/**
  * Returns the value in the volume field, or Units.UNKNOWN_VALUE if blank.
  *
  * @return The entered volume.
  */
 public double getVolume() {
   double amount = volume.getAmount();
   if (amount == Units.UNKNOWN_VALUE)
     steps.add(Function.latex("\\text{" + name + " volume} =  ?"));
   else steps.add(Function.latex("\\text{" + name + " volume} = " + amount + " L"));
   return amount;
 }
 /**
  * Returns the value in the molarity field, or Units.UNKNOWN_VALUE if blank.
  *
  * @return The entered molarity.
  */
 public double getMolarity() {
   double amount = molarity.getAmount();
   if (amount == Units.UNKNOWN_VALUE)
     steps.add(Function.latex("\\text{" + name + " molarity} =  ?"));
   else
     steps.add(Function.latex("\\text{" + name + " molarity} = " + amount + "\\frac{mol}{L}"));
   return amount;
 }