Consider a 1.50 mol kg-1 solution of phosphoric acid (H_3PO_4) in water, a) What

Question

Consider a 1.50 mol kg-1 solution of phosphoric acid (H_3PO_4) in water, a) What is the theoretical molality of ions in this solution? b) If the solution boils at 100.95 degree C, calculate the actual concentration of ions in the solution. Pure water has a boiling point elevation constant of 0.51 degree C kg mol-1 and boils at 100.00 degree C. c) Based on your answers to parts a) and b), which one of the following best describes how phosphoric acid mainly exists in this solution? Clearly indicate your choice with a tick. Magnesium oxalate (MgC_2CO_4) is an ionic compound consisting of magnesium and oxalate ions. It has relatively low solubility in water. The K_SP of magnesium oxalate is 8.6 times 10^-5 at 25 degree C. a) Write an equation for the reaction of magnesium oxalate dissolving into water and forming its constituent ions. b) Write the equilibrium expression (K_sp) for magnesium oxalate, based on the above equation. 100. mL each of 0.02 mol L^-1 sodium oxalate (Na_2C_2O_4) and 0.01 mol L^-1 magnesium nitrate are mixed. c) What is the concentration, in mol L^-1, of oxalate ions in the mixed solution? d) What is the concentration, in mol L^-1, of magnesium ions in the mixed solution? e) What is the reaction quotient for magnesium oxalate in this system? Will a precipitate form? Explain your answer.

Explanation / Answer

Ans 65.

a)The phosphoric acid dissociates as

H3PO4 = 3H+ + PO43-

So there are overall 4 ions per molecule dissociated .

Since the molality = no. of moles / mass of solvent in kg

The theoretical molaity of H3PO4 = (1.50 x 4) / 1

= 6mol/kg

b) The elevation in boiling point is given as

Delta Tb = kb m

100.95 - 100 = 0.51 m

0.95 = 0.51 m

m = 1.86 m

So the actual concentration of ions in the solution is 1.86 mol/ kg

c) Since the actual concentration of ions in the solution as calculated in part b is closest to real number 2 , which indicates that the phosphoric acid exists in the solution as H2PO4- and H+

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