Question

Which of the following solution should be mixed with 50.0 mL of 0.050 M HF solution to make an effective buffer?

A) 50.0 mL of 0.10 M NaOH
B) 25.0 mL of 0.10 M NaOH
C) 50.0 mL of 0.050 M NaOH
D) 25.0 mL of 0.050 M NaOH

Answer 1

An effective buffer for WHAT?
Buffers are weak acids and salt of the weak acid (or weak base and salt of the weak base). You have 50 x 0.05 = 2.5 millimoles HF which is the weak acid. If you mix NaOH with that you will produce NaF which is the salt. However, you can't add TOO much NaOH else you have no acid left. Look at each of the following and pick one that leaves some of the acid while it is making some of the salt. I'll give you a hint: the answer is not C.
A is 50 x 0.1M = 5 mmols NaOH
B is 25 x 0.1M = 2.5 mmols NaOH
C is 50 x 0.05M = 2.5 mmols NaOH
D is 25 x 0.05M = 1.25 mmols NaOH

.........HF + NaOH ==> NaF + H2O
A.......2.5...5.........x.....x
B.......2.5...2.5.......x.....x
C.......2.5...2.5.......x.....x
D.......2.5...1.25......x.....x

Answer 2

SIR PLEASE EXPLAIN THE ANSWER