When 5g of calcium is burnt in 2g of oxygen, then 7 g of calcium oxide of produced. What mass of calcium oxide will be produced when 5g of calcium is burnt in 20 g of oxygen ? Which law of chemical combination will govern your answer ?

Answer

To solve the problem, we need to determine how much calcium oxide (CaO) will be produced when 5g of calcium is burnt in 20g of oxygen. We will also identify the law of chemical combination that governs this reaction.

### Step-by-Step Solution:

1. **Understanding the Reaction**:
The reaction between calcium (Ca) and oxygen (O2) produces calcium oxide (CaO). The balanced chemical equation for this reaction is:
\[ 2 \text{Ca} + \text{O}_2 \rightarrow 2 \text{CaO} \]

2. **Calculating the Molar Masses**:
- Molar mass of Calcium (Ca) = 40 g/mol
- Molar mass of Oxygen (O2) = 32 g/mol (16 g/mol for each O atom)
- Molar mass of Calcium Oxide (CaO) = 40 g/mol (Ca) + 16 g/mol (O) = 56 g/mol

3. **Determining the Limiting Reagent**:
- From the balanced equation, 2 moles of Ca react with 1 mole of O2.
- For 5g of Ca, the number of moles of Ca is:
\[ \text{Moles of Ca} = \frac{5 \text{ g}}{40 \text{ g/mol}} = 0.125 \text{ moles} \]
- According to the stoichiometry, 0.125 moles of Ca would require:
\[ \text{Moles of O2 required} = \frac{0.125 \text{ moles Ca}}{2} = 0.0625 \text{ moles O2} \]
- The mass of O2 required is:
\[ \text{Mass of O2 required} = 0.0625 \text{ moles} \times 32 \text{ g/mol} = 2 \text{ g} \]
- Since we have 20g of O2 available, which is more than enough, calcium (Ca) is the limiting reagent.

4. **Calculating the Mass of CaO Produced**:
- From the balanced equation, 2 moles of Ca produce 2 moles of CaO.
- Therefore, 0.125 moles of Ca will produce 0.125 moles of CaO.
- The mass of CaO produced is:
\[ \text{Mass of CaO} = 0.125 \text{ moles} \times 56 \text{ g/mol} = 7 \text{ g} \]

5. **Conclusion**:
When 5g of calcium is burnt in 20g of oxygen, 7g of calcium oxide (CaO) will be produced.

### Law of Chemical Combination:
The law that governs this answer is the **Law of Conservation of Mass**, which states that mass is neither created nor destroyed in a chemical reaction. The total mass of the reactants (calcium and oxygen) will equal the total mass of the products (calcium oxide).