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Thread: Grams and molecules question

  1. #1 Grams and molecules question 
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    Hi, i am given a question that says;

    How many molecules are there in 21.6g of CH4?

    what i did;
    6 * 10^23 molecules =16g
    x molecules = 21.6g
    (then i crossed multiplied)
    x = 8.1 * 10^23 molecules

    what i would like to know is; is that answer correct?
    Any assistance will be greatly appreciated!
    thank you!


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  3. #2  
    Moderator Moderator AlexP's Avatar
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    You got it. For future problems on this same topic, you can just post them in this same thread. It'll keep things a little neater in the forum.


    "There is a kind of lazy pleasure in useless and out-of-the-way erudition." -Jorge Luis Borges
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  4. #3  
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    Ok thank you....i just have 1 more question ;

    How many grams are in 4.5 moles of Li2O?

    this is what i did;
    30g = 1 mole
    x = 4.5 moles
    (then i cross multiplied)
    x =135g

    Is my answer correct??
    And the method that i am using, is it a correct and valid method to use so that i am guaranteed to get the correct answer?
    any help would be greatly appreciated!!
    thank you!
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  5. #4  
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    It is a correct method and if you are consistent in using it you will get a correct answer. In this case your answer is correct except you rounded off the molecular weight of lithium and therefore ended up with 135 which rounds up to 140 when you consider significant figures. Had you used 6.941 as Lithium's molecular weight you would have gotten 134.469 which when rounded to 2 significant figures would have given you 130 the more correct answer.
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  6. #5  
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    Ok, but is it still correct although i didn't use the exact value for Li?

    thank you~~!!!! :-D
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  7. #6  
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    Yes the process is correct.
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  8. #7  
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    And what about the answer?
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  9. #8  
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    Oh sorry, except for round off, it is correct.
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  10. #9  
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    Ok, thank you soooo very much!!!!! :-D
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