Mole concept, can anybody help?

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[Tay Guan Piang]

To all Chemistry teachers,

There's a question i came across when i read through Quick Review D (Page 39, Chemistry F4 Text Book). The question sounds as below:

4. 1.12 dm³ of hydrogen gas, H₂ and 1.12 dm³ of oxygen gas, O₂ are mixed together in a closed container at STP. What is the total number of molecules in the container? What is the total mass of the gases in the container?

Solution that i had made:
Total volume of gases = 1.12 dm³ + 1.12 dm³ = 2.24 dm³
No. of moles of gases = 2.24 dm³ / 22.4 dm³ mol^-1 = 0.1 mole

total number of molecules in the container = 0.1 mole x avo. constant = 6.02 x 10²² molecules

BUT
total mass of the gases = 0.1 moles x (2 g mol^-1 + 32 g mol^-1) = 3.4g (this is the one i calculated)

the answer given in the book = 1.7g

anything wrong with my calculation? do correct me if i've done wrong. Thanks

[Ting Tin Chung]

To all Chemistry teachers,

There's a question i came across when i read through Quick Review D (Page 39, Chemistry F4 Text Book). The question sounds as below:

4. 1.12 dm³ of hydrogen gas, H₂ and 1.12 dm³ of oxygen gas, O₂ are mixed together in a closed container at STP. What is the total number of molecules in the container? What is the total mass of the gases in the container?

Solution that i had made:
Total volume of gases = 1.12 dm³ + 1.12 dm³ = 2.24 dm³
No. of moles of gases = 2.24 dm³ / 22.4 dm³ mol^-1 = 0.1 mole

total number of molecules in the container = 0.1 mole x avo. constant = 6.02 x 10²² molecules

BUT
total mass of the gases = 0.1 moles x (2 g mol^-1 + 32 g mol^-1) = 3.4g (this is the one i calculated)

the answer given in the book = 1.7g

anything wrong with my calculation? do correct me if i've done wrong. Thanks

my calculation as follow :
the number of mole for Hydrogen gas = 1.12 / 22.4 = 0.05 mol
the number of mole for oxygen gas = 1.12 / 22.4 = 0.05 mol

total number of molecules = (0.05 X N_A) + (0.05 X N_A)
                              = 6.02 X 10²²

total mass of gas = (0.05 X 2) + (0.05 X 32)
                        = 0.1 + 1.6
                        = 1.7 g_#

[Tay Guan Piang]

Oh, thanks a lot, Mr.Ting. your calculation just reminded me that we should take into consideration that that is two different gas. so the moles should be calculated separately before total them up. thanks and thanks again

[Ting Tin Chung]

Oh, thanks a lot, Mr.Ting. your calculation just reminded me that we should take into consideration that that is two different gas. so the moles should be calculated separately before total them up. thanks and thanks again

You are welcome Mr Tay...

[Mohd Fikri Bin Ismail]

thanx for that solution.. now i can give some of exmple to explain more to my student

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