contributes approximately 1 AMU. Here's the calculation:

Number of protons (p) = 50

Number of neutrons (n) = 70

Mass (in AMU) = (Number of protons * Mass of a proton) + (Number of neutrons * Mass of a neutron)

Mass = (50 * 1 AMU) + (70 * 1 AMU)

Mass = 50 AMU + 70 AMU

Mass = 120 AMU

Therefore, the mass of the Sn atom with 50 protons and 70 neutrons is 120 AMU.