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Thread: please help..chemistry

  1. #1 please help..chemistry 
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    Hello there,
    could someone PLEASE tell me the right calculation for a chemistry excercise? It is simple but I am not good in it at all...
    I am provided with 0.5g of trehalose in a 50ml volumetric flask and I know that the relative molecular mass of the trehalose is 378.3g/mol. I need to work out the number of moles present! Can anyone help, please?
    Thank you


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  3. #2  
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    I am not quite sure if I am on the right track..I mean to calculate the molarity tells me the number of moles of a substance in one litre of solution, but now I have less than a litre (50ml) and I got here this formula which says if less than a litre of slution is used, the formula is different but can't figure out how to do it :-( [/code]


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  4. #3  
    Universal Mind John Galt's Avatar
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    If there are, as you say, 0.5g in 50ml how many grams will there be in one litre?

    If there are 378.2 g of trehalose in one mole, how many moles will there be in the litre you now have?
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  5. #4 Stoichiometry 
    Forum Sophomore NimaRahnemoon's Avatar
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    Ophiolite is right, I think that is what your teacher wants, here is a different way to do it:

    To get mol:
    (.5g)*(1 mol/378.2g)= # of mol

    To get volume per mol assuming that there is 50 mL of trehalose :
    (50 mL/.5g)*(1L/1000mL)*(378.2g/ 1mol)= L/mol

    I need to work out the number of moles present!
    If "# of mol" is all you need, then the .5 mL is useless information, I figured he might want L/mol.
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  6. #5  
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    reading that, confused me. if the solution has .5 grams of trehalose, no matter what the dilution , it will still be .5g of trehalose.
    my reasoning right or wrong? (note this is jsut a question of mine own they are right,i think)
    Stumble on through life.
    Feel free to correct any false information, which unknown to me, may be included in my posts. (also - let this be a disclaimer)
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  7. #6  
    Universal Mind John Galt's Avatar
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    To maintain the same concentration in 1 litre that I have in 50ml I need to have 1000/50 times as much. i.e. 20 x 0.5 gms = 10 gms.

    Remember that the molarity is defined as the number of moles (at a given concentration) in one litre.

    1 mole = 378.2 g
    Therefore, 10 g is 10/378.2 moles, which is the molarity.

    I haven't touched this stuff for over three decades, so I'm encased in ferrous and ferric oxides. Feel free to correct!
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  8. #7  
    Forum Sophomore NimaRahnemoon's Avatar
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    Ahh it makes sense now. Ophiolite is right. Hmm I just realized molarity is the inverse of density .

    Here is how you do it using dimensional analysis:

    (.5g/50 mL)*(1000mL/1L)*(1mol/378.2g ) = mol/L

    GoodGod3rd it's pretty much saying you have .5g in 50 mL. You want to find molarity which is mol/L. So you gotta do the necessary conversions.

    I need to work out the number of moles present!
    Erm by seeing the second post, I guess not...
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  9. #8  
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    .5 * 20 = g/L

    answer over 378.2 = m/L
    Stumble on through life.
    Feel free to correct any false information, which unknown to me, may be included in my posts. (also - let this be a disclaimer)
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