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Two gas tanks containing compounds A and B are connected by separate valves to two ends of a 10 meter glass tube as shown below. When both valves are thrown open at the same time A and B travel down the tubes towards each other. A white line appears when they meet and b has only gone 3.534 meters

Given that A and B are one of the compounds below determine what A and B’s identities are.
Possible compounds that can be A and B: HF, HCl, HBr, HI (acids) NH3, NCl3, NF3, PH¬3, PCl3 (bases)

  • chem -

    rate A travels = 6.466 m/s
    rate B travels = 3.534 m/s (You can use any time period; I chose 1 s for convenience).
    (6.466/1/3.534/1) = sqrt(MB/MA) where MA = molar mass A and MB = molar mass B.
    Solve for ratio of B/A. I get something like 3.0 but you need to do it more accurately.
    Then look up the molar mass of the acid and bases and put them in a table or whatever you find easy to do.
    HF =abou5 20......NH3 = ?
    HCl = ?...........NCl3. ?
    HBr = ?...........NF3 = ? etc.
    HI = ?
    Then look at ratios. Some will obviously not fit but some will be close enough to do on a calculator. I
    think NCl3 and HCl are close for A and B BUT I don't know if they will react to form a white ring or not. In addition, NCl3 can be explosive. Check my thinking.

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