Give reason : the atom of the element produce more than one spectral line.

answer:

when atoms are excited they emit light of certain wavelengths which correspond to  different  colors. each  element produces  a unique set of  spectral lines. since no two  elements  emit the same  spectral lines,  elements  can be identified by their line spectrum.

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