At 298 K , a 1 litre solution containing 10 mmol of $\mathrm{Cr}_2 \mathrm{O}_7^{2-}$ and 100 mmol of $\mathrm{Cr}^{3+}$ shows a pH of 3.0 . Given : $\mathrm{Cr}_2 \mathrm{O}_7^{2-} \rightarrow \mathrm{Cr}^{3+} ; \mathrm{E}^0=1.330 \mathrm{~V}$ and $\frac{2.303 \mathrm{RT}}{\mathrm{F}}=0.059 \mathrm{~V}$ The potential for the half cell reaction is $x \times 10^{-3} \mathrm{~V}$. The value of $x$ is $\_\_\_\_$ .