Monday

January 26, 2015

January 26, 2015

Posted by **Julia** on Monday, April 16, 2007 at 4:16pm.

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?

I already have A.

I have the following equation

2CH2 + 3O2 ==> 2CO2 + 2H2O

I know I must find the percentage of CO2 and H2O in a 1:1 ratio, but I want to know if I am doing the percentage correct before I move one to anything else.

1 mol CO2 will have a mass of 44.01 g.

1 mol H2O will have a mass of 18.016 g.

I did 846.7-765.3= 81.4g to get the gain in mass which is the weight of CO2 & H20.

Then for CO2 44g/81.4 *100 = 54.066%

H20:18.016/81.4g *100 =13%

But shouldn't the percentages somehow add up to 100?

The pressure of compound B + pressure of oxygen before spark ignition is 11.98 atm. After ignition, pressure is 6.02. Therefore, 11.98-6.02= 5.96 is how much reacted (due to pressure). Use PV = nRT to calculate the number of mols at the appropriate T and V. Keep several numbers (i.e., don't try to round too severely--wait until the end of the problem). This n=number of mols of B + oxygen. You will need this number later.

Next, calculate the grams gain in mass for the ascarite and magnesium perchlorate. I found 81.4 grams. How do we split that up to see how much of it is due to CO2 and how much is due to H2O? One way to do it is to look at the products of the balanced equation. Remember that was

2CH2 + 3O2 ==> 2CO2 + 2H2O

We have H2O and CO2 in 1:1 ratio.

So assume we had 2 mols CO2 and 2 mols H2O, we would have 88 mols CO2 and 36 mols H2O for a total of 124 grams. What percent is 88/124 and 36/124? Apply those percentages to the 81.4 gram gain of and that will give you grams CO2 and grams H2O. Check to make sure they add to 81.4 g.

You want to do three things with these g CO2 and g H2O.

a). Calculate g carbon in the orginal sample. Calculate g H in the original sample. Add C and H to give you the mass of the original sample, B.

b)Calculate g oxygen used to make the CO2 and calculate g oxygen used to make the H2O. Add them together, convert to mols. That will be the mols oxygen needed to convert all the C to CO2 and all the H to H2O. Now subtract mols O2 from the mols you found above for mols B + mols O2. Obviously,

mols B + mols oxygen = ??

mols oxygen = ??

subtracting gives mols B.

c)Now you have grams B and you have mols of B. Convert that to grams/mol which is the molar mass B and that will identify compound B. You already know the empirical formula is CH2 from work you did last week.

Post your work if you get stuck.

Thank you..I will try the rest!

1 mol CO2 will have a mass of 44.01 g.

1 mol H2O will have a mass of 18.016 g.

I did 846.7-765.3= 81.4g to get the gain in mass which is the weight of CO2 & H20.

Then for CO2 44g/81.4 *100 = 54.066%

H20:18.016/81.4g *100 =13%

But shouldn't the percentages somehow add up to 100?

44/81.4 is taking the mols ratio. That isn't what you want.

1 mol CO2 = 44.01 g

1 mol H2O = 18.016 g

Total = 62.026 g

fraction CO2 = 44.01 g/62.026 g = 0.7095, THEN 0.7095 x 81.4 = 57.757 g CO2. etc.

So I have:

n=pv/rt

n=(5.96)(10.68)/(0.082057*295) = 2.6295 moles B+ O

______________________________

70.95% CO2 =57.75g

29.05% H2O = 23.65g

_____ ________

100% 81.4g

a)

(57.75/44.01)*(1molC/1molCO2)(12.01gC)=15.76g C

(23.65g H2O/18.016g H2O)(2H/1H2O)(1.008gH)= 2.65 g H

18.4g original B

b)

(57.73g CO2/44.01g)(2O/1C)(16gO)= 41.98g O

(23.65H2O/18.016)(1/1)*16gO0=21.00gO

21.00+41.98=62.98g O2

62.98gO2/32 = 1.97moles O2

2.6295moles-1.97 = 0.6595 moles B

molar mass B: 27.9 g/mol

CH2=14.026g/mol

27.9/14.026 = 2

B= C2H4

is all that right?

You have it A-OK.

So C2H4 is what?

You now know element A and compound B, now you can do the remainder of the problem.

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