Answer to Question #218963 in General Chemistry for ari

Question

(a) In the standardization of hydrochloric acid using sodium tetraborate, a student performs the following steps: (i)A certain quantity of sodium tetraborate was weighed accurately and placed in a 250 mL volumetric flask. (ii) 25 mL of this stock solution was then titrated with the hydrochloric acid using bromocresol green as an indicator. 

If the average titre value obtained for this titration was 10.5 mL and the concentration of hydrochloric acid was determined to be 0.4 M, calculate the initial mass of sodium tetraborate that was weighed out. Comment on why phenolphthalein is not an appropriate indicator for this titration.

(b) A sample contains 5.88 % by mass of the compound: C6H3Cl(CO2H)2.

(i)What mass of sample will be required to react completely with 3.924 g of sodium hydroxide?  

(ii)Give the repeating unit when the compound, C6H3Cl(CO2H)2, reacts with C4H8(CH2NH2)2.  

Answer 1

Na₂B₄O₇ (aq) + 2 HCl (aq) + 5 H₂O (l) → 4 H₃BO₃ (aq) + 2 NaCl (aq)

M acid = 0.4M

Because the equivalence point of a weak base-strong acid system is likely going to be outside of the color indicator range of phenolphthalein. Phenolphthalein has a color change point ranging from pH 8.2–10, which is suitable for acid-base combinations where the equivalence point will be in that range, but the system you describe is likely to have an equivalence point occur at a much lower pH, since the conjugate acid of a weak base is a relatively strong acid.