Question

if 10 ^ 21 molecules are removed from 200 milligrams of Co2 then find the number of moles of Co2 left in the container

Answer 1

Molar mass of CO2=44g
given mass=200mg=0.2g
No.of moles=0.2/44=1/220
No. of moles=No.of moles/Avagadro's constant
1/220=No. of moles/6.022×10^23/220=No.of moles
=2.73×10^21 molecules
Now from these 10^21 molecules are removed
2.73×10^21-10^21
1.73×10^21 molecules are now left
no. of moles=molecules/avagadros constant
(1.73×10^21)/(6.022×10^23)
=0.00288

Answer 2

molar mass of CO2= 44
mass= 200g or 0.2 kg
no of moles= 0.2/44 = 1/220

so, no of mole= no of molecules/ avagadros no
or no of molecules= no of moles/ avagdros no
= 6.022 x 10²³/ 220 
= 2.73 x 10²¹

as 10²¹ molecules has been removed 
1.73 x 10²¹ will be left behind 

No of moles= no of molecules / avagadros no
= 1.73 x 10²¹/ 6.022 x 10²³
= 0.00288