Hello!
To solve this problem we are going to use the Henderson-Hasselbach equation and clear for the molar ratio. Keep in mind that we need the value for Acetic Acid's pKa, which can be found in tables and is 4,76:
[tex]pH=pKa + log ( \frac{[CH_3COONa]}{[CH_3COOH]} ) [/tex]
[tex] \frac{[CH_3COOH]}{[CH_3COONa}= 10^{(pH-pKa)^{-1}}=10^{(4-4,76)^{-1}}=5,75 [/tex]
So, the mole ratio of CH₃COOH to CH₃COONa is 5,75
Have a nice day!
