The vapour pressure of a solution having 2.0g of a solute X(molar mass 32gmol−1) in 100g of CS2 (vapour pressure 854 Torr) is 848.9 Torr.The molecular formula of the solute is
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Molar mass of CS₂ = 64 + 12 = 76gmol⁻¹
Mole fraction of solvent :
Let M be molar mass of solute X
(100 / 76) / { (100/76) +( 2/M)}
= 1.316 / 1.316 + (2g / M)
Therefore :
848.9 Torr = {(1.316) / (1.316 + 2g / M)} × 854
848.9 = 1123.69 /(1.316 + 2g / M)
848.9 (1.3158 + 2g / M) = 1123.69
1116.98 + 1697.8 / M = 1123.69
1697.8 = 1123. 69 M - 1116. 98 M
6.71 M = 1697.8
M = 1697.8 / 6.71
M = 253.0 gmol⁻¹
253 / 32 = 7.9 = 8 (rounded off to a whole number)
Thus there are 8 S atoms in the solute.
The molecular formulae of X is thus :
X₈
Mole fraction of solvent :
Let M be molar mass of solute X
(100 / 76) / { (100/76) +( 2/M)}
= 1.316 / 1.316 + (2g / M)
Therefore :
848.9 Torr = {(1.316) / (1.316 + 2g / M)} × 854
848.9 = 1123.69 /(1.316 + 2g / M)
848.9 (1.3158 + 2g / M) = 1123.69
1116.98 + 1697.8 / M = 1123.69
1697.8 = 1123. 69 M - 1116. 98 M
6.71 M = 1697.8
M = 1697.8 / 6.71
M = 253.0 gmol⁻¹
253 / 32 = 7.9 = 8 (rounded off to a whole number)
Thus there are 8 S atoms in the solute.
The molecular formulae of X is thus :
X₈
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