On a day when the barometric pressure is 740 torr, a balloon containes (only) 8.
ID: 900212 • Letter: O
Question
On a day when the barometric pressure is 740 torr, a balloon containes (only) 8.00 L of a gaseous binary compound of mystery element X and fluorine at a certain temperature. When the entire contents of the balloon undergoes decomposition on this same day, the products are collected in separate balloons -- one containing 2.00 L of X8 (g) and the other containing 40.00 L of F2(g) -- both baloons maintained at the same temperature as the original balloon.
a) Determine the molecular formula of the binary compound.
b) On this same day at the same temperature, a separate balloon, filled with (only) 2.00 L of H2 (g), has a mass of 0.0400 grams. If the 2.00 L balloon of X8 (g) has a mass of 5.0706 grams, determine the atomic weight in (g/mol) and the identitiy of element X.
c) Write down the balanced reaction depicting the decomposition of the gaseous binary compound into X8(g) and F2(g) --identifying X in accordance with your results from (b).
Explanation / Answer
Number of moles of the Binary compound can be determined by using ideal gas equation.
PV=nRT
P= 740 torr, V= 8L R= 62.36 Ltorr/mole/K n=?
n = PV/RT = 740 torr * 8L/62.36 Ltorr/mole/K * T = 94.93/T moles
moles of X8 formed after decomposition = 740 torr * 2L/62.36 Ltorr/mole/K * T = 23.73/T moles
moles of F2 formed = 740 torr * 40L/62.36 Ltorr/mole/K * T = 478.66 /T moles
94.93/T moles of binary compound contains 23.73/T moles of X8. Number of X8 per mole of the binary compound = 1/3.9~ 1/4
1 mole X8 contain 8 atoms of X. So number of atoms of x in 1/4 mole of X8 = 1/4*8 = 2
94.93/T moles of binary compound contains 478.66/T moles of F2. Number of F2 per mole of the binary compound = 5/1
1 mole F2 contains 2 atom. Number of F atom in 5 mole = 5*2 = 10
Molecular formula of the compound = X2F10 = XF10
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moles of H2 gas present = mass/mol.wt = 0.04/2 = 0.02 moles
PV = nRT
T = PV/nR wher P = 740 torr, V=2 L, n = 0.02 moles R = 62.36 L.Torr/mole/K
T = 740 torr *2/0.02* 62.36 L.Torr/mole/K = 1186.6 K
moles of X8 present in the balloon
n = PV/ RT
= 740 * 2/62.36 * 1186.6 = 0.02 moles
moles = mass/molar mass
molar mass of the element =5.0706/0.02 = 253.53
atomic weight = 253.53/8 = 31.6
The element is Phosphorus (P)
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8PF5 ----------> P8 + 20F2
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