Calculate the concentration of an aqueous solution of non electrolyte at $300\text{ K}$ if its osmotic pressure is $12\text{ atm}$.
$[ \text{R} = 0.0821\text{ atm dm}^3\text{ K}^{-1}\text{ mol}^{-1} ]$
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Use $T$ in Kelvin always! (K = $^\circ$C + 273.15).