Yield And Percentage Yield
In a reaction where 4 g of hydrogen gas reacts with excess chlorine to form HCl, the actual yield obtained is 128 g. What is the percentage yield of HCl? (Molar masses: H(_2) = 2 g/mol, HCl = 36.5 g/mol)
Select the correct option:
Solution
87.7%
Percentage yield is the ratio of actual yield to theoretical yield, expressed as a percentage. The balanced equation is (\text{H}_2 + \text{Cl}_2 \rightarrow 2\text{HCl}). Moles of (\text{H}_2 = 4/2 = 2) mol. From stoichiometry, 1 mol (\text{H}_2) gives 2 mol HCl, so 2 mol (\text{H}_2) gives 4 mol HCl. Theoretical mass of HCl (= 4 \times 36.5 = 146) g. Percentage yield (= (128/146) \times 100 = 87.7%). Option 75.2% would imply an actual yield of about 109.8 g, which does not match. Option 80.0% would require a theoretical yield of 160 g — that would mean over 4 mol HCl from 2 mol (\text{H}_2), violating stoichiometry. Option 95.4% would require a theoretical yield of about 134.2 g, inconsistent with the calculation. This question applies the NCERT concept of theoretical and percentage yield in quantitative analysis. Plausibility check: (128/146 = 0.877), so percentage yield (= 87.7%), and since (128 < 146), the yield is less than 100%, as expected in a real reaction.
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About This Question
- Subject
- chemistry
- Chapter
- some basic concepts in chemistry
- Topic
- yield and percentage yield
- Difficulty
- Medium
- Year
- 2025
Solution
Correct Answer:
87.7%
Percentage yield is the ratio of actual yield to theoretical yield, expressed as a percentage. The balanced equation is (\text{H}_2 + \text{Cl}_2 \rightarrow 2\text{HCl}). Moles of (\text{H}_2 = 4/2 = 2) mol. From stoichiometry, 1 mol (\text{H}_2) gives 2 mol HCl, so 2 mol (\text{H}_2) gives 4 mol HCl. Theoretical mass of HCl (= 4 \times 36.5 = 146) g. Percentage yield (= (128/146) \times 100 = 87.7%). Option 75.2% would imply an actual yield of about 109.8 g, which does not match. Option 80.0% would require a theoretical yield of 160 g — that would mean over 4 mol HCl from 2 mol (\text{H}_2), violating stoichiometry. Option 95.4% would require a theoretical yield of about 134.2 g, inconsistent with the calculation. This question applies the NCERT concept of theoretical and percentage yield in quantitative analysis. Plausibility check: (128/146 = 0.877), so percentage yield (= 87.7%), and since (128 < 146), the yield is less than 100%, as expected in a real reaction.
This medium difficulty chemistry question is from the chapter some basic concepts in chemistry, covering the topic of yield and percentage yield. It appeared in the 2025 exam.
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