One problem with the Economic Sector of Land Use and Forestry is: 1.People like to climb trees.
2.Trees are used to build homes.
3.Cutting trees down stops them from removing carbon dioxide from the atmosphere.
4.More land can be used for farming.

plz help.

i do not get it   at all

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