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 potassium ion, K⁺, signifies that potassium has lost one electron, resulting in a positive charge. The carbonate ion, CO₃²⁻, consists of one carbon atom and three oxygen atoms, with an overall charge of -2 due to the additional electrons.

1. Filter the mixture: Use filter paper in a funnel to separate the solid barium sulfate from the liquid.
2. Wash the solid: Rinse the residue (barium sulfate) on the filter paper with distilled water to remove any impurities.
3. Dry the solid: Allow the barium sulfate to air dry or gently heat it to obtain a pure, dry sample.

A magnesium atom has two electrons in its outer shell (2.8.2) and can lose these two electrons to achieve a stable electron configuration, thereby forming a Mg²⁺ ion. An oxygen atom has six electrons in its outer shell (2.6) and can gain two electrons to complete its outer shell, becoming an O²⁻ ion. Thus, in magnesium oxide, one Mg²⁺ ion pairs with one O²⁻ ion, resulting in an ionic compound.

To calculate the relative formula mass of MgO:

• Relative atomic mass of Mg = 24
• Relative atomic mass of O = 16

The formula mass of MgO is: $24 + 16 = 40$

Therefore, the relative formula mass of magnesium oxide, MgO, is 40.