Question

A solution containing `Na_(2)CO_(3)` and `NaOH` requires `300 ml` of `0.1 N HCl` using phenolphthalein as an indicator. Methyl orange is then added to the above titrated solution when a further `25 ml` of `0.2 N HCl` is required. The amount of `NaOH` present in solution is `(NaOH=40, Na_(2)CO_(3)=106)`
A. (a)0.6 g
B. (b)1.0 g
C. (c )1.5 g
D. (d)2.0 g

Answer 1

Correct Answer - b
(I) Phenolphthalein indicates partial neutralisation of
`Na_(2)CO_(3) rarr NaHCO_(3)`
Meg. of `Na_(2)CO_(3) + "Meq. of " NaOH` = Meq. of HCl
`W/Exx1000+W/Exx1000=NV`
`("Suppose" Na_(2)CO_(3)="a gm", NaOH="b g")`
`a/106xx1000+b/40xx1000=300xx0.1 " "…(1)`
(II) Methyl orange indicates complete neutralisation
HCl HCl
`N_(1)V_(1)=N_(2)V_(2), 25xx0.2=0.1xxV_(2)` so `V_(2)=50` ml excess
`:. a/53xx1000+b/40x1000=350xx0.1" " ...(2)`
From (1) and (2), `b=1 g`