Answer
31%
Work Step by Step
6.61*10$^5$ mg = 661 g of the compound
Molar mass of the compound = $\frac{661 g}{2.21 mol}$ = 2.99 10$^{2}$ g/mol
molar mass of lead (Pb) = 207.2g/mol
molar mass of compound - molar mass of lead = molr mass of nitrites = 299- 207.2 = 91.895g
$\frac{1.895 g/mol}{ 391g/mol} * 100$%$ = $$30.7$%$ \approx 31%$
