The reaction for the fusion of chromite ore \( {FeCr}_2{O}_4 \) with sodium carbonate (\( {Na}_2{CO}_3 \)) in the presence of oxygen (\( {O}_2 \)) is given by:
\[
{FeCr}_2{O}_4 + {Na}_2{CO}_3 + {O}_2 \rightarrow {Fe}_2{O}_3 + 2 \, {Cr}_2{O}_3 + 2 \, {Na}_2{O}
\]
The molar mass of the water-insoluble product is the sum of the molar masses of the components involved.
The product formed here is chromium oxide, \( {Cr}_2{O}_3 \), with a molar mass of:
\[
2 \times 52 ({Cr}) + 3 \times 16 ({O}) = 104 + 48 = 152 \, {g/mol}
\]
Thus, the molar mass of the water-insoluble product is \( 196 \, {g/mol} \).