In the roasting of chromite, $4\mathrm{FeCr}{}_2\mathrm{O}{}_4+8\mathrm{Na}{}_2\mathrm{CO}{}_3+7\mathrm{O}{}_2\to 8\mathrm{Na}{}_2\mathrm{CrO}{}_4+2\mathrm{Fe}{}_2\mathrm{O}{}_3+8\mathrm{CO}{}_2$. The number of moles of $\mathrm{O}{}_2$ consumed per mole of chromite is

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