** Using WeBWorK **

I have put such a context together, and have added it to the pg/macros directory. You can get it from the HEAD version of the CVS repository. There are comments at the top of the file that explain how it works, which I also provide below.

I'm not a chemist, so I don't know if this is exactly what you need, but it looks like it should handle the problems that you illustrated in your initial question. Let me know if it needs adjusting.

Here is the documentation from the file:

This file implements a Context in which checmical reactions can be specified and compared. Reactions can be composed of sums of integer multiples of elements (possibly with subscripts), separated by a right arrow (indicated by "-->"). Helpful error messages are given when a reaction is not of the correct form. Sums of compounds can be given in any order, but the elements within a compound must be in the order given by the correct answer; e.g., if the correct answer specifies CO_2, then O_2C would be marked incorrect.

To use the context include

    loadMacros("contextReaction.pl");
    Context("Reaction");

    $R = Formula("4P + 5O_2 --> 2P_2O_5");

The Reaction Context also allows you to create parts of reactions. E.g., you could create

    $products = Formula("4CO_2 + 6H_2O");

    loadMacros("contextReaction.pl");
    Context("Reaction");

    $reactants = Formula("2C_2H_6 + 7O_2");
    $products  = Formula("4CO_2 + 6H_2O");

    Context()->texStrings;
    BEGIN_TEXT
    \($reactants \longrightarrow\) \{ans_rule(30)\}.
    END_TEXT
    Context()->normalStrings;

    ANS($products->cmp);

Note that sums and products are not simplified in any way, so that Formula("O + O") and Formula("2O") and Formula("O_2") are all different and unequal in this context.

All the elements of the periodic table are available within the Reaction Context. If you need additional terms, like "Heat" for example, you can add them as variables:

    Context()->variables->add(Heat => $context::Reaction::CONSTANT);

Hope that does what you need.

Davide

Second, the LimitedFraction context that you set at the bottom has no effect. This is because MathObjects retain the context in which they were created, so since $ans1, $ans2, and $ans3 where created in the Fraction context, they continue to be in that context even after you change the current context to LimitedFraction.

The proper way to do this is to create them in LimitedFraction context initially (without the requireReducedFractions flag set, so the correct answer can be entered as an improper fraction) and then set the requireReducedFractions flag after they are created.

Here is the code:

    #DESCRIPTION
    ##Type of
    #ENDDESCRIPTION

    DOCUMENT();
    loadMacros(
    "PGstandard.pl",
    #"PGchoicemacros.pl",
    #"PGgraphmacros.pl",
    "MathObjects.pl",
    # "compoundProblem.pl",
    #"contextCurrency.pl",
    #"contextInequalities.pl",
    #"unionTables.pl",
    # "unionLists.pl",
    #"unionMacros.pl",
    #"contextLeadingZero.pl",
    "contextFraction.pl",
    #"answerHints.pl",
    #"problemPanic.pl",
    );
    #for currency use Context("Currency") then Currency($A);
    #Then, in the text use $DOLLAR $a
    TEXT(beginproblem());
    $showPartialCorrectAnswers = 1;

    Context("LimitedFraction");

    $a1=random (3,7,2);
    $b1=random (4,8,4);
    $x=Compute("$a1/$b1");
    $a2=random (4,8,2);
    $b2=random (5,7,2);
    $y=Compute("$a2/$b2");
    $a3=random (3,9,3);
    $b3=random (5,10,5);
    $z=Compute("$a3/$b3");
    $ans1=$x-$y;
    $ans2=$x+2*$y;
    $ans3=$x+$y+$z;
    #warn "a1=$a1, b1=$b1, x=$x, a2=$a2, b2=$b2, y=$y, a3=$a3, b3=$b3, z=$z, a1=$ans1";

    Context()->flags->set(requireProperFractions => 1);


    BEGIN_TEXT
    Evaluate each expression if \(x =\frac{$a1}{$b1}\), \(y=\frac{$a2}{$b2}\), and
    \(z=\frac{$a3}{$b3}\).$BR
    Your answer should be a reduced fraction or a mixed number with the $BR
    faction part reduced (a mixed number can be entered in the form 2 3/4 to
    mean \(2\frac{3}{4}\)).$PAR
    $PAR
    a) \(x-y=\) \{ans_rule(6)\}
    $PAR
    b) \(x+2y=\) \{ans_rule(6)\}
    $PAR
    c) \(x+y+z=\) \{ans_rule(6)\}
    END_TEXT

    ANS($ans1->cmp);
    ANS($ans2->cmp);
    ANS($ans3->cmp);

    ENDDOCUMENT();

Davide