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IB Chemistry HL 2.1.3 Limiting Reactant Questions

Use real HL exam questions to compare reactants, identify the limiting reagent and calculate theoretical product yield.

Syllabus
First assessment 2025
Course
Chemistry HL
Level
HL

Exam points

  • Compare each reactant amount with the balanced-equation coefficients to identify the limiting reactant and any excess reagent.
  • Use the limiting reactant to determine theoretical product amount or yield and calculate the amount of excess reactant remaining.
  • Show the stoichiometric comparison before reporting the chemically possible product mass, volume or amount after the limiting reactant is consumed.

Learn this topic

Study 2.1.3—Limiting reactant conceptsOpen the corresponding Concept lesson for explanations, examples and exam connections.

IB Chemistry HL 2.1.3 Limiting Reactant Questions question 1

[Maximum number: 2]

Phosphine (IUPAC name phosphane) is a hydride of phosphorus, with the formula PH3\mathrm{PH}_{3}.

Question (a)

(a)

2.478 g of white phosphorus was used to make phosphine according to the equation:

P4( s)+3OH−(aq)+3H2O(l)→PH3( g)+3H2PO2−(aq)\mathrm{P}_{4}(\mathrm{~s})+3 \mathrm{OH}^{-}(\mathrm{aq})+3 \mathrm{H}_{2} \mathrm{O}(\mathrm{l}) \rightarrow \mathrm{PH}_{3}(\mathrm{~g})+3 \mathrm{H}_{2} \mathrm{PO}_{2}^{-}(\mathrm{aq})
[ 2 ]

Question (i)

(i)

This phosphorus was reacted with 100.0 cm3100.0 \mathrm{~cm}^{3} of 5.00 moldm−35.00 \mathrm{~mol} \mathrm{dm}^{-3} aqueous sodium hydroxide. Deduce, showing your working, which was the limiting reagent.

[ 1 ]

Question (ii)

(ii)

Determine the excess amount, in mol , of the other reagent.

[ 1 ]
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