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Question

A 19.6 g of a given gaseous sample contains 2.8 g of molecules d=0.75 g L1 11.2 g of molecules (d=3g L1) and 5.6 g of molecules (d=1.5 g L1). All density measurements are made at STP. Calculate the total number of molecules (N) present in the given sample. Report your answer in N×1023.
Assume Avogardro's number as 6×1023.

A
3
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B
2
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C
4
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D
2.5
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Solution

The correct option is A 3
Total weight = 19.6 g
(a) 2.8 g of molecules
d=0.765 g L1 Volume=Massd=2.80.75L at STP
1 mol 22.4 L at STP
Moles =2.80.75×122.4 Molecules =2.80.75×122.4×6×1023=1×1023
(b) 11.2 g of molecules
d=3 g L1
Volume=Massd=11.23L at STP
1 mol 22.4 L at STP
Moles =11.23×122.4 Molecules =11.23×122.4×6×1023=1×1023

(c) 5.6 g of molecules
d=1.5 g L1
Volume=Massd=5.61.5L at STP
1 mol 22.4 L at STP
Moles =5.61.5×122.4 Molecules =5.61.5×122.4×6×1023=1×1023
Molecules =1×1023
Total number of molecules (a)+(b)+(c)=3×1023N=3

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