Final answer:
To find the number of molecules in 25 cm3 of oxygen at STP, take 1/20th of the number of molecules in 500 cm3 of hydrogen since they are at the same conditions and use Avogadro's law.
Step-by-step explanation:
The number of molecules in a gas at standard temperature and pressure (STP) is dependent on the volume of the gas. According to Avogadro's law, equal volumes of different gases, at the same temperature and pressure, contain the same number of molecules. Since 500 cm3 of hydrogen at STP contains n molecules, 25 cm3 of oxygen at STP, which is 1/20 of the initial volume of hydrogen gas, will contain 1/20 of the number of molecules as in 500 cm3 of hydrogen.
Given that 1 cm3 of a gas at STP has 2.68 × 1019 molecules, the number of molecules in 25 cm3 of oxygen at STP can be calculated by multiplying 2.68 × 1019 by 25. Since the volume of hydrogen is given as 500 cm³, we know that it contains n molecules. Therefore, the number of molecules in 25 cm³ of oxygen is also n, since both gases are at the same conditions of temperature and pressure.