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# How many oxygen molecules are present in 113.97 liters of oxygen gas at STP?

Use dimensional analysis,

113.97 liters (1 mol of O2/22.4 liters) (6.02*10^23 molecules/1 mol of O2)

Can you finish it?

At STP, the molar volume of an ideal gas is 22.4 L. In other words, the volume of 1 mol of a gas like oxygen is 22.4 L. (mol/22.4L) or (22.4L/mol)

The problem gives you 113.97 L of oxygen. Using dimensional analysis, you can find the number of moles of oxygen.

113.97 L x (1 mol/22.4L) = 5.0879 mol of oxygen.

The problem asks for the number of oxygen molecules and the number of molecules in a mol is 6.02 x 10^23.

5.0879 mol oxygen x ((6.02 x 10^23) oxygen molecules/ mol oxygen ) = 3.06 x 10^24 molecules

First let's figure how many moles of oxygen are present.

At STP, 1 mole occupies 22.4 liters.  So,

113.97 liters * (1 mole / 22.4 liters) = 5.09 mole.

Now there are 6.02e23 molecules per mole. So multiplying this by the number of moles, we get

(6.02 x 1023 molecules / mole)(5.09 mole) = 3.06 x 1024 molecules.

Hope this helps.