The formula is mass of compound x divided by total mass of solution multiplied by ten to the six or million. Now, since this is a ratio that units cancel and Ppm is unitless, so now it's still an example using parts per million. In this example, we start with 25 bowls of water in a cup. Here's our cup. The blue dots are the water molecules, nothing else exists. You want to find them out in grams of HCL to add to create a 90,000 parts per million solution.