The document describes an experiment to determine the mass of unknown substances. It details that a cup alone weighed 3.5g, and a cup with an unknown substance weighed 32.5g, allowing the calculation of the unknown substance's mass as 29g. Similarly, a cup alone weighed 3.5g and with aluminum weighed 17g, giving the aluminum a mass of 17g - 3.5g = 13.5g.