Kaplan General Chemistry Subject Test 2

[Meas]

Kaplan General Chemistry Subject Test 2:

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!

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[dentrilla]

Kaplan General Chemistry Subject Test 2:

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?
Can't remember this topic atm, don't have my notes on me either.. i'll try to answer this tomorrow
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?

132 grams of CO2 means we have 3 moles of CO2. CO2 is the only molecule on the right side of the equation with carbon, this means there must be 3 carbons on the left side of the equation aswell(assuming its balanced, which it must be in nature). 3C's weigh 36 grams

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?

Boron is made up of two istopes, one that weighs ~10 and another that weighs ~11. The atomic weight, as stated on the periodic table, is an average of all naturally occurring isotopes in nature.

If it was 50:50, Borons weight on the periodic table would be 10.50
If it was 40:60, 10.6
Since the actual weight is 10.8, assuming that these are the only 2 istopes it must be 20:80 (20% of the 10amu, and 80% of the 11amu)
(11*0.80)+(10*0.20) = 10.80

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!

 

[Meas]

Hey dentrilla,

I really appreciate your help! I have all my questions answered for practice tests 1, 2, & 5 (just one pressure question left) and I am now only left with 3 & 4! Thank you soooo much for your help!!!

Also thanks to everyone who have responded to my kaplan practice test threads!

 

[suhshi]

For question 9

Dalton;s law of partial pressures says that the pressure exerted by ideal gas in a mixture is the same as it would be if the gas were to occupy the same volume alone


Use Boyles Law

P1V1 = P2V2

P2 is partial pressure of neon

(300)(100)/500 = 60

 

[dentrilla]

Hey dentrilla,

I really appreciate your help! I have all my questions answered for practice tests 1, 2, & 5 (just one pressure question left) and I am now only left with 3 & 4! Thank you soooo much for your help!!!

Also thanks to everyone who have responded to my kaplan practice test threads!

no problem

For question 9

Dalton;s law of partial pressures says that the pressure exerted by ideal gas in a mixture is the same as it would be if the gas were to occupy the same volume alone


Use Boyles Law

P1V1 = P2V2

P2 is partial pressure of neon

(300)(100)/500 = 60

 

[Meas]

Thanks everyone! Using P1V1 = P2V2 is a lot easier to calculate, but I don't know why the partial pressure formula will not work though (partial pressure of A = total pressure X mole fraction of A).