9701 MCQ Solution 41

Question 41 : [Physical > Atoms, Molecules & Stoichiometry]

The use of the Data Booklet is relevant to this question.

What is the number of molecules in 500 cm3 of oxygen under room conditions?

A   1.25 x 10 22

B   1.34 x 10 22

C   3.0   x 10 22

D   3.0   x 10 26

Reference: Past Exam Paper - June 2003 Paper 1 Q1

Solution 41:

Answer: A.

Solution : since its room conditions 1 mole of any gas occupy 24dm3 therefore we can use the formula volume= no.of moles x 24dm3. since we are given the volume of oxygen lets find the no.of moles . but before proceeding its easy to convert the volume that is given in cm3 to dm3 , therefore we now calculate like this : no.of moles : 500x10 -23 / 24 = 0.0208.

now we got the number of moles its easy to find the number of molecules or atoms , we can apply Avogadro constant , since its a gas , 02 then the number of moles we got which we found it to be 0.0208 x 6.02 x 10 23 = 1.25 x 10 22. if we found the number of moles and we applied Avogadro constant for diatomic molecules such as oxygen we get the number of molecules if we want to find the number of atoms we multiply the number of molecules we got by 2 . but for single atomic elements such as K , Na , we can apply the Avogadro constant to get the no. of atoms directly.