Physics and chemistry students learn several methods to determine the density of materials. While measuring the mass of materials is usually simple, volume measurements are more complex. For simple shapes the volume may be determined by measuring its geometry; for more complex shapes students often use Archimedes' principle. However, neither of these methods works well for soluble materials like salts. In water, the ions present in a salt dissociate, eliminating the possibility of using volume displacement. It is also difficult to measure the size of small salt crystals due to the irregularity in the crystal sizes and the air between crystals. Here, we modify Boyle's law experiment to find the volume of salt crystals and thus accurately determine their density.