#7, thinking it is d but not sure why. could someone explain it? Thank you 7.) W

Question

#7, thinking it is d but not sure why. could someone explain it? Thank you

7.) Which is the best colored indicator to use in the titration of0.1 M CH ofCH3COJH 1 .8 × 10-5, Kb of CH3CO2 = 5.6 x 10-10) ,CO2H(aq) with NaOH(aq)? Why? (Ka ,CO,H(aq) with NaOH(ad)? Why? K. Indicator Bromcresol Green Bromthymol Blue Phenolphthalein Ka 4.8 6.8 9.2 a. Bromcresol Green. The equivalence point for a weak acid titration occurs at low pH. b. Bromthymol Blue. The pH at the equivalence point is 7.0. c. Bromcresol Green. The pKa of CH3CO2H and the pKa of the indicator are similar. d. Phenolphthalein. The pKa of CH;CO2 and the pKb of the indicator are similar. e. Phenolphthalein. The pH at the equivalence point is near the pKa of the indicator. .) The solubility of BaCrOA(s) in water is 3.7 milligrams in 1.0 L at 25 °C. Calculate the value of Ksp for BaCrO4.

Explanation / Answer

Q;7: During the titration of CH3COOH with NaOH, the pH at equivalence point is around 9. Also the pKa of phenolphthalein is around 9 (9.2). Hence phenolphthalein is the best indicator becaue the pH at the equivalence point is near to the pKa of the indicator. Hence option (e) is correct.

Q.8: Given the solubility of BaCrO4 = 3.7 mg/L = 3.5 mg x ( 1 g / 1000 mg) / L = 3.5x10-3 g/L

Molar mass of BaCrO4 = 253.32 g/mol

Hence molar solubility of BaCrO4, S = 3.5x10-3 g /253.32 g/mol /L = 1.38x10-5 mol/L or 1.38x10-5 M

The dissociation reaction of BaCrO4(s)

BaCrO4(s) -----> Ba2+(aq) + CrO42-(aq)

------------------------ S, ----------- S

Hence Ksp = [Ba2+(aq)]x[CrO42-(aq)] = SxS = S2 = (1.38x10-5 M)2 = 1.91x10-10 (answer)

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