CH3COOH + NaOH -----> CH3COONa + H2O
The number of mole CH3COOH is equal to the number of mole NaOH in this reaction, so we can write
M(CH3COOH)*V(CH3COOH) = M(NaOH)*V(NaOH)
M(CH3COOH)= M(NaOH)*V(NaOH)/V(CH3COOH)
M(CH3COOH)= 0.774 M*41.6 mL/50 mL ≈ 0.644 M
Answer is M(CH3COOH) ≈ 0.644 M.