# Question Video: Calculating the Number of Moles of Gas Molecules at STP Given a Volume Chemistry

Under standard temperature and pressure (STP), a gas occupies a volume of 2 L. How many moles of gas molecules are there? Give your answer to 2 decimal places.

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### Video Transcript

Under standard temperature and pressure, STP, a gas occupies a volume of two liters. How many moles of gas molecules are there? Give your answer to two decimal places.

Avogadroβs law states that at constant temperature and pressure, the volume and number of moles of a gas are directly proportional. This proportion can be expressed by the equation π equals ππ subscript π, where π is the volume in liters, π is the number of moles, and π subscript π is the molar gas volume, a proportionality constant that indicates the volume occupied by one mole of gas at a specific temperature and pressure. Molar gas volume has the unit of liters per mole. In this question, the gas is under standard temperature and pressure, abbreviated STP. Standard temperature and pressure are zero degrees Celsius and one atmosphere, respectively. At STP, one mole of any gas will occupy a volume of 22.4 liters and have a molar gas volume of 22.4 liters per mol.

Looking at the question, we see that the gas occupies a volume of two liters, and we want to determine how many moles of gas molecules there are. We also know that as the gas is under standard temperature and pressure, we can use 22.4 liters per mol as the molar gas volume. We can then substitute the volume and molar gas volume into the equation and rearrange to solve for the number of moles. We perform the calculation and determine that the number of moles is equal to 0.08928 moles. But the question asked us to give our answer to two decimal places. Rounding appropriately, we have determined that there are 0.09 moles of gas molecules in two liters of gas under standard temperature and pressure.