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Check my homework (moles/stoichiometry)

  1. Aug 30, 2011 #1
    Question 29b)

    I have the actual question in the attachment.

    I go through the calculations as follows

    Mg + O2 ---> MgO

    then i balance equation

    2Mg + O2----> 2 MgO

    since we are given the mass of the magnesium and oxygen reactants, i prooced to finding the number of moles for magnesium and oxygen

    for Mg
    =2.6/24.3 (Molar mass)

    =0.1 mols of Mg

    now O2
    =1.6/32(Molar mass)

    = 0.05 mols

    now that we have the number of moles of Mg and O2

    i continue and find the limited and excess reactant(although i think both are limited, since there is only enough of Mg and O2 to do the reaction, so I use Mg in cross multiplication with MgO to find the number of MgO moles.

    2 mols of Mg ----->2 mols of MgO
    0.1 mole ---------> x




    so now we know the number of moles for MgO is 0.1

    finally we calculate the mass of MgO

    mass = number of moles x molar mass

    molar mass of MgO is 24.3(mg) + 16(Oxygen)

    24.3 + 16 = 40.3

    mass = 0.1 x 40.3

    =4.03 g

    according to the law of conservation of mass, both sides reactant and product should be equal and in my calculations they are almost equal, the slight difference (4.2g) on reactants and 4.03g on products side is there because the course curriculum uses different decimal values for molar mass of each element and i use wikipedia (course says Oxygen has molar mass of 16, whereas wikipedia says its 15.9) the same goes for magnesium, book says a value and wikipedia has a slightly different value(again very slightly, which probably accounts for the slight difference in mass)

    so the book and i are off by very little, does this check out?

    Attached Files:

  2. jcsd
  3. Aug 30, 2011 #2


    User Avatar

    Staff: Mentor

    2.6g and 1.6g are off, if anything, 2.6g of Mg reacts with 1.7g of oxygen. The difference is too large to be explained just by the differences in molar masses.

    That's problem with the question, not with your calculations; however - these values are faulty, so it is hard to tell what they expect you to do. Could be the idea is that you are to use these masses in calculations - note that up to d everything can be calculated using given numbers (even if they are incorrect). Only e will be difficult to solve.

    Wikipedia doesn't say oxygen has a molar mass of 15.9, it says 15.9994 - which rounds up to 16.
  4. Aug 30, 2011 #3
    hm so your saying that the question has typos in it? because thats been known to happen in this textbook
  5. Aug 31, 2011 #4


    User Avatar

    Staff: Mentor

    What I am saying is that it can be both a typo, or an intentional use of a slightly off value (similar to experimental error). Impossible to say.
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