Calculating the pH of the endpoint in a titration of weak acid and strong base
Would it be correct to assume that the concentration of $\ce{CH3COO-}$ to be the concentration of ethanoic acid that was present at the start of the titration ($\mathrm{0.100~mol~dm^{-3}}$), given that at the equivalence point, all $\ce{CH3COOH}$ had reacted to form a solution containing only the conjugate base $\ce{CH3COO-}$? 


You can easily rearrange the $\text{K}_\text{a}$ equation to solve for the information that you need to answer that question. 

$$\text{K}_\text{a} =\frac{\ce{[H^+][A^-]}}{\ce{[HA]}} $$ 
or
$$\frac{\ce{[HA]}}{\ce{[A^-]}} = \frac{\ce{[H^+]}}{\text{K}_\text{a}} = 1.06\times10^{-4}$$

Since $1.06\times10^{-4}$ is insignificant compared to 0.100 you can assume that essentially all the HA is converted to $\ce{A^-}$. 

NOTE: This does not mean that $\ce{[HA] = 0}$