A solution contains both NaOH and Na2CO3. 20cm3 of this solution
required 14.40 cm3 of 0.700 mol dm-3 HCl (aq) to decolorise
phenolphthalein. After adding a few drops of methyl orange to the
resulting solution, a further 4.50 cm3 of the acid was required to
turn methyl-orange from yellow to orange. Calculate the
concentration in gdm-3 of NaOH and Na2CO3.
The first endpoint (ie. phenolphthalein color change) indicated
neutralization (protonation) of the stronger base hydroxide
ion OH- (unlike the carbonate(IV) ion, the hydroxide ion isn't
stabilized by either induction nor resonance). The second endpoint
(ie. methyl orange color change) indicated neutralization
(protonation) of the weaker base carbonate(IV) ion CO3 2-.
Accordingly, determine the no. of moles, hence molarities, of
both bases. Bear in mind that HCl is a monoprotic acid, NaOH is a
monoprotic base, while Na2CO3 is a diprotic base.