AP Chem

posted by John

Use the following information to identify element A and compound B, then answer questions a and b.

I'm so stuck on this problem and don't know where to begin. I need to have this problem done by Wednesday. PLEASE HELP!!!

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 15⁰C. When the container is evacuated and refilled with a certain element (A) at a pressure of 745 torr and a temperature of 26⁰C, 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 (10.68L) with excess oxygen gas. The vessel is placed in a constant-temperature bath at 22⁰C. The pressure in the vessel is 11.98 atm. In the bottom of the vessel is a container that is packed with Ascarite and a dessicant. Ascarite is asbestos impregnated with sodium hydroxide; it quantitatively absorbs carbon dioxide:
2NaOH (s) + CO2 (g) „³ Na2CO3 (s) + H2O (l)
The dessicant is anhydrous magnesium perchlorate, which quantitatively absorbes 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 the 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 stell vessel is carefully 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 10.0L of A and 8.60: 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?

  1. DrBob222

    I always called these problems railroad problems for I thought they were designed to "railroad" me out of the class. These must be done is small parts. Use the initial information about nitrogen to determine the volume of the container using PV = nRT. Knowing that, and the informatin about element A, the atomic mass of element A can be determined. Then begin work on the other parts. If you get stuck post what you have done and we can do another part or two. Good luck.

  2. Eric

    Ok, i found the volume of the container to be 0.714L and then used PV=nRT to find that there were 0.0285 moles of A which gives the molar mass of A to be 70.8g/mol. Would that be Ga?

  3. John

    Is this Gallium?

  4. DrBob222

    My best guess is, "probably not." Is Ga a solid, liquid, or gas at these conditions? It has a melting point of about 30 degrees C so that wouldn't be a gas at the conditions cited, would it (26 degrees C)? I obtained the same molar mass, at least within 0.1, as you did.

  5. John

    Then would it be Cl2?

  6. DrBob222

    probably.
    Next you want to determine the empirical formula of gas B.

  7. John

    The formula should be H2C right?

  8. DrBob222

    right. That is the empirical formula. Of course we are aware that CH2 doesn't exist so gas B must be a dimer, trimer or tetramer of CH2. It can't be a pentamer because 1-pentene is a liquid. So the possibilities are C2H4, C3H6, or C4H10.
    What do you do next?

  9. John

    Im not sure what to do now?

  10. DrBob222

    Next step is the combustion of gas B.
    The container is filled with gas B and oxygen to a combined total pressure of 11.98 atm (pressure gas B + pressure oxygen = 11.98 atm). The mixture is combusted, the pressure settles to 6.02 atm, MEANING that 11.98 atm - 6.02 atm = mols B + oxygen used. I would substitute that pressure in PV = nRT and determine the number of mols used up in the combustion.

  11. John

    2 MOLS of CO2 formed and 2 mols of H2O formed

  12. DrBob222

    Yes, there are 2 mols CO2 and 2 mols H2O formed PER MOLE gas B used BUT you don't know how many mols that is.
    1. How many mols gas B + oxygen were used in the combustion of gas B?2. How many grams of CO2 and H2O were formed in the combustion of gas B?

  13. amy

    help i am working on this too. and i can not figure it out.

  14. Anonymous

    What would you predict for the rate if 2.5 mL of water were mixed with 2.5 mL of potassium iodate solution, then 2 mL of the sodium meta-bisulfite starch solution were added?

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