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Homework Help: What is hydrated?

  1. Nov 26, 2004 #1
    The problem is:

    A hydrated sample of MgSO_3 weighs 1.500g before heating and 0.737g after heating. What is the formula for the hydrate?

    what exactly is a hydrate?
  2. jcsd
  3. Nov 26, 2004 #2


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    From what I can remember, a hydrated compound is a chemical compound that absorbs water molecules into its chemical structure in a very definite way. Hence the term, "hydrated" compound. The corresponding "dry" compound into which no water molecules have been absorbed is termed the "anhydrous" compound, and it usually has different visual appearance and properties.

    Sometimes hydration reactions occur spontaneously (a compound will slowly absorb or release water from or into the atmosphere). However, hydration is not the same thing as a substance simply becoming 'wet' i.e. a substance is not necessarily 'hydrated' just because you dumped a bunch of water on it. As I said before, a chemical reaction must occur in which the water molecules are incorporated into the lattice structure of the compound in very specific proportions. For example, if a hydrated compound has five water molecules per formula unit (I just gave five as an example..you have to look up how many it is for this compound), then the chemical formula denoting this structure would be:

    [tex] \text{MgSO}_3 \bullet 5\text{H}_{2}\text{O} [/tex]

    There are five water molecules for every pair of Mg2+ and SO32- ions.

    If you are wondering what the nature of the chemical bond between these water molcules and the compound in question is, then I'm afraid I don't know.
  4. Nov 26, 2004 #3
    im assuming the problem is asking me to figure out what the water molecule per unit is then? our teacher didnt teach us about these things yet >.< so i have no idea on how to do this problem :grumpy:

    OOOOOOOOO this just occured to me as i waslooking at the problem

    since its 1.500g when its hydrated, and after heating it, it becomes .737g. that means all the weight that was lost is H20 right?

    from that i know the mass of H20 in that compound, then um..now what? :yuck:
    Last edited: Nov 26, 2004
  5. Nov 26, 2004 #4
    A chemical formula (what the question is asking for) includes mole ratios between parts of the molecule. How can you find the ratio of moles of [itex]MgSO_{3}[/itex] to moles of water using the masses?
  6. Nov 26, 2004 #5


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    How many moles of MgSO3 are in 0.737g? I get an answer around 0.0070614 moles. I'll store that as "A" in my calculator.

    Take the change of masses to have the mass of water.
    1.5 - 0.737 = 0.763g water. That's about 0.04234 moles of water. Store that value as "B".

    Now take a ratio between the water and the MgSO3.
    B/A = 5.99622 ~= 6

    MgSO3 * 6H2O
  7. Nov 26, 2004 #6


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    Excellent! You might not have been taught this stuff, but you figured out what the problem was asking, and what the key concepts and info are that you need to solve it.

    Think about it this way...you have a question that's asking about the amount of a substance that forms part of a compound, in terms of numbers of molecules, as opposed to mass. So what is a conveniently large number for expressing numbers of molecules? The mole. Convert everything into amounts in moles, instead of mass in grams, that way you can directly compare the relative amounts of various substances making up the compound. Try that, and let me know how it goes...
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