(2) It is known that to weigh 12g sodium hydroxide, the weight and roaming code should be added first, and then sodium hydroxide should be added to the balance, so the operation sequence is BA;
(2) (1) Add sodium hydroxide solution into dilute hydrochloric acid drop by drop to react to generate sodium chloride and water. When 25g sodium hydroxide solution is added, the sodium hydroxide is excessive, so the solute in the solution includes the generated sodium chloride and excess sodium hydroxide.
Chemical formula: NaCl, NaOH；;
(2) 20g of sodium hydroxide solution was dropped into 20g of dilute hydrochloric acid, and the reaction was complete.
Let the mass of HCl in this dilute hydrochloric acid be x.
HCl+NaOH = NaCl+H2O
36.5? 40
x？ 20g×8%？
36.540=X20g×8%
X= 1.46g
The mass fraction of dilute hydrochloric acid is:1.46g 20g×100% = 7.3%.
A: The mass fraction of solute in this dilute hydrochloric acid is 7.3%.
(3) When sodium hydroxide absorbs water, the actual mass of sodium hydroxide is less, so the actual solute mass fraction of the prepared sodium hydroxide solution is smaller, which leads to the increase of sodium hydroxide solution needed for the reaction. The more hydrogen chloride calculated according to the equation, the greater the solute mass fraction.
So the answer is: (1)① 12g,138; ② Ba; ② ① Sodium chloride and sodium hydroxide; ②7.3%; ③ Larger.