A chemist was to determine Ksp for calcium hydrogen phosphate (CaHPO4). He prepa

Question

A chemist was to determine Ksp for calcium hydrogen phosphate (CaHPO4). He prepared a saturated solution of CaHPO4 . The equilibrium established is: CaHPO4 (s) <----> Ca 2+ (aq) + HPO4 2- (aq)

A 100.0 mL sample of the solution was titrated with 6.74 x 10^-4 M NaOH. It required 46.91 mL of the NaOH solution to reach the equivalence point in the tiration.

i) Write the balanced net ionic equation for the reaction between solutions of NaOH and CaHPO4 (include phase labels). *I had trouble finding a correct equation online*

B) A chemist then prepared a 0.050 M solution of calcium chloride. The calcium chloride solution was then saturated with calcium hydrogen phosphate. He took a 100.0 mL sample of resulting solution and titrated it with the same 6.47 x 10^-4 M NaOH solution.

i) Do you expect a chemist to use more or less of the NaOH solution to reach the equivalence point for this second titration? Explain your reasoning. this was the only problem i got wrong on the exam - appreciate the help

Explanation / Answer

i) Balanced equation for CaHPO4 with NaOH

CaHPO4(s) + 2NaOH(aq) ----> Na2HPO4(aq) + Ca(OH)2(aq)

ii) When the solution of CaCl2 is saturated with CaHPO4, we have more Ca2+ in solution which would react with free HPO4- to reform CaHPO4. Thus, we would need less of NaOH solution to titrate this solution.

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