Question

Calculate the number of oxygen atom present in 11.2 lit of CO2 at STP

Answer 1

NTP, or Normal Temperature and Pressure, conditions imply a temperature of 20∘C, or 293.15 K, and a pressure of 1 atm.

T=293.15 K
P=1 atm

Since you also know the volume the gas occupies under these conditions, you can easily solve for the number of moles present by using the ideal gas law equation, PV=nRT.

Once you know how many moles of oxygen gas you have, you can calculate the number of oxygen molecules by using the fact that 1 mole of any substance contains exactly 6.022⋅1023 molecules of that substance - this is known as Avogadro's number.

So, start by figuring out he number of moles you have

PV=nRT⇒n=PVRT

n=1atm⋅11.2L0.082(L⋅atmmol⋅K⋅293.15K)=0.4659 moles O2

This is equivalent to having

0.4659moles⋅6.022⋅1023molecules1mole=2.81⋅1023molecules

Since oxygen is a diatomic molecule, i. e. it takes two oxygen atoms to make up an oxygen molecule, the number of oxygen atoms will be twice as big
2.81⋅1023molecules⋅2 atoms1molecule=5.62⋅1023atoms