Question

Which of the following mixtures will be a buffer when dissolved in a liter of water?

	A.	0.3 mol NaCl and 0.3 mol HCl
	B.	0.1 mol Ca(OH)2 and 0.3 mol HI
	C.	0.2 mol HBr and 0.1 mol NaOH
	D.	0.4 mol NH3 and 0.4 mol HCl
	E.	0.2 mol H3PO4 and 0.1 mol NaOH

Answer 1

A) This is a combination of the spectator ion and strong acid. This cannot form buffer

B) and C) are examples of strong and strong base reaction. This cannot form buffer

C)

                                NH₃       +     H⁺                NH₄⁺

Initial                      0.4                0.4                0

Change                   -0.4             -0.4               +0.4

Final                          0                  0                0.4

Here, the base is completely consumed and cannot act as a buffer (Ratio of weak base to conjugate acid is not possible)

D)               

                      H₃PO₄     +      NaOH          NaHPO₄   + H₂O

Initial               0.2                    0.1                  0              

Change           -0.1                   -0.1               +0.1

Final                0.1                       0                  0.1

This combination has weak acid and conjugate base left over. Therefore, this is a buffer

Answer is D) 0.2 mol H₃PO₄ and 0.1 mol NaOH