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How can water have a molarity if theres no diff b/w solute and solvent?

  1. Sep 26, 2008 #1
    mental block about this please help.
    molarity is mols or a substance per Liter of a solution right?
    that is, mols of the solute in litres of solution.

    1. What is the solution?
    2. if its water, then how can you say water has a molarity?
     
  2. jcsd
  3. Sep 27, 2008 #2

    Borek

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    Don't think in terms of solution - ie separate solvent and solute. Concentration is just amount per liter, not per liter of water. Thus you have 55.55 moles of water per liter.
     
  4. Sep 27, 2008 #3
    Because molarity is how much stuff you have in a certain amount of volume. It doesn't matter if it is water or an HCl solution.
     
  5. Sep 27, 2008 #4
    So that means that compounds dont necessarily have set molarities right?
    for ex, H20 always has mass of 16 + 1 + 1 = 18, and 1 mole is always 18g, but its molarity depends on how many liters of volume 1 mole is contaained in, is that right?

    If yes, then why is water said to be 55.5M ??
     
  6. Sep 27, 2008 #5

    Redbelly98

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    Not quite. Molarity is how many moles are in 1 liter. For water, it happens to be 55.6. 1 liter of water contains 55.6 moles of water.
     
  7. Sep 28, 2008 #6

    Borek

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    To be exact, molarity of pure water is 1000/18.02 = 55.51

    However, in the 1M solution of acetic acid there is only 946.9 g of water per liter, so its molarity in such solution will be 846.85/18.02 = 52.56
     
  8. May 6, 2010 #7
    molarity=no.of moles per volume of soluton in liters
    but u did 1000/18.02 how?
     
  9. May 6, 2010 #8

    Borek

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    What is mass of 1L of water?

    How many moles in that mass?

    --
    methods
     
  10. Jul 15, 2011 #9
    Let me breakdown:
    Re: (1L = 1000 cm^3)
    water has an approximate density of 1g/cm^3 (temperature dependent)
    Therefore, 1000 cm of pure water has an approximate mass of 1000g
    Mass of 1 mole of water = 18.02 g
    Hence, no. of moles water in 1000 cm^3 of water = Total mass/mass of 1 mole = 1000 g/18.02g per mole = 55.55 moles
     
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