The formula mass of Na 2 S is 55 u.

The formula given seems to be a mix of elements and formatting symbols rather than a chemical compound. Na is sodium, but the rest does not correspond to recognizable chemical notation. Therefore, a formula mass cannot be accurately determined from the given string.

The formula mass of Na 2 S is 55 u.

The formula given seems to be a mix of elements and formatting symbols rather than a chemical compound. Na is sodium, but the rest does not correspond to recognizable chemical notation. Therefore, a formula mass cannot be accurately determined from the given string.