A solution of citric acid (H₃C₆H₅O₇) with a known concentration of 0.200 M H₃C₆H₅O₇ is titrated with a 0.750 M NaOH solution. How many mL of NaOH are required to reach the third equivalence point with a starting volume of 25.0 mL H₃C₆H₅O₇ , according to the following balanced chemical equation: H₃C₆H₅O₇ + 3 NaOH → Na₃C₆H₅O₇ + 3 H₂O

answer : third trial had the contaminated sample and the correct value for dhrxn should be 1000 kj/mol

explanation :

let us write down the given equations

equation 1 :

equation 2 :

equation 3 :

let us use hess's law to find out which 2 equations can be added to get the third equation.

let us reverse equation 2 and multiply it by 2. the new equation that we get is,

equation 4 :

let us add equation 1 & 4

the above equation is similar to equation 3 but has different value for dhrxn.

therefore third trial had the contaminated sample and the correct value for dhrxn should be 1000 kj/mol