At standard temperature and pressure (STP), a 50-gram sample of H_2O(l) and a 100-gram sample of H_2O(l) have:

• A. the same volume
• B. the same mass
• C. the same chemical composition
• D. the same number of molecules

Answer: (C)

Verified step by step guidance

1. Identify what is being compared: two samples of liquid water (H_2O) with different masses (50 g and 100 g) at standard temperature and pressure (STP).
2. Recall that mass is a measure of the amount of matter in a sample, so a 50 g sample and a 100 g sample do not have the same mass.
3. Understand that volume depends on the amount of substance and density. Since the masses are different and density of liquid water is constant at STP, the volumes will also be different, so they do not have the same volume.
4. Recognize that chemical composition refers to the types of atoms and their arrangement in the substance. Both samples are pure H_2O, so they have the same chemical composition regardless of mass or volume.
5. Consider the number of molecules: since the 100 g sample has twice the mass of the 50 g sample, it contains twice as many molecules, so they do not have the same number of molecules.