Calculating the molar mass of a gas given state function values
The mistake you made was the value of $R$. If pressure and volume are taken in $\mathrm{atm}$ and $\mathrm{L}$, then the value of $R$ is $0.0821$. (Not $0.82..$). 

Now, $$n=\frac{0.9645\times 0.255}{0.0821\times 821.15}=0.0036$$

$$M=\frac{1.22}{0.0036}=334.412\mathrm{g}$$