What mass of solid NaCH_3 CO_2 (molar mass = 82.0 g-mol^-1) should be added to 1

Question

What mass of solid NaCH_3 CO_2 (molar mass = 82.0 g-mol^-1) should be added to 1.0 L of 0.50 M CH_3 CO_2 H to make a buffer with a pH of 7.21? (pK_a of CH_3 CO_2 H = 7.21) (Must show work) 0.0 g 1.9 g 41 g 71 g 1.6 times 10^2 g If the ratio of acid to base in a buffer increases by a factor of 10, the pH of the buffer increases by 1. decreases by 1. increases by 10. decreases by 10. remains unchanged. Which of the following conditions is/are met at the equivalence point of the titration of a monoprotic weak base with a strong acid? The moles of acid added from the buret equals the initial moles of weak base. The volume of acid added from the buret must equal the volume of base titrated. The pH of the solution is less than 7.00. 1 only 2 only 3 only 1 and 3 only 2 and 3 only

Explanation / Answer

18)Henderson equation is

pH = pKa + log ( [conjugate base]/[Acid])

7.217 = 7.21 + log ( [ Acetate]/[Acetic acid])

[Acetate]/[Acid] = 1.02

If [Acetate] = 1.02 × [Acid]

= 1.02 × 0.50

= 0.51M

Molar mass of Sodium acetate = 82g/mole

0.51M means 0.51 mole per litre

Mass of sodium acetate = Mole × Molar mass = 0.51mole × 82g/mole = 41.82g

The required mass of CH3COONa = 41.82g

So, the answer is c) 41g

19) Ka = [H+] [ Conjugate Base]/[Acid]

[Conjugate Base]/[Acid] =Ka/[H+]

[H+] = Ka/ ( [Conjcate Base]/[Acid])

when [Conjcate base]/[Acid] increase by 10 fold [H+] will decreases by 10 fold. pH is -log[H+] , so pH will increase by 1

Therefore the answer is a) increase by 1

20) The answer is d) 1 and 3 only

At equvalance point stoichiometrically needed amount in a balanced equation is added, for example

NH4OH + HCl ------> NH4Cl + H2O

According to the stoichiometry 1mole of NH4OH require 1 mole of HCl

The NH4Cl formed partly react with water to H+ ion , So the pH will be less than 7

NH4+ + H2O ------> NH3 + H3O+

  

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