Mr (Na2CO3 * 10H2O) = 2 * Ar(Na) + Ar(C) + 3 * Ar(O) + 10 * (2 * Ar(H) + Ar(O)) = 2 * 23 + 12 + 3 * 16 + 10 * (2 * 1 + 16) = 46 + 12 + 48 + 10 * 18 = 46 + 12 + 48 + 180 = 286
Ответ: 286