A cylinder is filled with pure oxygen. The initial pressure is 1 Atm and the tem
ID: 540090 • Letter: A
Question
A cylinder is filled with pure oxygen. The initial pressure is 1 Atm and the temperature is OoC. Initially, the volume of the gas is 0.500 liter.
The cylinder contains a small piece of solid red phosphorus. It is ignited and burns completely to form a white solid powder (P2O5). The difference in volume between the two solids can be neglected.
After cooling down to initial pressure and temperature, the volume of the gas is found to be reduced to 0.47 liter.
What was the mass of the phosphorus in milligrams?
Hint: First write a balanced reaction equation involving P, oxygen and the oxide.
Then use the "n" in the gas law to find what the volume change V corresponds to.
Atomic mass for P: 31 g/mol
(Answer within 1% of correct value)
Explanation / Answer
4P(s) + 5O2(g) ---> 2P2O5(s)
4 mol P = 5 mol O2
before reaction.
no of mol of O2 = n = PV/RT
= (1*0.5/(0.0821*273.15)
= 0.0223 mol
After the reaction,
no of mol of O2 = n = PV/RT
= (1*0.47/(0.0821*273.15)
= 0.02096 mol
no of mol of O2 reacted = 0.0223 - 0.0209 = 0.0014 mol
no of mol of P reacted = 0.0014*4/5 = 0.00112 mol
amount of P reacted = 0.00112*31 = 0.03472 g
= 34.72 mg
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