One lit of an unknown gas weighs 1.25 grams at s.t.p one possiboe formula of gas Co2 Co O2 So2?

Molar volume of a gas at s.t.p. = 22.4 L/mol
No. of moles of the unknown gas = (1 L) / (22.4 L/mol) = 1/22.4 mol

Molar mass of the unknown gas = (1.25 g) / (1/22.4 mol) = 28 g/mol

Molar masses :
CO₂ = (12 + 16×2) g/mol = 44 g/mol
CO = (12 + 16) g/mol = 28 g/mol
O₂ = 16 × 2 g/mol = 32 g/mol
SO₂ = 32 + 16×2 = 64 g/mol

Hence, the possible formula is CO.

Since 1 mole of a gas is contained in 22 litres.
Then there is 1/22 = 0.04545... moles present

Using the equation ,
moles = mass(g) / Mr
Then it follows
Mr = mass(g) / Moles
Mr = 1.25g / 0.04545...
Mr = 27.5 ( Relative Molecular mass of the gas).

Mr's of
CO2 = 12 + 16 + 16 = 44
CO = 12 + 16 = 28
O2 = 16 + 16 = 32
SO2 = 32 + 16 + 16 = 64

Since we have an Mr of 27,5' , then the gas could be 'CO' ( Carbon Monoxide) as this is the nearest Mr to the calculated result.

since the problem specifies STP.. you could do this

.. 1.25g... .22.41 L
---- ----- x ----- ----- = 28.0 g / mol
.. ..1 L... ... 1 mol

so.. which of those gases has molar mass = 28.0 g/mol?.. CO is the only one right?

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that is correct Pisgahchemist.... but I just can't help thinking this problem was designed for those students who recognize 22.41 L/mol @STP... :)

those students who paid attention and remembered that equality and learned their DA, could glance at this problem, spend 10 seconds solving it, and move on to the next problem.

Ideal gas equation....

Using the mass of the gas and the volume at STP, find the molar mass of the unknown gas.

PV = nRT .............. ideal gas equation
PV = mRT / M........ n=m/M.... m=mass, M=molar mass
M = mRT / (PV)...... solve for molar mass
M = 1.25g x 0.08206 Latm/molK x 273.15K / 1.00 atm / 1.00L
M = 28.0 g/mol

CO2 .... 44.0 g/mol
CO ...... 28.0 g/mol
O2 ....... 32.0 g/mol
SO2...... 64.1 g/mol

========== Follow up ==========

Using 22.4L, the volume of one mole of an ideal gas at STP, is certainly one way to approach this problem. (It's 22.4L, not 22L) But, what if the gas is not at STP, what then? The approach that I've shown will work with any mass, pressure, volume and temperature.

answer is CO
its mol.wt = 28
so 22.4 liter weighs = 28 gm
1 liter will weigh = 28/22.4= 1.25 gm at stp