To determine the mass of oxygen that contains the same number of molecules as 42 g of nitrogen, we can use Avogadro's number, which represents the number of atoms, molecules, or ions present in one mole of a substance. Avogadro's number is approximately 6.022 x 10^23.

First, we need to determine the number of molecules in 42 g of nitrogen. To do this, we divide the given mass by the molar mass of nitrogen. The molar mass of nitrogen (N2) is approximately 28 g/mol.

Number of molecules in 42 g of nitrogen = (42 g) / (28 g/mol)

Number of molecules in 42 g of nitrogen ≈ 1.5 mol

Since we want the same number of molecules in oxygen (O2), we can use the molar mass of oxygen to calculate the mass required. The molar mass of oxygen is approximately 32 g/mol.

Mass of oxygen with the same number of molecules = (Number of molecules) x (Molar mass of oxygen)

Mass of oxygen ≈ (1.5 mol) x (32 g/mol)

Mass of oxygen ≈ 48 g

Therefore, 48 g of oxygen contains the same number of molecules as 42 g of nitrogen.