Chemical formulae | Exit Quiz | Oak National Academy

What does the chemical formula H₂O represent?

• Two hydrogen atoms and one oxygen atom.

• One hydrogen atom and two oxygen atoms.

• Two hydrogen molecules and one oxygen molecule.

• Two oxygen atoms and one hydrogen atom.

Answer explanation

H₂O represents two hydrogen atoms (H₂) and one oxygen atom (O), forming water.

In the formula CO₂, what does the subscript '2' indicate?

• There are two carbon atoms.

• There are two oxygen atoms.

• The compound is twice as heavy as oxygen.

• The compound is twice as reactive.

Answer explanation

In CO₂, the subscript '2' indicates that there are two oxygen atoms present in the compound.

It is important to write subscript values correctly, both in terms of the value, but also in their physical size and placement to accurately represent the correct number of each type of ...

Answer explanation

Subscript values in a chemical formula represent the correct number of each type of atom in the compound, which is crucial for accurately conveying its composition.

What is the difference between H₂O and H₂O₂?

• H₂O₂ has more hydrogen atoms.

• H₂O has more oxygen atoms.

• H₂O₂ has more oxygen atoms.

• H₂O has more hydrogen atoms.

• There's no difference. They have the same number of oxygen and hydrogen atoms.

Answer explanation

H₂O (water) has two hydrogen and one oxygen atoms, while H₂O₂ (hydrogen peroxide) has the same hydrogen atoms but an extra oxygen atom.

Which statement best defines a chemical formula?

• A description of a chemical reaction.

• A graphical representation of a molecule.

• A symbolic way to represent the elements in a chemical substance.

• A method to determine the pH of a substance.

Answer explanation

A chemical formula is a symbolic way to represent the elements in a chemical substance and their ratios.

Match the following chemical substances up to the key terms.

• Element

• O₂

• Compound

• Ne

• Mixture

• CO

• Molecule

• N₂ and CO₂

Answer explanation

Ne is only made up of one type of atom (element). CO is both a molecule and a compound, but O₂ is only a molecule so has to be labelled this way. Together N₂ and CO₂ make a mixture.