I posted this question a bit ago and received some help..but need more.. thanks
Use the following information to identify element A and come
pound B, then answer questions a and b.
An empty glass container has a mass of 658.572 g. It has a
mass of 659.452 g after it has been filled with nitrogen gas at a
pressure of 790. torr and a temperature of 15°C. When the container
is evacuated and refilled with a certain element (A) at a
pressure of 745 torr and a temperature of 26"C, it has a mass of
660.59 g.
Compound B, a gaseous organic compound that consists of
85.6% carbon and 14.4% hydrogen by mass, is placed in a stainless
steel vessel (10.68 L) with excess oxygen gas. The vessel is
placed in a constant-temperature bath at 22°C. The pressure in
the vessel is 1 1.98 atm. In the bottom of the vessel is a container
that is packed with Ascarite and a desiccant. Ascarite is asbestos
impregnated with sodium hydroxide; it quantitatively absorbs
carbon dioxide:
2NaOH(s) + CO2(g) -----> Na2CO3(s) H2O(l)
The desiccant is anhydrous magnesium perchlorate, which
quantitatively absorbs the water produced by the combustion reaction
as well as the water produced by the above reaction. Neither
the Ascarite nor the desiccant reacts with compound B or
oxygen. The total mass of the container with the Ascarite and
desiccant is 765.3 g.
The combustion reaction of compound B is initiated by a
spark. The pressure immediately rises, then begins to decrease,
and finally reaches a steady value of 6.02 atm. The stainless steel
vessel is carefdly opened, and the mass of the container inside
the vessel is found to be 846.7 g.
A and B react quantitatively in a 1: 1 mole ratio to form one
mole of the single product, gas C.
a. How many grams of C will be produced if 10.0 L of A and
8.60 L of B (each at STP) are reacted by opening a stopcock
connecting the two samples?
b. What will be the total pressure in the system?
To get element A:
I used pv=nrt
v=nrt/p
n= (O.88gN2)(1molN2/28.02gN2) = 0.0314mol
T=288k
P=1.0395atm
V=(0.0314)(0.082057)(288)/1.0395
= 0.714L which is the volume of the glass container, which = volume of A right??
then to get the moles of A i used
n=pv/rt
T=299k
V=0.714L
P=0.9803atm
n=0.0285moles A
mass A=660.59g-658.572g = 2.018g of A
molar mass A= g/mol = 2.018g/0.0285mol
=70.8g/mol A.. which the closes thing to is Ga 69.723g/mol. Did I do it right???
So far so good EXCEPT for the conclusion for the identity of A. Your molar mass is good and all the work leading up to the final number is good. However, here is a web site showing the melting point of Ga.
http://www.webelements.com/webelements/elements/text/Ga/heat.html
So you see, Ga is a solid at about 30^{o} and the gas you used to fill the container was at 26^{o}. That pretty well eliminates Ga, doesn't it? Have you thought of a diatomic gas so that 2 times the atomic mass would be close to 70.8?
Cl2 is the diatomic gas....
For compound B, assume that the percentages of Carbon and hydrogen are from 100g, so there are 85.6 g C, so 7.1274molC, 14.4g H, 14.285molH. So 14.285/7.1274 = 2/1 ratio
Empirical form: H2C ??
from there on I am completely lost with the Ascarite and desiccant stuff.
Cl2 probably is correct (I haven't worked all the problem). (By the way, put this information in your head for the future. There are ONLY 11 elements that are gases and 6 of those of the noble gases. The other 5 are H, N, O, F, and Cl. Cl, of course, is the only one close to 35.4. CH2 is correct for the empirical formula of the hydrocarbon. I will post a note about the ascarite part of the problem separately.
Some hints to get you started.
Write the equation.
2CH2 + 3O2 ==> 2CO2 + 2H2O
1. The pressure is 11.98 at the beginning (includes pressure of B + pressure of oxygen) and it decreases to 6.02 so 11.98-6.02 is how much the pressure decreased due to the burning of compound B. Use PV = nRT to calculate the number of mols that are gone. That will be due to the C in element B being converted to CO2, the H heing converted to H2O, and of course the decrease in oxygen.
2. You see 2 mol CO2 formed and 2 mols H2O formed. That will be absorbed by the ascarite and Mg(ClO4)2. The gain in weight of the inner container is due to the CO2 and H2O. I found that gain in mass to be 81.4 grams just be subtracting the mass after the burn minus the mass before the burn. Now, how to split that up into how much is CO2 and how much is H2O.
3. One way is to observe that CO2 and H2O are in a 1:1 ratio; therefore, the mols of CO2 and H2O absorbed by the ascarite + desiccant are 1:1 ratio.
1 mol CO2 will have a mass of 44 g.
1 mol H2O will have a mass of 18 g.
Calculate percent CO2 and percent H2O in a 1:1 mixture (which is what you have), then convert that to grams CO2 and grams H2O.
4. Use g CO2 and g H2O to determine how many grams C you started with and how many grams H you started with; add them together. This will be the mass of the element B. Then determine how many grams O used to convert the carbon to CO2 and grams O used to convert the H to H2O, add them together, change to mols, and subtract from total mols you found in step 1. This will be the number of mols of compound B.
The rest should be a snap.
After identifying compound B, THEN you can answer the remaining questions.
This question is now on page 3 or 4 and it gets harder to find each day. If you have any further questions, please repost on page 1 (a new post), include the entire question (you can just copy and paste it in), then show your work to the point you are stuck and I'll try to help you through it. I didn't go through the reactions but I have identified compound B.