Question

A 1.00 liter solution contains 0.24 M hypochlorous acid and 0.31 M sodium hypochlorite. If 0.160 moles of potassium hydroxide 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 potassium hydroxide.) A. The number of moles of HClO will decrease. B. The number of moles of ClO- will increase. C. The equilibrium concentration of H3O+ will remain the same. D. The pH will decrease. E. The ratio of [HClO] / [ClO-] will decrease.

Answer 1

No. of moles of hypochlorous acid (HClO) - Molartity x vol in lts = 0.24 M (mol/lt) X 1 lt = 0.24 moles   

No. of moles of sodium hypochlorite (NaClO) = 0.31 M (mol/lt) X 1 lt = 0.31 moles   

No. of moles of KOH = 0.16 moles

As the volume doesn't change upon the addition of KOH then here Sodium and potassium are spectator ions.

HClO ---> H⁺​ + ClO^-​    KOH ----> K⁺ + OH^-​

Neutralisation reaction : OH^-​ ​+ HClO​ ---> H₂​O + ClO^-​   

( moles) initial    0.16    0.24 0.31   

   change -0.16 -0.16    +0.16

   equilibrium    0 0.08 0.47   

This shows that number of moles of HClO decreases is true statement.

This also shows the number of moles of ClO^-​ will increase is also true statement.

The equilibrium concentration of H₃O⁺​ (only source is HClO) but we can see that HClO concentration is decreased so the equilibrium concentration of H₃O⁺​ also decreases So, the above statement " The equilibrium concentration of H3O+ will remain the same​" is not correct.

D) Initial pH is = pKa + log (base/acid) = 7.46 + log (0.31/0.24) = 7.46 + log 1.3 = 7.46 + 0.11 = 7.57

After addition of KOH, pH = pKa + log (base/acid) = 7.46 + log (0.47/0.08) = 7.46+0.77 = 8.23 This shows that pH increases but not decreasing so above given statement " The pH will decrease" is not correct.

E) Initial ratio of [HClO] / [ClO^-] = 0.24/0.31 = 0.77   

After addition of KOH, final ratio of [HClO] / [ClO^-] =​ 0.08/0.47 = 0.17

This shows that the above given statement " The ratio of [HClO] / [ClO-] will decrease.​" is true.