IB Chemistry SL R2.1 How much? The amount of chemical change questions

Exam-style multiple choice and written questions with full markschemes. Check multiple choice answers straight away, or type, photograph or draw a written answer and get it marked.

Type

Multiple choice Written

Difficulty

Easy Medium Hard

Question 1

Multiple choiceHard1 mark

Magnesium reacts with hydrochloric acid according to the equation below. The hydrogen is collected at the conditions for which the molar gas volume is $24.0\ \mathrm{dm^3\,mol^{-1}}$ Use $A_\mathrm{r}(\mathrm{Mg})=24.31$

\mathrm{Mg(s)+2HCl(aq)\rightarrow MgCl_2(aq)+H_2(g)}

Substance	Amount
Mg	$0.120\ \mathrm{g}$
HCl(aq)	$25.0\ \mathrm{cm^3}$ of $0.200\ \mathrm{mol\,dm^{-3}}$
$H_2$ collected	$56.4\ \mathrm{cm^3}$

Which option gives the limiting reactant and the percentage yield of $H_2$

Show answer and explanation

Answer: C. Compare the available amounts using the $1:2$ mole ratio of Mg to HCl, then calculate the yield relative to the theoretical $H_2$ volume.

$n(\mathrm{Mg})=0.120/24.31=4.94\times10^{-3}\ \mathrm{mol}$ $n(\mathrm{HCl})=0.0250\times0.200=5.00\times10^{-3}\ \mathrm{mol}$ The Mg would require $9.87\times10^{-3}\ \mathrm{mol}$ HCl, so HCl is limiting.
$n(\mathrm{H_2})=5.00\times10^{-3}/2=2.50\times10^{-3}\ \mathrm{mol}$ giving a theoretical volume of $60.0\ \mathrm{cm^3}$
Percentage yield $=(56.4/60.0)\times100=94.0\%$

A: Compares the raw mole amounts to label Mg limiting, ignoring the equation’s coefficients.
B: Reverses the yield ratio, calculating $60.0/56.4\times100=106\%$
D: Treats Mg as limiting and uses its amount to calculate the theoretical $H_2$ volume, giving $118.5\ \mathrm{cm^3}$ and a $47.6\%$ yield.
Remember: compare available amounts after accounting for stoichiometric coefficients to identify the limiting reactant.

Question 2

Paper 2Hard8 marks

Aspirin (acetylsalicylic acid) can be prepared by either of the reactions shown in Figure 1. Use the atom economy equation in the data booklet and the following relative atomic masses: $A_\mathrm{r}(\mathrm C)=12.01$ $A_\mathrm{r}(\mathrm H)=1.01$ $A_\mathrm{r}(\mathrm O)=16.00$ $A_\mathrm{r}(\mathrm{Cl})=35.45$

(a)

Calculate the atom economy of Route I, to 3 significant figures.

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(b)

Calculate the atom economy of Route II, to 3 significant figures.

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(c)

For each route, determine the mass of reactant material not incorporated into aspirin, per mole of aspirin formed, on the basis of the balanced reaction. Use your answers to (a) and (b) to assess the claim: “Route II must generate less waste in an industrial process because it has a higher atom economy.”

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