IB Chemistry HL 1.4.2 Relative Atomic Mass Ar and Relative Formula Mass Mr Questions

Use the carbon-12 standard to define relative atomic mass, then combine relative atomic masses from a formula to calculate relative formula mass and percentage composition.

Exam points

Define relative atomic mass by comparison with the carbon-12 standard and identify the reference species used for isotope masses.
Calculate relative formula mass or percentage composition from a chemical formula using supplied relative atomic masses.

[Maximum number: 1]

Two groups of students (Group A and Group B) carried out a project* on the chemistry of some group 7 elements (the halogens) and their compounds.

The students reacted ICl(l) with CsBr(s) to form a yellow solid, $\mathrm{CsICl}_{2}(\mathrm{~s})$ , as one of the products. $\mathrm{CsICl}_{2}(\mathrm{~s})$ has been found to produce very pure CsCl(s) which is used in cancer treatment.

To confirm the composition of the yellow solid, Group A determined the amount of iodine in 0.2015 g of $\mathrm{CsICl}_{2}(\mathrm{~s})$ by titrating it with $0.0500 \mathrm{moldm}^{-3} \mathrm{Na}_{2} \mathrm{~S}_{2} \mathrm{O}_{3}(\mathrm{aq})$ . The following data were recorded for the titration.

Table for Question IB Chemistry HL 1.4.2 Relative Atomic Mass Ar and Relative Formula Mass Mr Questions question 1 — IB Chemistry HL

Calculate the percentage of iodine by mass in $\mathrm{CsICl}_{2}(\mathrm{~s})$ , correct to three significant figures.

$\left(\frac{126.90}{330.71} \times 100\right)=38.4 \%$ ;

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IB Chemistry HL 1.4.2 Relative Atomic Mass Ar and Relative Formula Mass Mr Questions question 1