The hypothesis states that the fraction of
radioactive nuclei after n half-lives is 0.5”.

in this expression, the value 0.5 represents

a) the fraction of nuclei still radioactive after any
half-life cycle.
b)the fraction of nuclei still radioactive at the
start of the experiment.
c)the amount of time it takes half of the
radioactive atoms to decay.
d)the number of half-life cycles.

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