(1) When the mass of potassium nitrate in the experimental site is 2g and the mass of water is 5g, the solubility of potassium nitrate at 25.5℃ is 80g；; In the third experiment, the temperature was (32.5℃+33.0℃+33.0℃)3 = 32.8℃, the mass of potassium nitrate was 2g, and the mass of water was 7g. At this temperature, the solubility of potassium nitrate is 57.65438 0 g; . In the fifth experiment, the error of 30. 1℃ is large, so the temperature is (25.8℃+26.1℃) ⊙ 2 = 26.0℃;

(2) According to the temperature as solubility, the corresponding solubility curve can be drawn;

(3)① According to the solubility table of potassium nitrate,

Temperature/℃

② The reason for the error may be that there is an error in controlling the temperature, resulting in an error in the image;

(2)① The temperature at which potassium nitrate and sodium chloride have the same solubility is not only the possibility to judge whether the solubility values in this section are equal, but also the possibility that the two substances have the same solubility at 20 ~ 30℃.

② At 20℃, 200g of Na-Ni-L solution with 20% solute mass fraction was heated to evaporate 60g of water, and then cooled to 20℃. It is known that the mass of residual water is 200g-200g×20%-60g= 100g, and the solubility of sodium chloride is 36.0g at 20℃, so it is 100g at 20℃.

So the answer is: (1) experimental group No. The initial temperature of KNO3/g H2O/g crystal/℃ 222.25210 25.830.126.126.020.0 (2); (3)①; ② There is an error in temperature control; (2)①20~30℃; ②2g。