4 The relative formula mass of $$ \mathrm{X}_{2} \mathrm{CO}_{3} \cdot 10 \mathrm{H}_{2} \mathrm{O} $$ is 286. What is the relative atomic mass of element X ? [Relative atomic mass: $$ \mathrm{H}=1, \mathrm{C}=12, \mathrm{O}=16 $$ ] Jisim formula relatif bagi $$ \mathrm{X}_{2} \mathrm{CO}_{3} \cdot 10 \mathrm{H}_{2} \mathrm{O} $$ ialah 286. Berapakah jisim atom relatif bagi unsur X ? [Jisim atom relatif: $$ \mathrm{H}=1, \mathrm{C}=12, \mathrm{O}=16 $$ ] A 23 B 46 C 120 D 240

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To determine the relative atomic mass of element **X** in the compound $$ \mathrm{X}_{2} \mathrm{CO}_{3} \cdot 10 \mathrm{H}_{2} \mathrm{O} $$ with a relative formula mass of **286**, follow these steps:

1. **Break down the compound:**
   $$
   \mathrm{X}_{2} \mathrm{CO}_{3} \cdot 10 \mathrm{H}_{2} \mathrm{O}
   $$
   This consists of:
   - 2 atoms of X
   - 1 atom of C
   - 3 atoms of O from $$ \mathrm{CO}_{3} $$
   - 10 water molecules ($$ 10 \mathrm{H}_{2} \mathrm{O} $$), which contribute:
     - $$ 10 \times 2 = 20 $$ atoms of H
     - $$ 10 \times 1 = 10 $$ atoms of O

2. **Calculate the total mass contributed by non-X elements:**
   $$
   \text{Mass from C} = 12 \, \text{(since C = 12)}
   $$
   $$
   \text{Mass from O} = 3 \times 16 + 10 \times 16 = 48 + 160 = 208
   $$
   $$
   \text{Mass from H} = 20 \times 1 = 20
   $$
   $$
   \text{Total mass from C, O, H} = 12 + 208 + 20 = 240
   $$

3. **Set up the equation for the total formula mass:**
   $$
   2X + 240 = 286
   $$
   $$
   2X = 286 - 240 = 46
   $$
   $$
   X = \frac{46}{2} = 23
   $$

**Therefore, the relative atomic mass of element X is **23**.**

**Answer:**  
**A 23**
The relative atomic mass of element X is 23. **Answer:** A 23

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