Now we need to prepare a buffer that has a pH of 7.4. We need this specific pH to maintain our blood cells in culture. How should you prepare this buffer?
a) By mixing strong acid and then adjusting the pH with strong base
b) By mixing weak base with its conjugate acid and then adjusting the pH with strong
acid
c) By mixing weak acid with its conjugate base and then adjusting the pH with strong
base
d) By mixing strong base and then adjusting the pH with strong acid

b) By mixing weak base with its conjugate acid and then adjusting the pH with strong  acid.

Explanation:

Hello,

In this case, considering that the buffer required a pH greater than 7 where the basic conditions are present, we need a weak base and its conjugate base in order to assure such pH greater than 7. Moreover, by using a strong acid, we can gently add it in order to decrease the pH up to 7.4 via a properly measured neutralization of the necessary moles of base up to a hydrogen concentration of 3.98x10⁻⁸ (pH). Therefore, answer is b) By mixing weak base with its conjugate acid and then adjusting the pH with strong  acid.

Best regards.

the answer to your question is b