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CAIE A-Level Chemistry 2.1 Relative Masses of Atoms and Molecules

Practise defining and calculating relative isotopic, atomic, molecular and formula masses on the carbon-12 scale.

Syllabus
2028–2030
Course
Chemistry 9701
Level
AS

Exam points

  • define the unified atomic mass unit as one twelfth of the mass of one carbon-12 atom
  • distinguish relative isotopic mass from weighted relative atomic mass
  • calculate relative molecular or formula mass by summing the appropriate relative atomic masses

Covered syllabus points

2.1.1—Unified atomic mass unit2.1.2—Atomic mass, Ar, isotopic mass

2.1 Relative masses of atoms and molecules question 1

[Maximum number: 1]

Calcium, magnesium and radium are Group 2 elements. Radium follows the same trends as the other members of Group 2.

A sample of magnesium contains three isotopes, 25Mg,26Mg{ }^{25} \mathrm{Mg},{ }^{26} \mathrm{Mg} and X.

The percentage abundance of the three isotopes is shown in Table 1.1.

Table 1.1

Table 1.1

The relative atomic mass, ArA_{r}, is calculated by comparing the average mass of the isotopes of an element to the unified atomic mass unit.

Define the unified atomic mass unit.

2.1 Relative masses of atoms and molecules question 2

[Maximum number: 2]

Copper is used in electrical equipment. It has a melting point of 1085∘C1085^{\circ} \mathrm{C}.

Question (a)

(a)

The relative isotopic masses and natural abundances of the two isotopes in a sample of copper are shown in Table 1.1.

Table 1.1

Table 1.1

[ 2 ]

Question (i)

(i)

Define the unified atomic mass unit.

[ 1 ]

Question (ii)

(ii)

Define relative atomic mass, ArA_{\mathrm{r}}, in terms of the unified atomic mass unit.

[ 1 ]
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