JohnnyRocket,
To use these VIs, create a polynomial array for each chemical element:
H = [0, 0.999851, 0.00015]
C = [0,0,0,0,0,0,0,0,0,0,0,0,0.98938, 0.01078]
N = [...]
O = [...]
I = [...]
For C15H11I4NO4, take poly C and raise to power 15 using the power VI. Do the same for each element and then multiply the resulting polynomials. This polynomial will contain the relative abundance of molecular weight for the molecule e.g. array element N is the relative abundance of molecular weight N.
A quick test for good result is to sum the array elements; it should always be 1.0 +/- numerical accuracy if the sum of chemical elements arrays H,C,... are also 1.0