A 1.00 liter solution contains 0.42 moles nitrous acid and 0.32 moles sodium nitrite . If 0.16 moles of nitric acid are added to this system, indicate whether the following statements are true or false.
(Assume that the volume does not change upon the addition of nitric acid.)
A. The number of moles of HNO2 will decrease.
B. The number of moles of NO2- will remain the same.
C. The equilibrium concentration of H3O+ will increase.
D. The pH will decrease.
E. The ratio of [HNO2] / [NO2-] will increase

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

D. The pH will decrease.

Explanation:

Nitrous acid ( HNO₂ ) is a weak acid and NaNO₂ is its salt . The mixture makes a buffer solution .

pH = pka + log [ salt] / [ Acid ]

= 3.4 + log .32 / .42

= 3.4 - .118

= 3.282 .

Now .16 moles of nitric acid is added which will react with salt to form acid

HNO₃ + NaNO₂ = HNO₂ + NaNO₃

concentration of nitrous acid will be increased and concentration of sodium nitrite ( salt will decrease )

concentration of nitrous acid = .42 + .16 = .58 M

concentration of salt = .32 - .16 = .16 M

ratio of [HNO₂ ] / NO₂⁻]

= .42 / .32 = 1.3125

ratio of [HNO₂ ] / NO₂⁻] after reaction

= .42 + .16 / .32 - .16

= 58 / 16

= 3.625 .

ratio will increase.

Option E is the answer .

pH after reaction

= 3.4 + log .16 / .58

= 2.84

pH will decrease.

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