Chemistry (Check)

Calculate the mass of 15.0 L of NH3 at 27 degrees C and 900 mmHg.

Mass of NH3 = 12.2 g

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  1. My calculator reads 12.267 so I would round that to 12.3 g.

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  2. Mine doesn't round up. What did I do wrong?

    15.0 L
    27 degrees C + 273 = 300K
    900. mmHg / 760 mmHg = 1.18 atm

    1.18 atm x 15.0 L = n (.08206 Latm / mol K) 300K
    n = .719 moles NH3 x (17.03056g / 1 moleNH3) = 12.2449 g NH3

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  3. The difference:
    If I use 900/760 and round that number to 1.18 (which is correct), I get 12.22 which rounds to 12.2.
    However, if I set my calculator to read 6 places (which I do routinely for all of my work) then go through without taking anything out of the calculator; ie., all of it done in one step, I get 12.266 which rounds to 12.3. Doing it your way causing rounding errors and this is an example. It becomes worse in problems where you take the number out of the calculator several times. Sometimes those rounding errors cancel, sometimes they don't. I think they don't most of the time. Besides, I don't like to take the time to read the calculator and copy the number onto a sheet of paper, then I must re-enter it into the calculator. All of that takes extra time and I'm too lazy to do that. By the way, many read that number (before taking it outof the calculator) to ONE MORE PLACE than allowed (you would have read 1.184) then when you re-entered it you would get the answer I get (almost) of 12.26 THEN you round to the correct number of s.f. which would be 12.3. Hope this digression helps make sense.

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  4. It does. Thank you so much for all your help.

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  5. good. Come back anytime.

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