In an equimolar mixture of o2 and n2, are the mass fractions equal?

No

Explanation:

The mass fraction is defined as:

where:

The mass fraction of two substances (A and B), will be the same, ONLY if the mass of the substance A (mA) is the same as the mass of the substance B (mB).

An equimolar mixutre of O2 and N2 has the same amount of moles of oxygen and nitrogen, just to give an example let's say that the system has 1 mole of O2 and 1 mole of N2. Then using the molecuar weigth of each of them we can calculate the mass:

mA= 1 mole of O2 * 16 g/1mol = 16 g

mB=1 mole of N2 *28 g/1mol=28 g

As mA≠mB then the mass fractions are not equal, so the answear is NO.