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.
Originally posted by Gulabjamoon.bai: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.