Redox question help please

Discussion in 'Homework Help' started by Sstartling engineering, Apr 1, 2018.

  1. Sstartling engineering

    Thread Starter New Member

    Apr 1, 2018
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    Help on my question please!

    IMG_20180226_232313_665.JPG
     
    Last edited by a moderator: Apr 1, 2018
  2. wayneh

    Expert

    Sep 9, 2010
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    This is a question about balancing the chemical reactions, which relate moles of reactants and products, using the measured masses of both.

    So start by calculating what you know from the information given. How can you get the grams and moles of the gas produced, given the volume? Once you know the moles of gas, what does that tell you about the rest of the reactions?
     
  3. Sstartling engineering

    Thread Starter New Member

    Apr 1, 2018
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    Please can you solve completely? Because when I solved I didn't get any option in 2nd part question.
     
  4. wayneh

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    Sep 9, 2010
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    Yes, but it's not my homework.
    Well, what did you get? Can you balance the equations?
     
  5. jpanhalt

    Expert

    Jan 18, 2008
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    It might help to remember that 1 mole of an ideal gas at 273.15 °K (0°C) and 1 atm pressure occupies 22.4 L . You were probably told to assume NO2 was an ideal gas, which it probably isn't.
     
  6. Sstartling engineering

    Thread Starter New Member

    Apr 1, 2018
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    But I want answer only for 2nd question. Please can you solve it? I am having problem in that only.
     
  7. WBahn

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    Mar 31, 2012
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    We will NOT work your homework for you. It is YOUR homework.

    Consider that you have almost certainly your textbook explained how to do it, you've seen examples in your text book on how to do it, and you've been shown examples in class on how to do it. Something hasn't clicked. So there is no reason to assume that having yet one more problem worked for you is going to somehow magically make the light bulb appear over your head. YOU need to struggle with it and fight with it so that YOU have to run up against the specific roadblocks that are hindering you so that YOU can bust through them and achieve understanding.

    We can HELP you do that, but ONLY if YOU put in the effort and SHOW YOUR WORK so that we can see what you are doing and where you are going astray.
     
  8. wayneh

    Expert

    Sep 9, 2010
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    Same answer as before in #4: Show where you're stuck. If you've balanced the equations and done enough work to answer the first part, this second part should be easy. I suspect you're just not seeing the simple solution right in front of you.
     
  9. Sstartling engineering

    Thread Starter New Member

    Apr 1, 2018
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    This way I am getting 14.16 ml but not an accurate answer.
    Please help me to get correct answer.
     
  10. wayneh

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    Sep 9, 2010
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    You’ve neglected the consumption of acid by the zinc reaction. Notice how the question was constructed to slightly obscure the involvement of HNO3 in that rxn.
     
  11. Sstartling engineering

    Thread Starter New Member

    Apr 1, 2018
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    If I include the reaction with Zn also volume will come more than 16 ml.
     
  12. wayneh

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    Sep 9, 2010
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    Perhaps you used the wrong percentage of Cu? (I’m truly guessing and have not calculated my own answer.) But what if the brass is mostly Cu? Seems like it could be close.
     
  13. wayneh

    Expert

    Sep 9, 2010
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    I finally got around to looking at the calculations and now I see your dilemma. 2 grams of brass, even if it’s pure copper, is only 0.0315 moles and yet it appears that 0.0425 miles of NO2 were produced. Something is wrong here but I’m not seeing it.

    Oh wait, duh, the equations given are not correct and need to be corrected for the stoichiometry.
     
    Last edited: Apr 2, 2018
  14. jpanhalt

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    Jan 18, 2008
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    Last edited: Apr 3, 2018
  15. WBahn

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    Mar 31, 2012
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    The equations clearly are not balanced. Just look at the number of hydrogen atoms on both sides of each one.
     
  16. jpanhalt

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    Jan 18, 2008
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    I count electrons. I know they are small, but they tend to be what are considered in redox equations.
     
  17. WBahn

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    Mar 31, 2012
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    So the atoms don't have to be balanced, just the electrons....???
     
  18. jpanhalt

    Expert

    Jan 18, 2008
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    That is not what I said, nor would anyone who knows how to work REDOX equations interpret it that way. I have never seen anyone approach such problems by balancing "hydrogen atoms" (H· ) to balance the oxidation/reduction states. Sure, there has to be elemental balance, but that is not always sufficient.

    Nevertheless, I have learned from experience not to get drawn into a trivial, off-topic disagreement with you and will not contribute to this thread further. I do hope the TS takes my advice, writes the half equations, and balances the electronic states. The rest will fall into place from those equations.

    Edit: In practice, one needs only to write the oxidation state for each element in the equation, not the whole half equations. However, doing the latter will always work too.
     
    Last edited: Apr 3, 2018
  19. WBahn

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    Mar 31, 2012
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    The post I was responding to seemed to not be sure whether they were balanced or not (as indicted by "perhaps the equations are not really balanced"). So I was trying to point out that a casual glance at the equations show that they are not balanced -- it doesn't matter which particular atom is not balanced, if ANY atom is not balanced on left and right, then the equation is not balanced. In the case of both of these equations, it is seemed (to me) trivial to note that the hydrogen atoms are not balanced and therefore I commented that "clearly they are not balanced". That assertion was not in any way based on ANY implication that balancing hydrogen atoms is a SUFFICIENT condition to balance the oxi/redux states, only that it is a NECESSARY condition.

    You seemed to take issue with this claim (that the equations clearly are not balanced since the hydrogen atoms are not balanced) on the basis that it's the electrons that count.

    If my claim is wrong then the basis for that claim must also be wrong meaning that, somehow, balancing the atoms (including hydrogen) is NOT a necessary condition. I merely asked for confirmation of that.

    It appears now that you DO agree that balancing the atoms on each side is a necessary condition for the equation to be balanced, which then, admittedly, brings me back to wondering what the issue is with me commenting that clearly the equations are not balanced and that this can be determined just by looking at the hydrogen atoms on both sides of each one.
     
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