Isotopes and Relative Atomic Mass

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Isotopes and Relative Atomic Mass — Free MYP4 Chemistry Practice Questions

1QuestionDefinition and Examples of IsotopesConcept Practice

2 marks~3 minCriterion A

Iodine-123 (

^{123}

I) is a radioactive isotope used in hospitals to image a patient's thyroid gland. A small quantity is administered to the patient, and its gamma emissions are detected to produce a diagnostic image.

Outline what makes

^{123}

I an isotope of iodine. [1]

A patient undergoing this procedure expresses concern about radiation exposure. Discuss one limitation of using

^{123}

I for thyroid imaging that is directly related to this concern. [1]

Solutions

2QuestionDefinition and Examples of IsotopesAssessment Practice

5 marks~8 minCriterion C

Mass spectrometry of a chlorine sample produces two peaks:

m/z = 35

, relative abundance

= 75.8\%

m/z = 37

, relative abundance

= 24.2\%

A separate experiment determines the average atomic mass of chlorine to be

35.45 \text{ amu}

.

The weighted-average formula for atomic mass is:

\text{average atomic mass} = \frac{(m_1 \times a_1) + (m_2 \times a_2)}{100}

where

m_1

m_2

are isotope masses and

a_1

a_2

are their percentage abundances.

Calculate the average atomic mass of chlorine predicted by the two-isotope data. [2]

Deduce whether the experimentally determined value of

35.45 \text{ amu}

is consistent with chlorine existing as only two stable isotopes. Justify your reasoning using your answer to part (a) and the concept of experimental error. [3]

Solutions

3QuestionCalculating Relative Atomic Mass RAMAssessment Practice

6 marks~9 minCriterion D

A nuclear accident releases iodine-131 into a local water supply. Natural iodine consists entirely of iodine-127 (mass = 126.9045 u). A contaminated water sample contains 80% iodine-127 and 20% iodine-131 (mass = 130.9061 u) by number of atoms.

Calculate the relative atomic mass (RAM) of iodine in the contaminated sample. [2]

Deduce whether the altered RAM alone is sufficient to confirm the presence of iodine-131 contamination. Justify your reasoning with reference to the standard RAM of iodine. [2]

Evaluate the limitations of using RAM as a monitoring tool for iodine-131 contamination, with reference to the health risks posed by iodine-131 accumulation in the thyroid gland. [2]

Solutions

4QuestionStability of Isotopes and Radioactivity IntroAssessment Practice

4 marks~6 minCriterion B

The stability of an isotope depends on its neutron-to-proton ratio (

n/p

). Data for five isotopes are given below.

Isotope	H-1	H-2	He-4	C-14	U-238
Protons	1	1	2	6	92
Neutrons	0	1	2	8	146
$n/p$ ratio	0.00	1.00	1.00	1.33	1.59
Stability	stable	stable	stable	radioactive	radioactive

Describe the relationship between the

n/p

ratio and isotope stability, referring to specific values in the data above. [2]

Deduce whether oxygen-16 (8 protons, 8 neutrons) and oxygen-18 (8 protons, 10 neutrons) are stable or radioactive. Justify each prediction using the pattern from part (a). [2]

Solutions

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