Q.34.05 mL of phosphorus vapour weighs 0.0625 g at 546 °C and 0.1 bar pressure. What is the molar mass of phosphorus?

The Intuition: What Does an "Ideal Gas" Even Mean?

Imagine a box full of tiny, perfectly bouncy balls — millions of them — zipping around in straight lines, never sticking to each other or to the walls. They take up no space themselves (their own volume is zero), and when they collide, they don't lose any energy. That's an ideal gas: a model where the only thing that matters is the motion of the particles.

Real gases (like air, oxygen, or helium) behave almost like this at low pressures and high temperatures. The ideal gas is a simplification that lets us predict how a gas will respond when we squeeze it, heat it, or add more of it.

The Three Laws That Came Before

Before the full equation, scientists discovered three separate patterns:

Boyle's Law — If you keep the temperature and amount of gas fixed, squeezing the gas into a smaller volume makes the pressure go up. Double the pressure, half the volume. Mathematically: $P \propto \frac{1}{V}$ (at constant $n$ and $T$).

Charles's Law — If you keep the pressure and amount fixed, heating the gas makes it expand. Double the absolute temperature (in Kelvin), double the volume. So: $V \propto T$ (at constant $n$ and $P$).

Avogadro's Law — If you keep pressure and temperature fixed, doubling the number of gas particles doubles the volume. So: $V \propto n$ (at constant $P$ and $T$).

Each law holds a different variable constant. The genius move was to combine all three into one statement.

The Combined Statement: The Ideal Gas Equation

Putting the three proportionalities together:

Remove the proportionality sign by introducing a constant $R$ (the universal gas constant):

Or, more familiarly:

That's it. One equation that tells you everything about the state of an ideal gas.

What Each Symbol Means

• $P$ — Pressure of the gas (usually in pascals, Pa, or atmospheres, atm)
• $V$ — Volume the gas occupies (in cubic metres, m³, or litres, L)
• $n$ — Number of moles of gas (not number of molecules — one mole is $6.022 \times 10^{23}$ particles)
• $R$ — Universal gas constant. Its value depends on the units you use. The two you'll see most often:
  • $R = 8.314 \, \text{J mol}^{-1} \text{K}^{-1}$ (when using SI units: Pa, m³)
  • $R = 0.0821 \, \text{L atm mol}^{-1} \text{K}^{-1}$ (when using L and atm)
• $T$ — Absolute temperature, always in kelvin (K). Never in Celsius. To convert: $T(\text{K}) = T(°\text{C}) + 273.15$

Why This Equation Is So Powerful

If you know any four of the five quantities ($P$, $V$, $n$, $T$, $R$), you can find the fifth. That means you can:

• Find how much gas is in a container by measuring pressure, volume, and temperature.
• Predict what happens to pressure when you heat a sealed can. …