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Math Help - Calculating Molarity Help

  1. #1
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    Calculating Molarity Help

    Hi, i'm stuck on how to go about solving the following problem:


    Explain how you would make the following solutions. You should tell how many grams of the substance you need to make the solution, not how many moles.

    2 L of 6 M HCl
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  2. #2
    Forum Admin topsquark's Avatar
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    Quote Originally Posted by MajorJohnson View Post
    Hi, i'm stuck on how to go about solving the following problem:


    Explain how you would make the following solutions. You should tell how many grams of the substance you need to make the solution, not how many moles.

    2 L of 6 M HCl
    6 M HCl means that there are 6 moles of HCl per liter of solution. You want 2 L of solution so you need to mix 2 times 6 = 12 moles of HCl into 2 L of water. How many grams is 12 moles of HCl?

    -Dan
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  3. #3
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    12 mol * 36.45 g (the molar mass of HCl) = 437.4 grams
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    Forum Admin topsquark's Avatar
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    Quote Originally Posted by MajorJohnson View Post
    12 mol * 36.45 g (the molar mass of HCl) = 437.4 grams
    Looks good to me.

    By the way....I'm not great with Chemistry, but that's a pretty good amount of solute for a 2 L solution. You may need to consider heating the water as part of your preparation instructions.

    -Dan
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  5. #5
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    ^ Okay thanks.

    I happened to find this problem a pdf worksheet, I was using it to study for this exam I have coming up and just so happened to get stump on this particular problem. According to my professor, he's going to give us problems where we will have to make up our own solution, I guess similar to what I had done above. The only thing that will change is that he will add in cubic centimeters in the equation (which i'm assuming that i'll be converting to liters for whatever he is asking).


    I have a similar problem to the one above, but my answer is a bit off to the one they came up with:

    How many grams of CaCl2 would be required to produce a 3.5 M (molar) solution with a volume of 2.0 L?



    My calculations: 110.98 molar mass of CaCl2

    3.5 M x 2.0 L = 7 mols of CaCl2

    7 moles x 110.98 g = 776.86 grams

    They must have rounded this to 780 g because thats what they got for an answer.




    Last edited by MajorJohnson; March 7th 2011 at 02:46 AM.
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