NaOH与H3PO4的反应,因为比例的不同,有三种产物
3NaOH + H3PO4 = Na3PO4 + 3H2O,NaOH/H3PO4=3
2NaOH + H3PO4 = Na2HPO4 + 2H2O,NaOH/H3PO4=2
NaOH + H3PO4 = NaH2PO4 + H2O,NaOH/H3PO4=1
用数轴分析
-----1---------------2---------------3--------->表示NaOH/H3PO4
---NaH2PO4---------Na2HPO4----------Na3PO4
若,NaOH/H3PO4在1-2之间,则,同时有NaH2PO4、Na2HPO4
若,NaOH/H3PO4在2-3之间,则,同时有Na2HPO4、Na3PO4
若,NaOH/H3PO4小于1,则,NaOH太少,有NaH2PO4、H3PO4
若,NaOH/H3PO4大于3,则,NaOH太多,有Na3PO4、NaOH
现在,NaOH/H3PO4=(0.75*0.2)/(0.5*0.2)=3/2=1.5,介于1-2之间
所以,生成的盐是,NaH2PO4和Na2HPO4
用两个守恒做,设NaH2PO4为x,Na2HPO4为y
Na守恒,x+2y=0.75*0.2=0.15
P守恒,x+y=0.5*0.2=0.1
解得,x=0.05mol,y=0.05mol
最后的溶液体积=200+200=400mL=0.4L
所以,NaH2PO4和Na2HPO4的浓度都=0.05/0.4=0.125mol/L