“Parts per” notation is one way of expressing how much solute is in a solution. Common parts per expressions in aquatic science are parts per thousand (ppt), parts per million (ppm), and parts per billion (ppb). These expressions represent like units, such as grams per 1000 grams (ppt) or grams per 1000000 grams (ppm). For example, 35 parts per thousand salinity represents 35 grams of salt in 1000 grams of solution.

Understanding parts per notation is useful when creating solutions. The following equation is for a 35 ppt salinity solution containing 35 grams of salt per 1000 grams of saltwater.

A scientist wants to make 400 mL of 35 ppt saltwater. The scientist must determine how much salt and how much water to use to make this solution. The density of pure water is approximately 1 g/mL, so the mass of 1 mL of water is 1 g. Thus, 400 mL of solution will have a mass of 400 g. In the following equation, the scientist solves for x to determine the amount of salt in grams needed to make 400 mL of a solution of 35 ppt.

Cross-multiplying the fractions gives

Multiply the values and cancel the units on each side of the equation, giving

So, the scientist needs 14 grams of salt to make 400 g of 35 ppt saltwater. However, the total mass of the solution is 400 grams. To calculate the amount of water needed

To make 400 grams of 35 ppt saltwater, the scientist will need to dissolve 14 grams of salt in 386 grams of water.