Chemistry Lesson Note On Gas Law

Topic 3: Gas Laws

Class: SS1

---

Specific Objectives:

By the end of the lesson, students should be able to:

1. State and explain Boyle’s Law and Charles’ Law.
2. Perform calculations using the gas laws.
3. Explain the ideal gas equation and apply it to solve numerical problems.
4. Relate gas laws to real-life situations.

---

Instructional Materials:

• Balloons
• Syringes
• Graph/chart paper
• Ruler
• Calculator
• Chart showing the gas laws and their graphs

---

Lesson Content:

Step 1: Boyle’s Law

• Statement: At constant temperature, the volume of a fixed mass of gas is inversely proportional to its pressure.
• Mathematical Expression:

  P \propto \frac{1}{V} \quad \text{or} \quad PV = k

• = pressure

• = volume

• = constant

• (for calculations)

• Graph: A plot of P against 1/V gives a straight line; a plot of P against V gives a curve.

Example:
A gas occupies 200 cm³ at 750 mmHg. What volume will it occupy at 1000 mmHg (T constant)?

P_1V_1 = P_2V_2 \Rightarrow 750 \times 200 = 1000 \times V_2  
\Rightarrow V_2 = \frac{750 \times 200}{1000} = 150\text{ cm}^3

---

Step 2: Charles’ Law

• Statement: At constant pressure, the volume of a fixed mass of gas is directly proportional to its absolute temperature.
• Mathematical Expression:

  V \propto T \quad \text{or} \quad \frac{V}{T} = k  
  \Rightarrow \frac{V_1}{T_1} = \frac{V_2}{T_2}

• = volume

• = temperature in Kelvin

• Kelvin = °C + 273

• Graph: A plot of volume against temperature (K) gives a straight line.

Example:
A gas has a volume of 300 cm³ at 27°C. What will be its volume at 87°C (pressure constant)?
Convert to Kelvin: T₁ = 27 + 273 = 300 K, T₂ = 87 + 273 = 360 K

\frac{V_1}{T_1} = \frac{V_2}{T_2} \Rightarrow \frac{300}{300} = \frac{V_2}{360}  
\Rightarrow V_2 = \frac{300 \times 360}{300} = 360 \text{ cm}^3

---

Step 3: Ideal Gas Equation

• Formula:

  PV = nRT

• = pressure (in atm or Pa)
• = volume (in litres or m³)
• = number of moles
• = universal gas constant (0.0821 atm·L/mol·K or 8.314 J/mol·K)
• = temperature in Kelvin

Example:
Calculate the volume occupied by 2 moles of an ideal gas at 300 K and 1 atm pressure.

PV = nRT \Rightarrow V = \frac{nRT}{P} = \frac{2 \times 0.0821 \times 300}{1} = 49.26\text{ L}

---

Step 4: Combined Gas Law

When pressure, volume, and temperature all change:

\frac{P_1V_1}{T_1} = \frac{P_2V_2}{T_2}

Real-Life Applications:

• Inflating car tires (Boyle’s Law)
• Expansion of hot air balloons (Charles’ Law)
• Gas behavior in refrigeration and air conditioning systems
• Breathing mechanism in humans

---

Evaluation:

1. State Boyle’s and Charles’ laws.
2. A gas has volume 500 cm³ at 600 mmHg. What will be its volume at 800 mmHg (T constant)?
3. At 25°C, a gas has volume 250 cm³. What is its volume at 75°C (pressure constant)?
4. Define and apply the ideal gas law to find the number of moles of gas occupying 10 L at 273 K and 1 atm.
5. Explain how the lungs obey Boyle’s law.

---

Homework:

1. A sample of gas occupies 400 cm³ at 20°C. Find its volume at 80°C.
2. Calculate the pressure exerted by 0.5 mol of gas in a 2 L container at 273 K.
3. Explain how gas laws are important in aviation or meteorology.

---

Conclusion:

Gas laws explain how gases respond to changes in pressure, volume, and temperature. These laws are fundamental in understanding natural phenomena and designing systems that involve gases.