A 72.7 mL sample of oxygen was collected over water. The atmospheric pressure is 0.861 atm and the temperature of the water is 19.0 °C. What are the moles of oxygen?

Question

A  72.7 mL sample of oxygen was collected over water.  The atmospheric pressure is 0.861 atm and the temperature of the water is 19.0 °C.  What are the moles of oxygen?

Answer 1

We know PV=nRT, where n is the amount of moles.

If we rearrange this equation for n we obtain n= (PV)/(RT).

We first convert 19.0C to K and we get T=19.0C+273.15=292.15 K.

Now we are ready to plug everything in our equation, making sure that we convert the Volume to L.

n= (0.861 atm*0.0727 L)/(292.15 K*0.08206 atm*L/K*mol)=2.61*10^-3 moles.