The mass number of 3 isotopes of magnesium are 24,25,26.what is the mass number of the most abundant isotope of magnesium.​

mole fraction boron trifluoride :  0.496

mole franction chlorine pentafluoride gas : 0.504

Explanation:

This is a problem where we have to remember a definition to solve it right away and where there is some additional data which can make us lose sight of the easy solution.

Mole fraction of component A is defined as the ratio of the moles A over the total number of moles in the mixture:

Χₐ = nₐ/ ( na + nb + nc +)

For a two gas mixture it reduces to:

Χₐ = na/(na + nb)

we have the masses so we can convert them to moles as follows:

na = ma/MWₐ      

MW boron triflouride = 67.82 g/mol

nₐ = 8.62 g / 67.82 g/mol  = 0.127 mol

MW chlorine pentafluoride : 130.45 g/moL

nb = 16.9 g / 130.45 g/mol = 0.129 mol  

( MW chlorine pentafluoride : 130.45 g/moL)

Χₐ = 0.127 mol  / (0.127+0.129) mol = 0.496

Χb = 0.129 mol /  (0.127+0.129) mol =0.504   ( also since its 2 gases we can calculate the mole fraction of b by using the expression Xa + Xb =1)