iGCSE Chemistry (0620) Core:3.2 Relative masses of atoms and molecules: Exam Style Questions Paper 1- New Syllabus
▶️ Answer/Explanation
Detailed solution:
The balanced equation $2Mg + O_2 \rightarrow 2MgO$ tells us that 2 moles of magnesium atoms react with 1 mole of oxygen molecules. Using relative atomic masses, $A_r$ of Mg is 24 and $M_r$ of $O_2$ is 32, so 48g of magnesium reacts with exactly 32g of oxygen. This simplifies to a 3:2 mass ratio. So for 6g of magnesium, we set up the proportion: if 3 parts is 6g, then 2 parts is 4g. Hence, exactly 4g of oxygen is needed.
▶️ Answer/Explanation
Detailed solution:
The formula $\text{CaCO}_3$ contains one carbon atom. The relative atomic mass of carbon is 12. Since the $M_r$ of $\text{CaCO}_3$ is 100, carbon makes up $\frac{12}{100}$ of the total mass. So, in $100\text{g}$ of calcium carbonate, the mass of carbon is $\frac{12}{100} \times 100 = 12\text{g}$. Simple proportion gives us 12g.
▶️ Answer/Explanation
Detailed solution:
Relative atomic mass is the *average* mass of the isotopes of an element, taking into account their relative abundances, compared to $\frac{1}{12}$th the mass of a carbon-12 atom. It must mention “isotopes” and “average”, which option A does. The other options incorrectly say “total mass” or “average mass of an element” without specifying isotopes.