Question

A 1.00 liter solution contains 0.49 moles hydrocyanic acid and 0.38 moles sodium cyanide . If 0.19 moles of hydrochloric 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 hydrochloric acid.) A. The number of moles of HCN will increase. B. The number of moles of CN- will increase. C. The equilibrium concentration of H3O will increase. D. The pH will increase. E. The ratio of [HCN] / [CN-] will remain the same.

Answer 1

Answer:

A)true

B)false

C)true

D)false

E)false

Explanation:

Step 1: Data given

Volume = 1.00 L

Number of moles HCN = 0.49 moles

Number of moles NaCN = 0.38 moles

Number of moles HCl added = 0.19 moles

Step 2:

This is a buffer system consisting of a weak acid, HCN and the salt of that acid, NaCN.

Upon addition of hydrochloric acid (HCl), HCN is formed and NaCl is used up.

HCl + NaCN ⇔ NaCl + HCN

A. The number of moles of HCN will increase. This is true.

B. The number of moles of CN- will increase.

⇒ false, The number of moles of CN- will decrease b/c it is now part of HCN, a weak acid.

C. The equilibrium concentration of H3O will increase.  

 ⇒ True, The [H3O+] will increase slightly.

D. The pH will increase.

⇒ False, The pH will decrease slightly because acid is being added and being converted to HCN

E. The ratio of [HCN] / [CN-] will remain the same.

⇒ False. It will increase.