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Are volumes additive?

Discussion in 'MCAT Discussions' started by PensiveNoodle, May 2, 2007.

  1. PensiveNoodle

    PensiveNoodle Pensively eating ramen...

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    Hello all!

    If I added 5mL of EtOH and 5mL of Water, would I get 10mL total?

    I'm guessing that one would get less than 10mL. The EtOH and water mixture creates new hydrogen bonding which would bring molecules closer together, causing it to occupy less space/volume. Did I approach this correctly?

    Thanks!
     
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  3. Canadian Premed

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    You are way overthinking this. First, the number of Hydrogen bonding won't increase much, really really small tiny number! Second, Hydrogen bonding doesn't bring the molecules closer, if it did then how come water expands when it freezes? Third, may be I'm an idiot and I'm completely wrong...
     
  4. spicedmanna

    Moderator Emeritus

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    I concur. Water and ethanol are miscible, thus I would assume there would be no significant adjustment from norm in terms of volume in your example, particularly within the limits of the significant figures you presented in your post.

    It's true that volume is based on density and temperature, and to be truly accurate you would need to take these factors into account. However, unless, there is some indication that these need to be considered in your calculation, you are probably pretty safe in assuming that 5 mL H[sub]2[/sub]O + 5 mL ETOH = 10 mL of solution.
     
  5. mrmandrake

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    Generally speaking, volumes are not additive.
     
  6. ihdihd

    ihdihd Member

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    And this kind of overthinking is a pathology of being a very advanced MCAT test taker. For example, a physics PhD would have to limit their sphere of knowledge to what the rest of the 'world' knows to answer questions correctly on the MCAT (that or present item challenges by remembering any question where advanced and recent research shows 2 answers to be possible).
     
  7. Schaden Freud

    Schaden Freud MiSanthrope II

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    Volumes are not additive, but for MCAT purposes can be approximated as such.
     
  8. scottj72

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    This is actually a demonstration I have done in some of my chemistry classes. If you measure 1 L of water and then 1 liter of ethanol and then mix the two volumes you do in fact end up with a volume less than 2 L. I mainly done it to get engage my students to think about how the compounds were different and what was happening when the two liquids were mixed. But I guess it kinda depends on the context of what you are trying to get from the information.
     
  9. sehnsucht

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    It would be around 10mL, however, the more important question to ask along the lines of an MCAT question would be: by diluting x M EtOH, how much would your new concentration be?
     
  10. CATallergy

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    I think you should just decide if the final and initial are both aq./liq., and if so, assume that the volume stays roughly the same.
     
  11. Chargers

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    Are you sure that you didn't spill any of them in the process?:D
     
  12. MrDocNYU

    MrDocNYU Member

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    I don't actually know whether or not this is tested on the MCAT, but there is no law of conservation of volume. Mixing water and ethanol always leads you to a lower volume than the expected total. This has to do with certain physical properties of these solvents. So your eyes are not deceiving you when you see that the volume decreases. You can actually derive the relationship mathematically.
     

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