The standardized sodium hydroxide solution from (c) was used to titrate a propanoic acid solution of unknown concentration.
Calculate the concentration of the propanoic acid if \( 15.00~cm^{3} \) of its solution was neutralized with \( 20.00~cm^{3} \) of standardized sodium hydroxide.
If you did not determine the concentration of sodium hydroxide in part (c), use \( 0.2155~mol~dm^{-3} \) although this is not the correct answer.