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

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

Question 1

[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 ]

Question 2

[Maximum number: 1]

Which sodium compound contains 74.2 % by mass of sodium?

A

sodium carbonate

B

sodium chloride

C

sodium hydroxide

D

sodium oxide

Question 3

[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.

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