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

To find the ratio of magnesium atoms to sulfur atoms in magnesium sulfide, we consider the given mass of magnesium (24 g) and sulfur (32 g).

• Molar mass of magnesium (Mg) = 24 g/mol
• Molar mass of sulfur (S) = 32 g/mol

Number of moles of magnesium: ext{Moles of Mg} = rac{24 ext{ g}}{24 ext{ g/mol}} = 1 ext{ mol}

Number of moles of sulfur: ext{Moles of S} = rac{32 ext{ g}}{32 ext{ g/mol}} = 1 ext{ mol}

The ratio of magnesium atoms to sulfur atoms is therefore: rac{ ext{Moles of Mg}}{ ext{Moles of S}} = rac{1}{1}

Hence, the correct answer is A: 1:1.

From the simplest ratio of aluminium to oxygen atoms in aluminium oxide, which is 2 : 3, we can derive the empirical formula.

This means there are 2 aluminium atoms for every 3 oxygen atoms. Therefore, the empirical formula of aluminium oxide is:

Al₂O₃.

The empirical formula CH₂ indicates that the ratio of carbon to hydrogen is 1:2. To find the molecular formula, we must consider possible multiples of the empirical formula.

The molecular formula corresponding to other ratios can include:

• 1 (CH₂) = C₁H₂
• 2 (C₂H₄)
• 3 (C₃H₆)
• 4 (C₄H₁₀)

From the given options, the molecular formula corresponding to the empirical formula CH₂ is B: C₂H₆.

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

ext{Percentage Yield} = rac{ ext{Actual Yield}}{ ext{Theoretical Yield}} imes 100

Given:

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

So, substituting the values:

ightarrow rac{7.2}{9.0} = 0.8$$ Then: $$0.8 imes 100 = 80 ext{%}$$ The percentage yield of zinc oxide is **80%**.

The actual yield of a product in an experiment is usually lower than the theoretical yield due to several factors, including:

1. Incomplete Reactions: Some reactants may not fully convert to products due to the reaction not going to completion.

2. Side Reactions: Unintended side reactions can occur, producing by-products instead of the desired product.

3. Loss of Product: During the process of purification and extraction, some product may be lost.

4. Measurement Errors: Inaccuracies in measuring reactants or products can also contribute to a lower actual yield.

To calculate the percentage by mass of zinc in zinc oxide (ZnO), we first need the relative formula mass of zinc oxide, which we can determine as follows:

• Relative atomic mass of Zinc (Zn) = 65
• Relative atomic mass of Oxygen (O) = 16

So, $ext{Relative Formula Mass of ZnO} = 65 + 16 = 81$

Now, the percentage by mass of zinc in ZnO is given by the formula:

ext{Percentage by Mass of Zn} = rac{ ext{Mass of Zn}}{ ext{Mass of ZnO}} imes 100

Substituting the values:

ext{Percentage by Mass of Zn} = rac{65}{81} imes 100 = 80.25 ext{%}

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