Question

A 1.00 liter solution contains 0.51 M nitrous acid and 0.38 M sodium nitrite. If 25.0 mL of water are added to this system, indicate whether the following statements are TRUE (T) or FALSE (F). (Note the the volume MUST CHANGE upon the addition of water.)

A. The concentration of HNO2 will increase.

B. The concentration of NO2- will remain the same.

C. The equilibrium concentration of H3O+ will remain the same.

D. The pH will remain the same.

E. The ratio of [HNO2] / [NO2-] will increase.

Answer 1

Given 1.00 litre solution of 0.51 M nitrous acid and 0.38 M sodium nitrite is

HNO₂ + NaNO₂ NaNO₂ + HNO₂

here 25.0 mL of water (H₂O) is added in the solution.

nitrous acid is weak acid,

and sodium nitrate is conjugate base of weak acid (HNO₂), and remain in the solution as

NaNO₂ Na⁺ + NO₂^-

Answer A) TRUE

adding water(H₂O) will get consumed as,

NaNO₂ + H₂O HNO₂ + NaOH

here water is acting as weak acid. from the above reaction it is clear that the concentration of nitrous acid is increasing.

Answer B) FALSE

NO₂^- concentration will reduce. as the NO₂^- ions will attack H₂O to give HNO₂ as explained above.

Answer C) FALSE   

the nitrous acid (HNO₂) already present in the solution will react with water as,

HNO₂ + H₂O NO₂^- + H₃O⁺

therefore increasing the concentration of H₃O⁺ concentration of the solution.

Answer D) FALSE

As the H₃O⁺ concentration is increasing hence the pH value will not remain the same.

Answer E) TRUE

As it is clear from part B i.e the concentration of nitrous acid increases and NO₂^- decreases in the solution as,

As Na⁺ +NO₂^- + H₂O HNO₂ + Na⁺OH^-

So the ratio of [HNO₂]/[NO₂^-] will increase.