Question

A student dissolves some phosphoric acid (pK_a = 2.1, 6.8, 12.4) in water such that the total concentration of all forms of phosphate is 1.16 mmol L^-1. Some of the phosphoric acid dissociates into H₂PO₄^–. Which of the following statements is true about this process (select all that apply)? (Hint: consider which of the statements mean the same thing as each other.)

Answer 1

Correct Answers:      

ΔGrxn < 0,     

Q<K,     

The pH must be decreasing    and   

K > 1

Explanation:

ΔGrxn < 0

Since Some of the phosphoric acid dissociates into H₂PO₄^– hence the reaction is spontaneous in forward direction.

Q<K

there are more reactants than products. As a result, some of the reactants will become products, causing the reaction to shift to the right. Since H3PO4 dissociates into products H2PO4-.

The pH must be decreasing

since water ph initially at 7, but after addition of weak acid definitely decrease its ph by increasing H⁺ ion concentration as H₃O⁺.

ΔG° < 0 will not change as it is standard value.

K > 1, this means K > Q. This means you want to go forward in the reaction to achieve equilibrium.

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