use strict;
use warnings;
use Chemistry::MolecularMass;

my $chemical = 'H2O';
my $mm       = new Chemistry::MolecularMass;
my $mass     = $mm->calc_mass($chemical);

if ( defined $mass ) {
	print "The mass of $chemical is: $mass";
}
else {
	print "Unable to calculate the mass of $chemical";
}