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Chem Problems

Discussion in 'Members' Lounge' started by endlesszeal, Apr 28, 2006.

  1. endlesszeal

    endlesszeal Senior Member

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    In the lab, your supervisor asked you to prepare 1 liter of 0.5 M NaH2PO4 (sodium phosphate monobasic) solution. In the chemical cabinet of the lab, however, you only found a bottle of sodium phosphate monobasic monohydrate, NaH2PO4.H2O (FW = 138.0).
    In this case, you should weigh out .0 g of NaH2PO4.H2O to make 1 liter of the solution.


    Next day, a graduate student in the lab asked you to prepare 1 liter of solution X, following the lab recipe. The lab recipe book says you should dissolve 240 g of NaH2PO4 (anhydrous) in water to make 1 liter of solution X. However, the lab seems to have only NaH2PO4.H2O (monohydrate) in the chemical cabinet and you cannot find anhydrous NaH2PO4 powder anywhere.
    In this case, you should use .0 g of NaH2PO4.H2O to make 1 liter of solution X.




    I know these are review questions, but I have no idea how to do it because it throws in the h20. anyone can help?
     
  2. phunky.buddha

    phunky.buddha Admin with a big stick Admin VIP

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    Figure out the molar weight of the stuff in the jar, then see how much you'll need to add to water by weight to get the requested percentages. Just account for the water that's already attached to the molecule that you're trying to make a solution out of. Looks pretty easy to me...
     
  3. endlesszeal

    endlesszeal Senior Member

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    lets see..

    i goy 69.00g for the first part and 276.00g for the second.

    anyone can concour?
     
  4. pissedoffsol

    pissedoffsol RETIRED

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    [​IMG]

    highschool chem eludes me.

    i knew it was sodium phosphate though :p
     
  5. phunky.buddha

    phunky.buddha Admin with a big stick Admin VIP

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    [​IMG]

    It's been a while man... and I don't have a periodic table in front of me. Look it up in your book! That's what it's for.
     
  6. endlesszeal

    endlesszeal Senior Member

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    you dont need a table because the weight is already given. its 138g for the NaH2PO4 with H20. H20 is 18g. So the NaH2PO4 is 120g.
     
  7. phunky.buddha

    phunky.buddha Admin with a big stick Admin VIP

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    Ok, well there you go. So 69g for the first problem and 276g for the second one.
     
  8. preluderjs

    preluderjs Senior Member

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    Just out of curiosity, which chem class is that for? I took chem 202 (chem for engineers) a couple years ago and I can't do any of those problems.
     
  9. TommyTheCat

    TommyTheCat Gonzo Scientist

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    You must go to the worst eng. college ever. Its hardly even a real chemistry problem, its a math "word problem" with chemistry words.
     
  10. pills_PMD

    pills_PMD Super Moderator

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    word. pretty easy stuff
     
  11. endlesszeal

    endlesszeal Senior Member

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    haha, its not for chemistry.. its for yeast microbiology and its a quiz because the teacher felt that students taking his class should be able to do these kinds of problems or else it would be embrassing for him to have students that didnt have the abilities to do them.

    i havent done them for like 2-3 years so i was just double checking. the last chem class was organics and we didnt deal with those kinds of probelms, mostly mechanisms.
     
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