An unknown sample was prepared by placing a single analgesic tablet in a small beaker and adding 10 mL of methanol. After allowing the sample to dissolve, the contents of the beaker are quantitatively transferred to a 25 mL volumetric flask and diluted to volume with methanol. The sample is then filtered, and a 1.00 mL portion transferred to a 10 mL volumetric flask and diluted to volume with methanol. HPLC analysis gave a signal for caffeine of 21.47. Report the mg of caffeine in the tablet.

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