Answer
$$6.5 \times 10^{-3} \space M \space CO_2$$
Work Step by Step
$ CO_2 $ : ( 12.01 $\times$ 1 )+ ( 16.00 $\times$ 2 )= 44.01 g/mol
$$\frac{280 \space mg \space CO_2}{1 \space kg \space solution} \times \frac{1 \space g}{1000 \space mg} \times \frac{1 \space mol \space CO_2}{44.01 \space g \space CO_2} \times \frac{1027 \space kg \space solution}{1000 \space L \space solution} =6.5 \times 10^{-3} \space M \space CO_2$$