MolMass is a java applet which calculates all possible molecule masses on the basis the isotopes of the constituent atoms. The program lets you enter the molecule you want to investigate (e.g. CH4). When hitting the calculate button it will calculate the different molecule masses and their abundancy.

Consider CH4 as an example. Since there are the isotopes ¹²C and ¹³C, and ¹H and ²H involved, there are a number of possible masses that can be produced. When you would run this example in the program the results are as follows: There will be a chance of 98.83% of producing CH4 with a mass of 16. In addition there will be a chance of 1.17% of producing CH4 with a mass of 17. The reason why masses of 18-20 don't appear is that their abundancy is very very small.

To run MolMass you need to run a java enabled browser.

Screen prints:

Relative abundancies of CH4 masses	Abundancies scaled to the highest

Please find more screen shots here.