Question 3

Magnesium nitrate has completely decomposed into magnesium oxide, nitrogen dioxide gas and oxygen gas when heated at room condition. If 12 g of magnesium nitrate was used in the reaction, calculate

a) The mass of magnesium oxide formed

b) The volume of nitrogen dioxide gas released

[RAM: Mg, 24; N, 14; O, 16]

[Molar volume of gas: 24 dm³mol^-1]


Tips:
In solving numerical problems regarding chemical reactions, always:
1. Highlight all the information given in the question.
2. Write a balanced chemical equation of the reaction.
3. Identify the substance that you can find its number of moles.
4. Refer chemical equation for the ratio of moles of substances involved in working your way out to find either their mass, volume or molarity / concentration.

Answer:

a) The mass of magnesium oxide formed

Step 1: Write balanced chemical equation of the reaction

              2Mg(NO₃)₂      →   2MgO   ✛     4NO₂   ✛  O₂
                 12 g               

Step 2: The number of moles of magnesium nitrate

= Mass / RMM of magnesium nitrate
= 12 g / [24 + 14x2 + 16x6] gmol^-1
= 12 g /  148 gmol^-1
= 0.081 mol

Step 3: Find the number of moles of magnesium oxide

From chemical equation
2 mol of Mg(NO₃)₂  produces 2 mol of MgO
Therefore, 0.081 mol of Mg(NO₃)₂  produces 0.081 mol of  MgO

Step 4: Find the mass of magnesium oxide formed

= Number of moles x RMM of magnesium oxide
= 0.081 mol x  [24 + 16] gmol^-1
= 0.081 mol x  40 gmol^-1
= 3.24 g of magnesium oxide

 
b) The volume of nitrogen dioxide gas released

Step 1: Find the number of moles of nitrogen oxide gas

From chemical equation
2 mol of Mg(NO₃)₂  produces 4 mol of NO₂
Therefore, 0.081 mol of Mg(NO₃)₂  produces 0.162 mol of NO₂

Step 2: Find the volume of nitrogen dioxide gas released

= Number of moles x Molar volume of gas
= 0.162 mol x  24 dm³mol^-1
= 3.888 dm³ or  3888 cm³ of nitrogen dioxide gas

Chemical Formula of Ionic Compound

Ionic compound is generally a compound that is made up of metal ions and non metal ions. Constructing chemical formula of ionic compound is actually pretty easy once you have all the cations (positively charged ions) and anions (negatively charged ions) in your mind. Diagram below shows the list of common cations and anions that you need to know.

At first sight, you may have thought that there are a lot of ions to remember, but since learning chemistry will make you an expert in writing chemical formulas and chemical equations, there will be a time where you realize that these images of ions just naturally pop in your head whenever you need them because you are just that good.

Example: Constructing chemical formula of Barium Sulphate

Barium sulphate is made up of barium ion (metal ion) and sulphate ion (non metal ion). Diagram below shows the basic steps to construct chemical formula of barium sulphate.

It is very easy, isn't? Now you can use the same steps to construct chemical formula of other ionic compounds such as aluminium oxide Al₂O₃, copper(II) nitrate Cu(NO₃)₂ and sodium carbonate Na₂CO₃. You must do this exercise with other ionic compounds until you won't need a piece of paper to write the steps down because again, you are just that good.

Additional note:
Other than chemical formula of ionic compounds, as a chemistry student you also need to make yourself familiar with chemical formula of water H₂O and few common acids as well as gases such as nitric acid HNO₃, hydrochloric acid HCl, sulphuric acid H₂SO₄, carbon dioxide gas CO₂, ammonia gas NH₃, methane gas CH₄, sulphur dioxide gas SO₂ and nitrogen dioxide gas NO₂.