If you were to measure the mass of magnesium and hydrochloric acid before combining them in the test tube, how would that mass compare to the mass of reactants left in the test tube after the reaction? Explain your answer and how it corresponds to the law of conservation of mass.

The mass of product left in the test tube will be less than the initial measured mass of the reactants.

Explanation: This is because one of the products of the reaction is a gas (hydrogen) and it escapes as the reaction happens in an open system.

This is the reaction,

Mg(s) + 2HCl(aq) > MgCl2(aq) + H2(g)

So, evidently, only MgCl2 is left in the reaction test tube together with unreacted reactants.

But, the hydrogen gas that escapes accounts for the lesser mass at the end of the reaction.

The mass in the test tube at the end of the reaction +

the mass of Hydrogen gas that escapes = the mass of reactants before the reaction; consolidating the law of conservation of mass.

given:

mass of na = 115 g

excess cl2

to determine:

mass of nacl produced

explanation:

given reaction is-

2na(s) + cl2(g) → 2nacl(s)

since cl2 is in excess, na will be the limiting reagent

as per the reaction stoichiometry na: nacl = 1: 1

i.e. moles of na reacted = moles of nacl formed

now, # moles of na = mass of na/atomic mass

                                  = 115 g/23 g.mol-1 = 5 moles

therefore, moles of nacl = 5

molar mass of nacl = 58 g/mol

mass of nacl = 5 moles * 58 g.mol-1 = 290 g

ans: amount of nacl produced = 290 g