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

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Question 1

Multiple choiceEasy1 mark

A process makes ethanol using the balanced reaction $\mathrm{C_2H_5Br + NaOH \rightarrow C_2H_5OH + NaBr}$ Which proposed change would increase the atom economy, rather than only the percentage yield?

Show answer and explanation

Answer: C. Atom economy is set by the balanced equation and compares the mass of desired product with the total reactant mass. A pathway that incorporates a greater proportion of reactant mass into ethanol therefore has greater atom economy.
A: recovering more ethanol increases the experimental yield, not the atom economy.
B: reducing unreacted bromoethane changes conversion, while the reaction stoichiometry remains unchanged.
D: a catalyst changes the reaction rate, not the stoichiometric mass fraction forming ethanol.
Remember: atom economy depends on the reaction equation; percentage yield depends on the amount actually obtained.

Question 2

Paper 2Medium7 marks

A student prepares magnesium hydroxide by mixing aqueous magnesium chloride with aqueous sodium hydroxide. The reaction is

\ce{MgCl2(aq) + 2NaOH(aq) -> Mg(OH)2(s) + 2NaCl(aq)}

Figure 1 shows the quantities used and the mass of solid collected after filtration and drying.

Use relative atomic masses from the data booklet where required.

(a)

Calculate the amount, in mol, of NaOH used. Give your answer to three significant figures.

[1]

(b)

Using your answer to (a) and Figure 1, determine the limiting reactant and calculate the theoretical mass, in g, of $\ce{Mg(OH)2}$ Show your working.

[3]

(c)

Calculate the percentage yield of $\ce{Mg(OH)2}$ to three significant figures.

[1]

(d)

Explain one reason why an experimental yield could be lower than the theoretical yield, and one reason why its apparent value could be higher.

[2]

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