Using exercise package correctly

Question

Please correct my code as necessary. It is not compiling due to

! Package exercise Error: You don't respect the hierarchy of questions.

See the exercise package documentation for explanation.
Type  H <return>  for immediate help.
 ...                                              

l.13 \begin{Question}W
                      hat is the symbol for copper?\end{Question}
? 

The code is as follows:

\documentclass[12pt]{article}
\usepackage[a4paper]{geometry}
\usepackage{mhchem}


\usepackage[lastexercise]{exercise}

\begin{document}
\section{Naming of Chemical Compounds - A ``DIY'' Tutorial}

\begin{Exercise}[title=DIY]\end{Exercise}
\begin{ExePart}Question 1\end{ExePart}
\begin{Question}What is the symbol for copper?\end{Question}
\begin{Answer}\ce{Cu}\end{Answer}

\end{document}

It could be that I've missed something but the documentation does not even provide an example of the package's usage.

Answer 1

I managed to get it to work using the ExerciseList environment:

\documentclass[12pt]{article}
\usepackage[a4paper]{geometry}
\usepackage{mhchem}


\usepackage[lastexercise]{exercise}

\begin{document}
\section{Naming of Chemical Compounds - A ``DIY'' Tutorial}

\begin{ExerciseList}
  \Exercise[title=DIY]
  \ExePart{Question 1}
  \Question{What is the symbol for copper?}
  \Answer{\ce{Cu}}
\end{ExerciseList}

\end{document}

Use of \ExePart or redefinition of \QuestionNB

That said, you shouldn't need the \ExePart just to print "Question 1". It might be better to redefine \QuestionNB instead, like this:

\renewcommand{\QuestionNB}{Question~\arabic{Question}.\ }

Theorically, that should do what you want to achieve, although in practice, it's kind of a mess. I feel the reason is that the label is hardcoded in the package, and without a variable. This can however be "fixed" by resetting \QuestionIndent:

\setlength{\QuestionIndent}{7em}

So your example becomes:

\documentclass[12pt]{article}
\usepackage[a4paper]{geometry}
\usepackage{mhchem}


\usepackage[lastexercise]{exercise}

\begin{document}
\section{Naming of Chemical Compounds - A ``DIY'' Tutorial}

\renewcommand{\QuestionNB}{Question~\arabic{Question}.\ }
\setlength{\QuestionIndent}{7em}

\begin{ExerciseList}
  \Exercise[title=DIY]
  \Question{What is the symbol for copper?}
  \Answer{\ce{Cu}}
\end{ExerciseList}

\end{document}

which works for me (although I would personally find it nicer to put questions on the next line -- note: I'm not using the mhchem package):

After playing a bit with more Questions/Answers, I do think it's missing a linebreak somewhere:

I'm also not very sure of the spacing: answers end up being closer to the next question than to the question they relate to...

Exercise title

In fact, it seems the problem with the linebreak comes from the fact that you're using \Exercise[title=DIY] instead of simply \Exercise{DIY}, which defines the exercise name. When I change this:

\documentclass[12pt]{article}
\usepackage[a4paper]{geometry}
\usepackage{mhchem}


\usepackage[lastexercise]{exercise}

\begin{document}
\section{Naming of Chemical Compounds - A ``DIY'' Tutorial}

\renewcommand{\QuestionNB}{Question~\arabic{Question}.\ }
\setlength{\QuestionIndent}{7em}

\begin{ExerciseList}
  \Exercise{DIY}
  \Question{What is the symbol for copper?}
  \Answer{\ce{Cu}}
  \Question{What is the symbol for aluminum?}
  \Answer{\ce{Al}}
  \Question{What is the symbol for iron?}
  \Answer{\ce{Fe}}
\end{ExerciseList}

\end{document}

I get a linebreak:

and it is not necessary anymore to redefine \QuestionIndent:

Answer 2

\Question is a command, not environment. That's why you get the errors.

\documentclass[12pt]{article}
\usepackage[a4paper]{geometry}


\usepackage[lastexercise]{exercise}

\begin{document}
\section{Naming of Chemical Compounds - A ``DIY'' Tutorial}



\begin{Exercise}[title=DIY,label=E1]
\ExePart[name=simple]
\Question{What is the symbol for copper?}\label{Q1}
\Question{What is the symbol for oxygen?}\label{Q2}

\ExePart[name=difficult]
\Question{What 42?}
\end{Exercise}

\begin{Answer}[ref=E1]\ref{Q1}. Cu  \quad\ref{Q2}. O \end{Answer}
\end{document}

Answer 3

Answers must be out of the list. This way, counters work fine.

\documentclass[12pt]{article}
\usepackage[a4paper]{geometry}


\usepackage[lastexercise, answerdelayed]{exercise}


\begin{document}
\section{Naming of Chemical Compounds - A ``DIY'' Tutorial}


\begin{ExerciseList}
  \Exercise[title=DIY, label=E1]
  \ExePart{Question 1}
  \Question{What is the symbol for copper?}
  \Question{What is the symbol for iron?}
  \ExePart{Question 1}
  \Question{What is the symbol for copper?}
  \Question{What is the symbol for iron?}

  \Exercise[title=DIY, label=E2]
  \ExePart{Question 1}
  \Question{What is the symbol for oxigen?}
  \Question{What is the symbol for hidrogen?}
  \ExePart{Question 1}
  \Question{What is the symbol for oxigen?}
  \Question{What is the symbol for hidrogen?}



\end{ExerciseList}



  \begin{Answer}[ref=E1]
  \ExePart{Question 1}
  \Question{Cu}
  \Question{Fe}     
  \ExePart{Question 1}
  \Question{Cu}
  \Question{Fe}     
  \end{Answer}

  \begin{Answer}[ref=E2]
  \ExePart{Question 1}
  \Question{O}
  \Question{H}      
  \ExePart{Question 1}
  \Question{O}
  \Question{H}      
  \end{Answer}


\paragraph*{ Solutions:}   Now, let's see te solutions
\shipoutAnswer 
\end{document}