As mentioned in previous threads, I am posting questions I have about subject tests that I did. Hope to get responses you all!

9) A 300ml flask containing nitrogen at a pressure of 100 torr & a 200 mL flask containing oxygen at a pressure of 200 torr are connected such that the gases are allowed to fill the combined volume of 2 flasks. What is the partial pressure of nitrogen in the combined volume?

A: 60 torr

--I used Pressure of nitrogen / total pressure = # of moles of nitrogen / total # of moles. Plugging in #s, I have 100/300 = 1/3. Then using, partial p of nitrogen / (total p X mole fraction of nitrogen), I have (300)(1/3) = 100 torr.

20) Combustion of a 50 gram sample of an unknown hydrocarbon yields 132grams of CO2 & 126 grams of H2O. About how many grams of carbon were contained in the original sample?

A: 36grams

--I have no idea how to even start this question!

30) Boron found in nature has an atomic weight of 10.811 & is made up of isotopes 10B (mass 10.013 amu) & 11B (mass 11.0093 amu). What is % of naturally occurring obron is made up of 10B & 11B respectively?

A: 20:80

--Atomic weight has a unit of gram/mol. I am a little confused what amu is & what unit it has.

Thanks in advance!