What kind of joint is pictured in the diagram shown?
A)A semimoveable joint, allowing back and forth movement
B)A moveable joint, allowing back and forth movement
C)A semimoveable joint, allowing twisting
D)Amoveable joint, allowing twisting

a:

by the combination of an unlimited number of elements

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