Atoms and Molecules

Given:
- Total mass of compound = 0.24 g
- Mass of boron = 0.096 g
- Mass of oxygen = 0.144 g

Formula used:
$$\text{Percentage by weight} = \frac{\text{Mass of element}}{\text{Total mass of compound}} \times 100$$

Step 1: Percentage of Boron
$$\% \text{ Boron} = \frac{0.096}{0.24} \times 100 = 40\%$$

Step 2: Percentage of Oxygen
$$\% \text{ Oxygen} = \frac{0.144}{0.24} \times 100 = 60\%$$

Verification: $40\% + 60\% = 100\%$ ✓

Answer: The compound contains 40% Boron and 60% Oxygen by weight.

Given (first case):
- Mass of carbon = 3.0 g
- Mass of oxygen = 8.00 g
- Mass of CO₂ produced = 11.00 g

Observation from first case:
Carbon and oxygen combine in the ratio:
$$\text{C : O} = 3.0 : 8.0 = 3 : 8$$
This means 3 g of carbon requires exactly 8 g of oxygen to produce 11 g of CO₂.

Second case:
- Mass of carbon = 3.00 g
- Mass of oxygen available = 50.00 g

Step 1: Find oxygen required to burn 3.00 g of carbon completely.
From the fixed ratio, oxygen required $= 8.00$ g.

Step 2: Since 50.00 g of oxygen is available but only 8.00 g is needed, oxygen is in excess. Carbon is the limiting reactant.

Step 3: Mass of CO₂ formed
$$= 3.00 + 8.00 = 11.00 \text{ g}$$

Answer: 11.00 g of carbon dioxide will be formed.

Law governing this: This is governed by the Law of Constant Proportions (Law of Definite Proportions), which states that in a chemical compound, elements are always present in definite proportions by mass, regardless of the source or method of preparation.

Definition:
A polyatomic ion is a group of atoms carrying a net electric charge (positive or negative). These atoms are covalently bonded together and the group as a whole acts as a single ion.

Examples:

| Ion | Formula | Charge |
|-----|---------|--------|
| Ammonium ion | $\text{NH}_4^+$ | $+1$ |
| Hydroxide ion | $\text{OH}^-$ | $-1$ |
| Carbonate ion | $\text{CO}_3^{2-}$ | $-2$ |
| Sulphate ion | $\text{SO}_4^{2-}$ | $-2$ |
| Nitrate ion | $\text{NO}_3^-$ | $-1$ |
| Phosphate ion | $\text{PO}_4^{3-}$ | $-3$ |

These ions participate in ionic bonding just like monoatomic ions.