[Vileblood]

Alright so i was out the whole week we were learning about gas laws and now have to do a few problems i can't seem to understand.

Any or all help will be appreciated greatly.

1) When a steel cylinder of oxygen gas with a volume of 14.50 L was used to supply oxygen to an oxyacetylene torch, the pressure of the cylinder changed from 208.00 atm to 201.00 atm. The temperature of the oxygen was at 25.00 degrees C when both reading were made.
a) write a complete balanced equation with phases for the combusion of acetylene gas (C2H2) if the only products are carbon dioxide and water vapor (and heat).
b) how many moles of oxygen (O2) were consumed in the reaction?
c) how many grams of oxygen (O2) were consumed?
d) how many moles of acetylene were burned in the reaction?

so temperature is constant, so i can use boyles law: P1V1= P2V2 correct? but what do i use this equation for?
balanced equation is 2 C2H2 + 5 O2 > 4 CO2 + 2 H2O

2) An antimalarial drug sample has a mass of 0.2394 g and is made up of carbon, hydrogen, oxygen, and nitrogen. This compound was reacted so that all its nitrogen was converted to 18.90 mL nitrogen gas at 23.80 degrees C and 746.0 torr. The vapor pressure of water when the nitrogen was collected at 23.80 degrees C was 22.110 torr.
a) calculate the percent nitrogen in the sample
b) when 6.478 mg of the compound burned in pure oxygen, 17.57 mg of carbon dioxide and 4.319 mg of water were obtained. Calculate the % of the other elements in this compound.
c) what is the empirical formula of this compound?
d) if the molecular mass is 324 g/mol, what is the molecular formula?

i will have to convert torr to atm right? and to do that 1.000000 atm = 760.000 torr
746.0 torr/760.000 torr= 0.9816 atm
22.110 torr/760.000 torr= 0.029092

[Stovn0611]

For #1, do you expect the steel cylinder's volume to change? Try using a different equation involving all of the given data(volume, final pressure, temperature), then use stoichiometry to solve b, c, and d.

For #2, it is not necessary to convert to atm. Since all of the compound's nitrogen was converted to nitrogen gas, the compound has no more nitrogen in it and all the nitrogen that was in it originally is now in a gaseous form. Find the partial pressure of the nitrogen gas, then use the other data to solve for the amount of nitrogen gas(the same as the amount of nitrogen in the original compound)

2. b) The reaction would look something like this CHON + O2 -> CO₂ + H₂O
(not balanced and not correct subscripts for CHON)

Atoms are conserved in every reaction so consider how the amount of carbon dioxide produced relates to the amount of carbon in the original compound and how the amount of water relates to the amount of hydrogen in the original compound. Now you have the percentages of nitrogen(part a), carbon, and hydrogen. The only other element in the compound is oxygen and the percent composition of each element in a compound must sum to 100% so you can solve for the oxygen percentage.

c) Now that you have the mass percent composition of the compound, try this - assume you have 100 g of the compound, then what are the amounts of each element in that compound? Remember that formulas tell mole ratios so you'll have to convert the amount of each element in the compound from grams to moles then find a ratio to determine the empirical formula.

d) Find the molar mass given by the empirical formula, then think about how the subscripts of the empirical formula will have to be changed to give a molar mass of 324 g/mol

[Vileblood]

For #1, before i do the calculations i just wanna make sure..i should use the ideal gas law PV= nRT correct? and solve for n (number of moles) so n= PV/RT and also i will have to convert atm to kPa in order to use this equation right?

[Stovn0611]

Yes, you should use the Ideal Gas Law, but you do not necessarily need to convert anything

The ideal gas law is PV=nRT

R is a constant that has differing units based on what pressure unit you want to use( 0.08206 L*atm/(mol*K), 62.4 L*mmHg/(mol*K), or 8.3145 L*kPa/(mol*K))

[Vileblood]

do i use the difference in pressure in the equation and when i get n, that's the number of moles for the whole reaction? err sorry i'm just really confused.

[Stovn0611]

Think about it this way - you started with

n=(208*14.5)/(0.08206*298)

and end with

n=(201*14.5)/(0.08206*298)

So to find how much was used up you subtract the final amount from the initial

(208*14.5)/(0.08206*298) - (201*14.5)/(0.08206*298) = (208-201)(14.5)/(0.08206*298) = (7)(14.5)/(0.08206*298) which represents the moles of oxygen used in the reaction. So yes - you use the difference in pressure in the Ideal Gas Law to solve for the moles of oxygen used in the reaction.

[Vileblood]

Alright thank you, that's what i did. Here are the values i got:
b)4.15 moles of O2 consumed
c)26.6 g O2 consumed
d) 1.66 moles C2H2 burned

[Stovn0611]

How did you get your answer to part c? Are you using the correct molar mass of oxygen to convert 4.15 moles of O2 to grams?

[Vileblood]

i did 4.15 moles O2 x 1 mole O2 x 32.00 g/mole =  26.6 g O2
                   1        5 moles O2          1 mole O2

[Stovn0611]

Why use 1 mole O2/5 moles O2? The number of moles of oxygen used in the reaction can be directly converted to the number of grams of oxygen used in the reaction using only the conversion factor 32.00 g O2/1 mole O2.

[Vileblood]

hm guess i don't know why i did that but now i got 133. g O2. Thank you
now for problem #2, sorry but how do i find partial pressure?

[Vileblood]

could i do this: 0.78 (air composition) x 746.0 torr= 580 torr

[Stovn0611]

Use Dalton's Law of Partial Pressures

P_total = P₁ + P₂ where 1 and 2 are the two gases contributing to the pressure.

In this case, the two substances exerting pressure are the water vapor and nitrogen gas and you are given the vapor pressure of water for this temperature so you know the partial pressure of water, and can use Dalton's Law of Partial Pressures to solve for the partial pressure of the nitrogen gas.

[Vileblood]

So the partial pressure of the nitrogen gas is 723.9 torr and to find the percent nitrogen in the sample i would just divide that partial pressure by the total pressure (746.0 torr) and get 97.04%?

[Vileblood]

or is it not that simple? could it be that i have to use the ideal gas law again and solve for n, then once i get moles of n2 convert to grams and divide that amount by the mass of the sample (0.2394 g) x 100 to get %nitrogen?