3. (a) Which of the following is the formula of a cation? Put a cross (✓) in the box next to your answer - Edexcel - GCSE Chemistry - Question 3 - 2015 - Paper 1

The identification of the potassium ion, which has the formula K+, and the carbonate ion, CO3²-, serves as a basis for understanding ionic compounds involving these ions.

To obtain a pure, dry sample of barium sulfate:

1. Filter the Mixture: Use filter paper and a funnel to separate the solid barium sulfate from the liquid. Pour the mixture through the filter paper.
2. Wash the Solid: Rinse the solid barium sulfate on the filter paper with distilled water to remove any impurities.
3. Dry the Solid: Allow the residue to dry completely, ensuring you have a pure sample of barium sulfate.

In magnesium oxide (MgO), magnesium atoms lose two electrons to achieve a stable electronic configuration, forming Mg²+ ions. Oxygen atoms gain two electrons, resulting in O²- ions. Thus, Mg²+ and O²- ions are formed, which then attract each other to form the ionic compound magnesium oxide.

To calculate the relative formula mass of MgO:

• The atomic mass of magnesium (Mg) is 24.
• The atomic mass of oxygen (O) is 16.

Thus, the relative formula mass is calculated as follows: $ext{Relative formula mass of MgO} = 24 + 16 = 40$.