Nitrite (NO₂^-) can be determined by oxidation with excess Ce⁴⁺, followed by back titration of the unreacted Ce⁴⁺. A 4.030-g sample of solid containing only NaNO₂ (FM 68.995) and NaNO₃ was dissolved in 500.0 mL. A 25.00-mL sample of this solution was treated with 50.00 mL of 0.118 6 M Ce⁴⁺ in strong acid for 5 min, and the excess Ce⁴⁺ was back-titrated with 31.13 mL of 0.042 89 M ferrous ammonium sulfate.

Calculate the weight percent of NaNO₂ in the solid.

2Ce** + NO, + H,0 2Ce* + NO, + 2H* Ce*+ + Fe2+ - Ce* + Fe+