A strip of magnesium is placed into a test tube with hydrochloric acid to form magnesium chloride and hydrogen. The mass of the reactants is measure to be 12g. After the reaction is complete, the mass of the system is 11.8g. Knowing the Law of the Conservation of Mass, what happened to the rest of the mass?

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