Chemistry
Ideal Gas Law
The ideal gas law PV = nRT relates pressure, volume, moles, and temperature of an idealized gas. R is the universal gas constant (0.0821 L·atm/(mol·K) or 8.314 J/(mol·K)). Real gases deviate at high pressure and low temperature, but most chemistry problems use the ideal form.
How to Approach Ideal Gas Law
Convert to consistent units
Temperature MUST be in kelvin (K = °C + 273.15). Pressure must match R's units (atm with R = 0.0821, or Pa with R = 8.314). Volume in liters or m³ matching.
Identify what's known and unknown
You'll have three of P, V, n, T given and one unknown. Rearrange PV = nRT to isolate the unknown: P = nRT/V, V = nRT/P, n = PV/(RT), T = PV/(nR).
Plug in and verify
Substitute carefully with units. Cancel them — they should leave you with the correct unit on the unknown. If they don't, you have a unit-conversion mistake.
Frequently Asked Questions
What's the difference between R values?+
Same constant, different units. R = 0.0821 L·atm/(mol·K) for pressure in atm. R = 8.314 J/(mol·K) for SI units. Pick the one whose units match your problem and stick with it.
When does the ideal gas law fail?+
At high pressure (molecules are forced close enough to interact) or low temperature (molecules are slow enough that intermolecular forces matter). Use van der Waals equation for real gases under those conditions.
What's STP?+
Standard Temperature and Pressure — historically 0°C and 1 atm, more recently 25°C and 100 kPa (IUPAC). At STP (old definition), one mole of any ideal gas occupies 22.4 L.
Related Topics
More step-by-step guides in Chemistry and adjacent subjects.
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