Which of the following is true for balancing equations?

the number of products should be equal to the number of reactants.
the properties of products should be the same as the properties of reactants.
there must be as equal number of compounds on both sides of the equation.
there must be as equal number of atoms of each element on both sides of the equation.

the last option, there must be as equal number of atoms of each element on both sides of the equation.

Explanation:

During a chemical reaction, the elements (kind of atoms) are not transformed into different elements. Each element has to keep its identity. Sodium atoms will remain being sodium atoms, and the same is true for the 118 elements listed in the periodic table (all the elements known).

The chemical change (chemical reaction) consists in the rupture of the chemical bonds in the reactant compounds and the formation of new bonds to constitute one or more new compound.

Atoms are not created nor destroyed, in a chemical reaction, they just bond in different ways which is how different compounds are formed.

Since, atoms are not created nor destroyed there must be the same number of atoms of each element at the begining (reactant side) than at the end (product side).

That is the basis for balancing equations: you must make sure that equal number of atoms of each element on both sides of the equation.

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

c. orbital shape i hope this is correct