Calculations Involved Masses
Conservation of mass:
The law of conservation states that no atoms are lost or made during a chemical reaction so,
the mass of the product equals the mass of the reactants.
Example,
When hydrogen molecules react with chlorine molecules, they make hydrogen chlorine
molecules:
H2+Cl2 HCl
This equation shows the reactants and products, but it isn’t balanced.
H2+Cl2 2HCl
This balanced equation shows that 1 hydrogen molecule reacts with 1 chlorine molecule to form 2
molecules of hydrochloric acid.
Closed Systems:
A Precipitation reaction is a simple example of conservation of mass
A Precipitate (insoluble solid) is formed during the reaction
The reactants and products remain inside the closed reaction making it easy to see that the
overall mass hasn’t changed.
Example;
Mass products = Mass reactants
AgNO3(ag)+NaCl(ag) AgCl(s)+NaNO3(ag)
Non-closed Systems:
Example;
When magnesium is heated in a crucible it reacts with oxygen and forms magnesium oxide:
2Mg+O2 2MgO
This equation shows that 2 magnesium atoms react with 1 oxygen molecule to form 2 magnesium
oxide compounds
Here are the results from the reaction:
Mass in g
Mass of crucible at the start of the reaction 0.23
Mass of crucible at end of reaction 0.41
Some reactions may appear to involve a change in mass but this can be explained because a
reactant or product is usually a gas and its mass has not been taken into account.
In this example, the mass of the magnesium oxide produced is greater than the mass of the
original metal.
Conservation of mass:
The law of conservation states that no atoms are lost or made during a chemical reaction so,
the mass of the product equals the mass of the reactants.
Example,
When hydrogen molecules react with chlorine molecules, they make hydrogen chlorine
molecules:
H2+Cl2 HCl
This equation shows the reactants and products, but it isn’t balanced.
H2+Cl2 2HCl
This balanced equation shows that 1 hydrogen molecule reacts with 1 chlorine molecule to form 2
molecules of hydrochloric acid.
Closed Systems:
A Precipitation reaction is a simple example of conservation of mass
A Precipitate (insoluble solid) is formed during the reaction
The reactants and products remain inside the closed reaction making it easy to see that the
overall mass hasn’t changed.
Example;
Mass products = Mass reactants
AgNO3(ag)+NaCl(ag) AgCl(s)+NaNO3(ag)
Non-closed Systems:
Example;
When magnesium is heated in a crucible it reacts with oxygen and forms magnesium oxide:
2Mg+O2 2MgO
This equation shows that 2 magnesium atoms react with 1 oxygen molecule to form 2 magnesium
oxide compounds
Here are the results from the reaction:
Mass in g
Mass of crucible at the start of the reaction 0.23
Mass of crucible at end of reaction 0.41
Some reactions may appear to involve a change in mass but this can be explained because a
reactant or product is usually a gas and its mass has not been taken into account.
In this example, the mass of the magnesium oxide produced is greater than the mass of the
original metal.
