CBSE Class 12-science Answered
6% of weight of non volatile solute in isopentane(M=72) at 300K has V.P of 1426mm of hg.20g of isopentane is now added to the solution and the resulting solution has vapour pressure(V.P) of 1445.calculate V.P of isopentane
Asked by banga71 | 17 May, 2017, 09:38: PM
Expert Answer
P = Pº x mole fraction
6% by mass means 6g of 100g solution = solute
94 g isopentane / 72 g/mole = 1.3 moles
adding 20g isopentane
6g solute in 120g solution where 114g = isopentane
114 g / 72 g/mole = 1.58 moles
1426 = Pº x 1.3 / (1.3 + y)
1445 = Pº x 1.58 / (1.58 + y)
set equations = to each other since Pº is constant and y (moles solute) is constant
1426 x (1.3 + y) / 1.3 = 1445 x (1.58 + y) / 1.58
y = 0.104 moles solute
P = Pº x mole fraction isopentane
1426 = Pº x (1.3 / (1.3 + 0.104))
Pº = 1540.1 mmHg
6% by mass means 6g of 100g solution = solute
94 g isopentane / 72 g/mole = 1.3 moles
adding 20g isopentane
6g solute in 120g solution where 114g = isopentane
114 g / 72 g/mole = 1.58 moles
1426 = Pº x 1.3 / (1.3 + y)
1445 = Pº x 1.58 / (1.58 + y)
set equations = to each other since Pº is constant and y (moles solute) is constant
1426 x (1.3 + y) / 1.3 = 1445 x (1.58 + y) / 1.58
y = 0.104 moles solute
P = Pº x mole fraction isopentane
1426 = Pº x (1.3 / (1.3 + 0.104))
Pº = 1540.1 mmHg
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