Đáp án đúng: B
Giải chi tiết:nX = 12,4 : 124 = 0,1 mol
Cấu tạo thỏa mãn của X là:
\(\begin{array}{l}\,\,\,\,\,\,\,\,\,\,\,(N{H_4})C{O_3}(N{H_3}C{H_3}) + 2NaOH \to N{a_2}C{O_3} + N{H_3} + C{H_3}N{H_2} + 2{H_2}O\\Bd:\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,0,1\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,0,3\\Pu:\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,0,1\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, \to \,\,\,\,\,\,0,2\,\,\,\,\,\,\,\, \to \,\,\,\,\,0,1\,\,\,\, \to \,\,\,\,\,0,1\, \to \,\,\,\,\,\,\,0,1\\Sau:\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,0\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,0,1\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,0,1\end{array}\)
Chất rắn gồm: 0,1 mol NaOH dư và 0,1 mol Na2CO3
=> m = 0,1.40 + 0,1.106 = 14,6 gam
Đáp án B