The calculation for the number of atoms in 15 liters of argon (Ar) at standard temperature and pressure (STP) involves dividing the volume by the molar volume of a gas (22.4 L) and then multiplying by Avogadro's number (6.022 x 10^23). The initial calculation yielded 4.0325 x 10^23 atoms, but the significant figures were incorrect. The final answer should reflect the appropriate number of significant figures based on the given data. The method used for the calculation was correct, but attention to significant digits is crucial for accuracy. Properly addressing significant figures will ensure the answer is valid.