π¨ Avogadro's Law Calculator
V1/n1 = V2/n2 at constant T and P.
Leave exactly one field empty to solve for it.
How to Use This Calculator
Enter three of the four values and leave one blank. The calculator solves V1/n1 = V2/n2 for the missing variable.
Avogadro's Law
V2 = V1 x (n2 / n1)
Equal volumes of gases at the same temperature and pressure contain equal numbers of molecules. This was the key insight that let chemists determine molecular formulas and atomic weights in the 1800s.
Worked Example
5.0 L of gas contains 0.20 mol. How many moles in 12.5 L? n2 = n1 x V2/V1 = 0.20 x 12.5/5.0 = 0.50 mol.
Why Equal Volumes Hold Equal Molecules
Avogadro's hypothesis was bold in 1811: at the same temperature and pressure, equal volumes of any gas contain equal numbers of molecules, regardless of what the gas is. A liter of hydrogen and a liter of carbon dioxide hold the same molecule count. This works because gas pressure comes from particle collisions, and at fixed T and P the spacing between molecules is the same for all ideal gases.
Molar Volume and STP
One mole of ideal gas occupies 22.4 liters at standard temperature and pressure (0 C, 1 atm). At room conditions (25 C, 1 atm) it is about 24.5 liters. Chemists use these numbers constantly to convert between gas volumes and moles without a calculator, and this tool handles the general case at any conditions.
Real Gases Deviate
Avogadro's law assumes ideal behavior: point particles with no attractions. Real gases deviate at high pressure (molecules have real volume) and low temperature (attractions matter). For most lab work near 1 atm and room temperature, the ideal approximation is within a few percent and perfectly adequate.
Breathing Example
Each breath you take follows Avogadro's law in reverse: your diaphragm expands lung volume, pressure drops, and air flows in to equalize. A deep breath moves about 0.5 liters containing roughly 0.02 moles of air, over 10^22 molecules. Scuba tank calculations use the same principle at much higher pressures.
Frequently Asked Questions
What is Avogadro's law?
At constant temperature and pressure, the volume of a gas is directly proportional to the number of moles: V1/n1 = V2/n2. Double the moles, double the volume. It is one of the four gas laws that combine into PV = nRT.
What is the molar volume of a gas at STP?
22.4 liters per mole at standard temperature and pressure (0 C, 1 atm). At room conditions (25 C, 1 atm) it is about 24.5 L/mol. This calculator uses your actual conditions.
When does Avogadro's law apply?
Only when temperature and pressure stay constant and the gas behaves ideally. It breaks down at very high pressures or very low temperatures where gas molecules interact.
How is this different from the ideal gas law?
Avogadro's law is a special case of PV = nRT with P and T fixed. If pressure or temperature changes too, use the combined gas law or the full ideal gas law instead.
Can I use it for gas mixtures?
Yes for the total: each gas contributes volume proportional to its moles (Dalton's law of partial pressures). Calculate each component separately and sum.