Calculate the relative formula mass of calcium carbonate, CaCO₃ - Edexcel - GCSE Chemistry - Question 4 - 2016 - Paper 1

To determine the ratio of magnesium atoms to sulfur atoms in magnesium sulfide:

• The molar mass of magnesium (Mg) is 24 g.
• The molar mass of sulfur (S) is 32 g.

We can find the number of moles:

$ext{Moles of Mg} = \frac{24 ext{ g}}{24 ext{ g/mol}} = 1 ext{ mol}$

$ext{Moles of S} = \frac{32 ext{ g}}{32 ext{ g/mol}} = 1 ext{ mol}$

Thus, the ratio of magnesium atoms to sulfur atoms is 1:1. Therefore, the answer is option A (1:1).

The simplest ratio of aluminium atoms to oxygen atoms in aluminium oxide is given as 2:3. Hence, the empirical formula can be derived from these ratios as:

• 2 Aluminium (Al)
• 3 Oxygen (O)

Thus, the empirical formula of aluminium oxide is ( ext{Al}_2 ext{O}_3 ).

The empirical formula CH₂ indicates that every unit of this hydrocarbon contains one carbon atom and two hydrogen atoms. The molecular formula could be derived by multiplying the subscripts by a whole number, n:

$ext{Molecular formula} = C_{n}H_{2n}$

For n=1, the molecular formula would be CH₂; for n=2, it would be C₂H₄; for n=3, it would be C₃H₆, and so forth. The answer is likely to be option B (C₂H₆) if we assume the simplest case. However, the exact choice should be confirmed from the options available.

To calculate the percentage yield of zinc oxide, we can use the formula:

$\text{Percentage Yield} = \left(\frac{\text{Actual Yield}}{\text{Theoretical Yield}}\right) \times 100$

Inserting the values from the problem:

• Actual Yield = 7.2 g
• Theoretical Yield = 9.0 g

Thus:

$\text{Percentage Yield} = \left(\frac{7.2}{9.0}\right) \times 100 = 80\%$

So, the percentage yield of zinc oxide is 80%.

The actual yield of a product is often lower than the theoretical yield due to several factors:

1. Incomplete Reactions: Not all reactants are converted into products in some cases.
2. Side Reactions: Unwanted side reactions may take place, consuming reactants and producing by-products instead of the desired product.
3. Loss During Processing: Some product may be lost during separation and purification processes, which reduces the final yield.
4. Measurement Errors: Inaccurate measurements can lead to discrepancies between expected and actual yields.

These factors contribute to the actual yield falling short of the theoretical yield.

The relative formula mass of zinc oxide (ZnO) can be calculated as:

• Zinc (Zn): 65
• Oxygen (O): 16

Thus, the relative formula mass of ZnO is:

$\text{Relative Formula Mass of ZnO} = 65 + 16 = 81$

The percentage by mass of zinc in zinc oxide can thus be calculated as:

$\text{Percentage by Mass of Zinc} = \left(\frac{\text{Mass of Zn}}{\text{Relative Formula Mass of ZnO}}\right) \times 100$

Substituting the values:

$\text{Percentage by Mass of Zinc} = \left(\frac{65}{81}\right) \times 100 \approx 80.25\%$

Therefore, the percentage by mass of zinc in zinc oxide is approximately 80.25%.