IB DP Chemistry -Reactivity 2.1 How much? The amount of chemical change- IB Style Questions For HL Paper 1A -FA 2025

IBDP Chemistry HL Paper 1A & 1B - All Chapters

Question

Chloroethane is produced by:
\[C_2H_6(g) + Cl_2(g) \rightarrow C_2H_5Cl(g) + HCl(g)\]
Molar masses: C2H6 = 30.08, Cl2 = 70.90, C2H5Cl = 64.52, HCl = 36.46 g mol−1
\[ \text{Atom economy} = \frac{\text{molar mass of desired product}}{\text{molar mass of all reactants}} \times 100 \]
What is the atom economy?
(A) 15.7%
(B) 46.6%
(C) 63.9%
(D) 99.9%
▶️ Answer/Explanation
Detailed solution

Atom economy = \(\frac{64.52}{30.08 + 70.90} \times 100\)
= \(\frac{64.52}{100.98} \times 100 \approx 63.9\%\)

✅ Answer: (C)

Question

\(0.20\) mol of magnesium is mixed with \(0.10\) mol of hydrochloric acid.
\( \text{Mg}(s) + 2\text{HCl}(aq) \rightarrow \text{MgCl}_2(aq) + \text{H}_2(g) \)
Which row correctly identifies the limiting reagent and the maximum possible yield of \( \text{H}_2 \)?
OptionLimiting reagentMaximum yield of \( \text{H}_2 \) / mol
AHCl0.10
BMg0.20
CHCl0.05
DMg0.10
▶️ Answer/Explanation
Detailed solution

The balanced reaction is:

\( \text{Mg}(s) + 2\text{HCl}(aq) \rightarrow \text{MgCl}_2(aq) + \text{H}_2(g) \)

To react completely with \(0.20\) mol of Mg, the required amount of HCl is:

\( 0.20 \text{ mol Mg} \times 2 = 0.40 \text{ mol HCl} \)

But only \(0.10\) mol of HCl is available. Therefore, HCl is the limiting reagent.

The reaction uses HCl and produces \( \text{H}_2 \) in a \(2:1\) ratio:

\( 0.10 \div 2 = 0.05 \text{ mol of } \text{H}_2 \)

Thus, the maximum possible yield of hydrogen is \( \mathbf{0.05\ mol} \).

✅ Answer: (C)

Question

Four identical containers under the same conditions are filled with gases as shown below. Which container and contents will have the highest mass?
▶️ Answer/Explanation
Detailed solution

All containers have the same:

  • volume
  • temperature
  • pressure

Under identical conditions, the number of moles in each container is the same (from the ideal gas law).

Therefore, the container with the largest molar mass will have the greatest total mass.

Molar masses:

  • \( M(\text{N}_2) = 28 \)
  • \( M(\text{O}_2) = 32 \)
  • \( M(\text{C}_2\text{H}_6) = 30 \)
  • \( M(\text{Ne}) = 20 \)

Oxygen has the largest molar mass, so its container will have the greatest mass.

✅ Answer: B

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