In a classic effusion experiment, ammonia (NH_3) travels farther than hydrogen chloride (HCl) in a glass tube. Which of the following best explains this observation?

• A. NH_3 has a higher density than HCl, allowing it to travel farther.
• B. NH_3 is more reactive than HCl, causing it to move farther.
• C. NH_3 has a lower molar mass than HCl, so it effuses faster according to Graham's law.
• D. NH_3 is a stronger acid than HCl, so it diffuses more rapidly.

Answer: (C)

Verified step by step guidance

1. Step 1: Understand the concept of effusion, which is the process by which gas particles pass through a tiny opening without collisions between particles. The rate of effusion depends on the speed of the gas molecules.
2. Step 2: Recall Graham's law of effusion, which states that the rate of effusion of a gas is inversely proportional to the square root of its molar mass. Mathematically, this is expressed as: \(\frac{r_1}{r_2} = \sqrt{\frac{M_2}{M_1}}\), where \(r\) is the rate of effusion and \(M\) is the molar mass.
3. Step 3: Identify the molar masses of ammonia (NH_3) and hydrogen chloride (HCl). Calculate or look up their molar masses: NH_3 has a molar mass of approximately 17 g/mol, and HCl has a molar mass of approximately 36.5 g/mol.
4. Step 4: Apply Graham's law to compare the rates of effusion of NH_3 and HCl. Since NH_3 has a lower molar mass, it will effuse faster and thus travel farther in the tube.
5. Step 5: Evaluate the other options and understand why they are incorrect: density alone does not determine effusion rate, reactivity and acidity do not affect the physical process of effusion.