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Homework Help: Chemistry - Calculating moles, mass and percentages

  1. Jan 4, 2009 #1
    One area of chemistry I seem to make a lot of mistakes on is the mathematical side. Calculating moles and grams and percentages and that kinda stuff. Heres a few questions on my last test I got wrong

    Q1.) Copper reacts with concentrated sulfuric acid according to the equation:
    Cu + 2H2SO4 -> CuSO4 + SO2 + 2H2O

    If 12.7g copper was used calculated:

    a) The number of moles of copper involved
    b) The mass of copper sulfate produced
    c) The number of moles of water produced
    d) Volume of sulfur dioxide formed at stp

    I just redid them there to see if Im still making errors. Heres my methods and answers
    a) since there are 63.5g in a mole of copper I just divided 12.7/63.5 and got 0.2 moles

    b) For this I just added up the mass of a mole of CuSO4 (160) then since 1 mole Cu makes 1 mole CuSO4 I multiplied 0.2 by 160 and get 32 grams

    c) For this one since 1 mole Cu produces 2 moles H2O I assume 0.2 Cu produces 0.4 moles H2O

    Q2) What is the empirical formula for the compound with the compisition, Copper 34.6%, Iron 30.5%, Sulfur 34.9%

    I was lost on that one. I assumed I find out the AMU of each of the elements but from there I didn't know how to assign coefficients to each element
  2. jcsd
  3. Jan 5, 2009 #2


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    Gold Member

    Q2) That is the correct idea, of the AMU, because you want number ratios of atoms in the compound. You then want to simplify the ratios as closely as possible to integer values (basic mathematics for fractions).

    Assume for simple arbitrary purposes, you have 100 grams of compound. Look for "formula weight" of element; divide the percent (NOT in the form of decimal fraction but as percentage units) by element's formula weight. Now you have the relative atom ratios, but you need to simplify this set of ratios.
  4. Jan 5, 2009 #3
    Thanks that explains the percentages question. I just looked back over my old test results and I noticed I answered 32 grams for the second question but the teacher marked it wrong. Was that a mistake on the teachers part? I just multiplied the mass of copper sulfate by 0.2 since it was 0.2 moles of copper put in.
  5. Jan 5, 2009 #4


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    I got 31.9g.
  6. Jan 6, 2009 #5
    A good idea when working with chemical quantity calculation it is always a good idea just to calculate the units of the end result, to check up if the result is right. And this is always a good idea. When you calculate diffusion potential through various gasses or liquids or for instance when calculating equilibrium constants. Just always check your formula, if the units you put in, match the ones you want in the end. That also corrects factor errors, when putting in for example enthalpy in kJ/mol, and entropy in J/mol*K, then the unit is not gonna be right in the end, unless you correct for it along the calculation.

  7. Jan 6, 2009 #6


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    Staff: Mentor

    No idea how it is related to the question in question, but I agree it is a good idea to check units to be sure of units.
  8. Jan 7, 2009 #7
    The relation is, for the questionasker to obtain a good tool to ensure results, and to make it a habit to take units in consideration when asking him/herself these questions. So not so much a 2+2=4 solution, but more a good tool to ensure the results.

    And since you guys had already helped sufficiently with the questions in question, then why not organize the questioneer's thoughts regarding all concepts and shades of calculating anything regarding physics and chemistry.

  9. Jan 8, 2009 #8
    Yeah Borek that must be it they expect precise answers. For matters like this is it best to go 2 spaces after the decimal point and round off or should you just throw in every single number?
  10. Jan 8, 2009 #9
    That depends how many digits you've been using for your calculation.. This is the bottleneck for how many digits your result should be in..

    It gives no point to calculate your result with nine decimals or something, and then giving the result with just one decimal. However this has never been my forza..
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