3 Q

[Ice-cream]

1. the radius of an argon atom is 4.2e-8 cm. What percent of the total volume of a 1 mol sample of argon gas at STP is occupied by the argon atoms? [0.83]

 

im really stuck on this one!

 

2. calculate the P[total] in atom for a mixture of 3 gases that have partial pressures of 72cmHg, 400mmHg and 0.65mHg respectively.[ans:2.33]

 

dont i just add those up and then divide by 760 to change to atm?? coz that doesnt give me the right ans.

 

3. A volume of 10cm^3 each of O2 gas and CO gas at STP are mixed in a 20cm^3 container and allowed to react at constant temperature to yield CO2. Calculate P[total] in atm after the reaction is complete. [ans: 0.75]

 

i thought P1V1 = P2V2 so for one of the gases, say O2, P2 would be 0.5 coz P1 = 1atm, V1 = 10, V2 = 20...then since volumes of both gases are equal, both would have partial pressure of 0.5 so total pressure would be 1.

[Aeternus]

1)

For the first question, it looks like its simply a case of working out the volume one atom takes up, converting this in dm^3 and then multiplying this out to calculate the volume 1 mole of these atoms would take up and then taking this as a ratio/percentage of 22.4dm^3 (1 Mole of Gas at STP).

 

So thats the volume of a sphere - (4/3) x pi x r^3 = (4/3) x pi x (4.2e-8)^3

Which is - 3.1033e-22 cm^3

Which is - 3.1033e-25 dm^3 (1dm^3 = 1000cm^3)

 

So then take that and multiply by Avogadros Constant (number of molecules/atoms of an element/compound in one mole of that element/compound).

 

So thats 6.022045e23 x 3.1033e-25 = 0.186887 dm^3

 

Then work that out as a percentage of 22.4 dm^3.

Which is (0.186887/22.4) x 100 = 0.8343% = 0.83% (2sf)

 

 

2)

 

Well for Question 2 you do just add but it looks like you have to convert all of them to mmHg first, so thats 720mmHg, 400mmHg, 650mmHg, which when summed is 1770. Divide that by 760 as you suggested and it comes to 2.33 (2dp/3sf).

 

 

3)

My guess on the third question is that as the formula for the reaction is -

 

2CO + O2 => 2CO2

 

And the number of moles of O2 and CO2 are actually the same, youll end up with some O2 left in the ratio 0.5:1 (O2:CO2) and assuming the CO2 has a partial pressure of 0.5 like the CO, as we now have have the number of atoms of O2 its partial pressure will be halfed (half the number of atoms to collide with etc etc) and so will be 0.25 so adding these together gives 0.75. Not sure if thats right but its a theory.