Question

what is the mole percentage of O₂ in a mixture of 7g of N₂ and 8g of O₂

Answer 1

molar mass of O₂ = 32g
8g of O₂  = 8/32 moles = 0.25 mole
molar mass of N₂ = 28g
7g of O₂  = 7/28 moles = 0.25 mole

$mole\ percentage\ of\ O_2 = \frac{moles\ of\ O_2}{moles\ of\ O_2+moles\ of\ N_2}*100 \\ \\mole\ percentage\ of\ O_2 = \frac{0.25}{0.25+0.25} *100=\boxed{50}$

Answer 2

Moles of O2 for 8 g of O2 = 8/32 = 1/4
Moles of N2 for 7 g of N2 = 7/28 = 1/4
Total moles = 1/4 + 1/4 = 1/2
Mole percentage of O2 = {(1/4) / (1/2)}*100 = 50%