Chemistry 2001 JAMB Past Questions - Question 3: 25cm³ of a gas X contains Z molecules at 15°C and 75mmHg. How many molecules will 25cm³ of another gas Y contain at the same temperature and pressure?

Correct Answer

C

Explanation

To solve this problem, we can use the ideal gas law, which states that PV = nRT, where P is the pressure, V is the volume, n is the number of moles, R is the gas constant, and T is the temperature in Kelvin.

First, we need to find the number of moles of gas X using the given information. We can use the formula n = PV / RT, where P = 75 mmHg, V = 25 cm³, R = 0.0821 L·atm/(K·mol), and T = 15°C = 15 + 273.15 = 288.15 K.

n = (75 mmHg) * (25 cm³) / (0.0821 L·atm/(K·mol) * 288.15 K)
  = (75 * 25 * 10^-3) / (0.0821 * 288.15)
  ≈ 0.0087 moles

Now, we can use Avogadro's law, which states that equal volumes of gases at the same temperature and pressure contain equal numbers of molecules. Therefore, the number of molecules of gas Y in 25 cm³ at the same temperature and pressure will also be Z, assuming both gases behave ideally.

So, 25cm³ of gas Y will contain the same number of molecules as gas X, which is Z.