Question

Which mass of which salt below should be dissolved in 0.20 M H₂CO₃ (K_a = 4.2*10^-7) to make 500 mL of an appropriate buffer solution with pH = 5.76? (Assume constant volume during dissolution).

a) 4.0 g NaHCO₃

b) 8.0 g NaHCO₃

c) 2.0 g NaHCO₃

d) 5.1 g Na₂CO₃

e) 2.6 g Na₂CO₃

I know the answer is C. Can someone please explain how to get this answer, and please explain why it would be NaHCO₃ instead of Na₂CO₃?

Thank you!

Answer 1

Here, NaHCO3 is used instead of Na2CO3 because equation used for buffer solution is of Salt and and weak acid and Na2CO3 is a strong base so sodium carbonate can't be an answer.

all the calculation is as follows: