Answer :

i) Calculation of number of molecules in each:

a. Given: Mass of He = 20g


Gram molecular mass of He = 4g


To calculate the number of molecules, first we apply the formula:





Number of gram molecules = 5


Now, number of molecules = 5 × 6.022 × 1023


= 3.011 × 1024


Thus, number of molecules of He is 3.011 × 1024


b. Given: Volume of NH3 at STP = 44.8L


Gram molecular mass of NH3 = 17g


First we will calculate the number of moles so that we can find the number of molecules.


As we know that 1 mole of any gas occupies 22.4 litres of volume.


1 mole of NH3 = 22.4 litre


22.4 × Number of moles = 44.8 litres



Number of moles = 2


Now, number of molecules = No. of moles × 6.022 × 1023


= 2 × 6.022 × 1023


= 1.204 × 1024


Thus, number of molecules of NH3 is 1.204 × 1024


c. Given: Volume of N2 at STP = 67.2L


Gram molecular mass of N2 = 28g


First we will calculate the number of moles so that we can find the number of molecules.


As we know that 1 mole of any gas occupies 22.4 litres of volume.


1 mole of NH3 = 22.4 litre


22.4 × Number of moles = 67.2 litres



Number of moles = 3


Now, number of molecules = No. of moles × 6.022 × 1023


= 3 × 6.022 × 1023


= 1.806 × 1024


Thus, number of molecules of N2 is 1.806 × 1024


d. 1 mol of H2SO4


Given: Number of moles = 1mol


Number of molecules = No. of moles × 6.022 × 1023


= 1 × 6.022 × 1023


= 6.022 × 1023


Thus, number of molecules of H2SO4 is 6.022 × 1023


e. Given: Mass of water = 180g


Gram molecular mass of H2O = 18g


To calculate the number of molecules, first we apply the formula:





Number of gram molecules = 10


Now, number of molecules = 10 × 6.022 × 1023


= 6.022 × 1024


Thus, number of molecules of H2O is 6.022 × 1024


The arrangement of the samples in increasing order of the number of molecules in each is:


44.8 L of NH3 at STP < 67.2 L of N2 at STP < 20 g of He < 1 mol of H2SO4 < 180 g of water


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