the weight of 350ml gas at 0 degree Celsius and 2 ATM pressure is 1 gram calculate the weight of one molecule of gas.

Your atom OXYGEN with MW= 16, I mean weight,

How it comes its very simple, let me show you the calculation part.

You must be familiar with ideal gas law, very famous

PV=nRT

now as you mentioned

Our P=2atm, V=350 ml, R= 0.082 lit.atm/Mol.K & T= 0'c or 273K

Put up all the value in above equation

Here is one tricky part where people often makes mistakes.

In equation n is equal to n/2 , because our gas is diatomic.

Thats it, you will get n i.e. no. of moles = 0.0625

Now, putting very simple equation i.e.

Weight= moles * MW

Where, weight= 1g as you mentioned in question.

That will turn out to be

MW = 16

which stands for OXYGEN.