Question

At STP weight of litre of a gas is 1.25 grams. The gas will be:
A. ${{N}_{2}}$
B. $CO$
C. ${{C}_{2}}{{H}_{4}}$
D. All of these

Answer 1

Hint: To solve the question you should know the value of volume at STP that is 22.4L. In question the weight is given for one liter and we have to calculate for 22.4L.

Complete step by step solution:
To solve this question you should know the value of physical properties at STP that you have learnt in your chemistry lessons. Like for pressure it is 760mm of Hg and likewise there are many more.
To know more about the value of volume at STP you should know about Avogadro's law.
According to Avogadro's law, 1 mole of every gas occupies 22.4L at STP and weighs equal to its molar mass.
In the question it is given that weight of one liter of gas is 1.25 grams
1L of gas = 1.25 g
Then for 22.4 L of gas it will occupy = $1.25\times 22.4=28g$$1.25\times 22.4=28g$
Now, from the question calculate the molar mass of each of the options given above.
(A).${{N}_{2}}$
To calculate its molar mass you should know the atomic mass of nitrogen.
Atomic mass of nitrogen = 14
So, the two molecules of nitrogen will have molecular mass = $2\times 14=28g$
(B). $CO$
Atomic mass of carbon= 12
Atomic mass of nitrogen =16
Now the molar mass of this molecule will be = $16+12=28g$
(C).${{C}_{2}}{{H}_{4}}$
Molar mass of this molecule will be = $2\times 12+4\times 1=28g$
In this way you can find the molar mass or weight and check the molecule which gets a similar result as you have found at STP.

Thus the correct option will be (D).

Note: Do not use the formula $moles=\frac{Volume\,of\,gas}{22.4L}$ this formula is to find the moles but here we have to find the weight at STP. Calculate the molar mass of each molecule carefully and not atomic weight.