Which of these compounds would be the best buffer at ph 5.0: formic acid (pka = 3.75), acetic acid (pka = 4.76), or ethylamine (pka = 9.0)? briefly justify your answer?

The equation which determine the pH of a buffer solution is Heisenberg equation which is . Now if the ratio of is less than 1 then the logarithmic term will be negative. Thus the pH will be less than the value of the acid. And if the ratio is more than 1 then the logarithmic part will be positive and the pH of the buffer will be more than the .

Now among the available acids the acetic acid  has the near the desired pH of the buffer solution. Thus to make the buffer the salt concentration needed to be little high. So acetic acid is suitable acid to make the buffer.