Formulae, equations and state symbols

The law of conservation of mass states:

• chemical reactions create new atoms

• atoms cannot be created or destroyed

• reactants vanish once a reaction takes place

• products appear when a reaction takes place

Answer explanation

Conservation of mass says that the mass at the end of the reaction is the same as the mass at the start of the reaction. This is because all the atoms are still present, they have only been rearranged, and are not created or destroyed.

This is why equations must be balanced (so you have the same number of atoms at the end of the reaction as at the beginning).

Which of the following symbols is not a correct indication of the phase of a substance in a reaction? 

• (s)=solid
• (g)=grams
• (l)=liquid
• (aq)= aqueous

Which is NOT a diatomic molecule?

• Fluorine
• Nitrogen
• Boron
• Bromine

Answer explanation

Remember you can use HON7 to memorise which elements exist as diatomic molecules... this is hydrogen, oxygen, nitrogen and all the Group 7 elements.

So chlorine for instance is Cl₂ when an element, no just Cl. Remember this is only for when it's by itself as an element, not when chlorine is found as part of a compound, like NaCl. (For compounds you need to use the valencies or ion charges and the crossing over method).

Which of the following would you expect to be monoatomic (exist as single atoms)?

• Calcium

• Hydrogen

• Scandium

• Krypton

Answer explanation

The noble gases (Group 8 or Group 0) exist as single atoms (are known as monatomic) because they already have a full outer shell of electrons so don't need to join up with any other atoms/ make any bonds. This makes them very unreactive. So when you write the formula of a noble gas, it's just the element symbol e.g. Ne or Ar or Kr.

Which compounds are the PRODUCTS of this reaction?

• CO₂ + H₂O

• C₆H₁₂O₆ + O₂

• C₆H₁₂O₆ + CO₂

• H₂O + O₂

Answer explanation

Remember products are what you have at the end of the reaction (what is made).

Which are the reactants in the following reaction?

2Mg + O₂ ---> 2MgO

• 2MgO

• 2Mg +O

• 2Mg + O₂

Answer explanation

Remember reactants are what you add together at the beginning of a reaction.

What is the correct formula for sodium carbonate?

• Na₂C

• NaCO₂

• NaCO₃

• Na₂CO₃

Answer explanation

Sodium is a 1+ ion, and carbonate is CO₃^2-, so you need two sodiums for every carbonate. Use the crossing over method to work out the formula.

What is the correct formula for copper (I) nitride?

• CuN

• Cu₂N

• Cu₃N

• CuNO₃

Answer explanation

Careful - nitride just means nitrogen (valency 3 or a 3- ion) whereas nitrate is an NO₃ (valency 1 or a 1- ion).

Because we are told it is copper (I) we know that copper is 1+.

So we need to cross over the numbers to get the formula.

What is the correct formula for Ammonium sulphate?

• NH₃SO₄

• (NH₃)₂SO₄

• NH₄SO₄

• (NH₄)₂SO₄

Answer explanation

Remember ammonia is NH₃ and ammonium is NH₄. Ammonia can exist by itself but ammonium is found as part of a compound because it's an ion (NH₄⁺).

For instance you could have ammonium chloride, NH₄Cl.

Sulphate is SO₄^2- so you need two of the NH₄⁺ (when you cross over the ion charges).

What is the correct formula for aluminium nitrate?

• AlN

• AlNO₃

• Al(NO₃)₂

• Al(NO₃)₃

Answer explanation

Nitrate is NO₃^- and aluminium is a 3+ ion (it's Group 3) so you need 3 of the nitrates.