5.解：设酚酞为指示剂时消耗盐酸V 甲基橙为指示剂时消耗盐酸V ,'.V1+V2=50.0ml ,V2=50.0-V1 mmNa-CO+mNaHCO=0.6850g 2Y1·Ca·M(Na2C0g)/2000+(W2-Vi)×C1·M(NaHC0)/1000 =[V·Cc·M(NazC0)+(50.0-2V)·Cc1·M(NaHC03)]/1000 =(0.200×106.0×V1+50.0×0.200×84.00-2×0.200×8400V1)/100 =(-12.4W1+840/100)=0.6850 -12.4V,=-155 .V1=12.5mL5. 解:设酚酞为指示剂时消耗盐酸V1 甲基橙为指示剂时消耗盐酸V2 ∴V1+V2=50.0mL ∵V2=50.0-V1 而mNa2CO3+mNaHCO3=0.6850g 2V1·CHCl·M(Na2CO3)/2000 +(V2-V1)×CHCl·M（NaHCO3）/1000 =[V1·CHCl·M(Na2CO3)+(50.0-2V1) ·CHCl·M（NaHCO3）]/1000 =(0.200×106.0×V1+50.0×0.200×84.00-2×0.200×8400V1)/100 =(-12.4V1+840/100)=0.6850 -12.4V1=-155 ∴V1=12.5mL