The mole fraction of water in a sulphuric acid solution is 0.85. What is the molality of the solution?
Answer & explanation
Correct answer: option 4
Mole fraction of water = 0.85
Mole fraction of H2SO4 in solution = 1 - 0.85 = 0.15
If n1 is the the number of moles of water and n2 is the no of moles of H2SO4 in solution.
Molality of solution means moles of H2SO4 present in 1000 g of water
n1 = \(\frac{1000}{18}\) = 55.55, n2 = ?
Mole fraction of H2SO4 = \(\frac{n_2}{55.55 + n_2}\)
0.15 = \(\frac{n_2}{55.55 + n_2}\)
0.15 n2 + 8.3325 = n2
8.3325 = 0.85 n2
n2 = \(\frac{8.3325}{0.85}\) = 9.8 m