Chemical Measurements

What does the law of conservation of mass state?

No atoms are lost or made during a chemical reaction so the mass of the products equals the mass of the reactants.

Following the law of conservation of mass, how can chemical reaction be represented?

By symbol equations which are balanced in terms of the numbers of atoms of each element involved on both sides of the equation.

What is the relative formula mass (Mr) of a compound?

The sum of the relative atomic masses of the atoms in the numbers shown in the formula.

Why might some reactions may appear to involve a change in mass?

This can usually be explained because a reactant or product is a gas and its mass has not been taken into account.

After a reaction, why might the mass appear to increase?

One of the reactants is a gas (that’s found in the air) while the products are all solids, liquids, or aqueous. Before the reaction, the gas is floating in the air so you can’t account for its mass. Wehn the gas reacts to form part of the product, it is now contained inside the reaction vessel - the mass ‘increasing‘.

After a reaction, why might the mass appear to decrease?

One of the products is a gas while all the reactants are solid, liquid or aqueous. Before the reaction, all the reactants are contained in the reaction vessel. If the vessel isn’t enclosed, the gas can escape from the reaction vessel as it’s formed. It’s no longer contained in the rection vessel so you can’t account for its mass - it‘decreases’.

What happens whenever a measurement is made?

There is always some uncertainty about the result obtained.

What is a chemical change?

A rearrangement of the atoms in the reactants, to form the products.

Why is it more difficult to measure the mass of a gas than the mass of a liquid or solid?

A gas can easily escape into the air.