\( \begin{array}{l}\text { When sodium hydrogencarbonate is heated, it decomposes to produce sodium carbonate, water } \\ \text { and carbon dioxide. } \\ \text { The equation for the reaction is } \\ \left.\text { [ } \mathrm{A}_{\mathrm{r}}: \mathrm{H}, 1 ; \mathrm{C}, 12 ; \mathrm{O}, 16 ; \mathrm{Na}_{3} 23\right] \\ \text { The relative molecular mass of sodium hydrogencarbonate is } 84 . \\ \text { (a) (i) Calculate the relative molecular mass of sodium carbonate. } \\ \text { (ii) Complete the following sentences. } \\ \text { (i) } \\ 4.2 \mathrm{~g} \text {. } \mathrm{CO} \\ 3\end{array}+\mathrm{CO}_{2}+\mathrm{H}_{2} \mathrm{O} \).

To solve the problem, we will follow the steps outlined in the prompt. ### Step 1: Extract Known Conditions - The reaction involves sodium hydrogencarbonate decomposing into sodium carbonate, water, and carbon dioxide. - The relative atomic masses (A_r) are given as: - H: 1 - C: 12 - O: 16 - Na: 23 - The relative molecular mass of sodium hydrogencarbonate (NaHCO₃) is 84 g/mol. ### Step 2: Calculate the Relative Molecular Mass of Sodium Carbonate Sodium carbonate (Na₂CO₃) consists of: - 2 Sodium (Na) - 1 Carbon (C) - 3 Oxygen (O) Using the relative atomic masses, we can calculate the relative molecular mass of sodium carbonate: \[ \text{Relative Molecular Mass of Na}_2\text{CO}_3 = 2 \times \text{A}_r(\text{Na}) + 1 \times \text{A}_r(\text{C}) + 3 \times \text{A}_r(\text{O}) \] Substituting the values: \[ \text{Relative Molecular Mass of Na}_2\text{CO}_3 = 2 \times 23 + 1 \times 12 + 3 \times 16 \] Now, let's calculate this step by step: 1. Calculate \(2 \times 23\): \[ 2 \times 23 = 46 \] 2. Calculate \(1 \times 12\): \[ 1 \times 12 = 12 \] 3. Calculate \(3 \times 16\): \[ 3 \times 16 = 48 \] 4. Now, sum these results: \[ 46 + 12 + 48 = 106 \] Thus, the relative molecular mass of sodium carbonate (Na₂CO₃) is \(106\) g/mol. ### Step 3: Complete the Sentences The problem states: - "4.2 g CO₃ + CO₂ + H₂O" This indicates that 4.2 g of sodium hydrogencarbonate decomposes to produce sodium carbonate, carbon dioxide, and water. ### Final Answers (a) (i) The relative molecular mass of sodium carbonate is \(106\) g/mol. (ii) The completed sentence is: "4.2 g of sodium hydrogencarbonate decomposes to produce sodium carbonate, carbon dioxide, and water."