To find the mass of oxygen gas that reacted with the iron nail, we can use the information about the mass of the iron nail and the mass of the rusted nail.
- The initial mass of the iron nail before rusting is 100 g.
- The mass of the rusted nail after rusting is 143 g.
The increase in mass due to the formation of rust is calculated as follows:
\[ \text{Mass of rust} = \text{mass of rusted nail} - \text{mass of iron nail} \]
\[ \text{Mass of rust} = 143 , \text{g} - 100 , \text{g} = 43 , \text{g} \]
This increase in mass (43 g) comes from the oxygen gas that reacted with the iron to form rust (iron oxide).
Therefore, the mass of oxygen gas that reacted with the iron nail is:
\[ \boxed{43 , \text{g}} \]