def El2Mass(Elements):
    """compute molecular weight from Elements 
    
    :param dict Elements: elements in molecular formula; 
        each must contain 
        Num: number of atoms in formula
        Mass: at. wt. 
    
    :returns: float mass: molecular weight.
    
    """
    mass = 0
    for El in Elements:
        mass += Elements[El]['Num'] * Elements[El]['Mass']
    return mass
