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1

Chapter 4 Relative atomic mass and the mole

E1. A sample of argon (Ar) known to contain isotopes of mass numbers 36, 39 and 40 is introduced in a mass spectrometer. The sample is bombarded with electrons to form positively charged ions. a Which ion, 36 Ar + or 40 Ar +, is likely to be deflected most in the magnetic field? 18 18 Give a reason for your answer. b Some atoms lose two electrons in the ionisation chamber. Which ion, 36 Ar + or 18

Ar 2+, is likely to be deflected most in the magnetic field? Give a reason for your answer.

36 18

AE1. a b Q1.

36 18

36 18

Ar + has the lighter mass, so will be deflected the most. Ar 2+ has the higher positive charge and so will be deflected the most.

Use the data in Table 4.2 on page 55 to calculate the relative atomic mass of: a oxygen b silver c hydrogen

A1. a Ar(O) =

(15.995 × 99.76) + (16.999 × 0.04) + (17.999 × 0.20) 100 = 15.999

b

Ar(Ag) =

(106.9 × 51.8) + (108.9 × 48.2) 100 = 108

c

Ar(H) =

(1.008 × 99.986) + (2.014 × 0.014) + (3.016 × 0.0001) 100 = 1.008

Heinemann Chemistry 1 (4th edition) © Reed International Books Australia Pty Ltd

Worked solutions to textbook questions

2

Q2.

The element lithium has two isotopes: • 6Li has a relative isotopic mass of 6.02. • 7Li has a relative isotopic mass of 7.02. The relative atomic mass of lithium is 6.94. Calculate the percentage abundance of the lighter isotope.

A2.

Let the percentage abundance of the lighter isotope be x%. ∴ percentage abundance of the heavier isotope will be (100 – x)%.

∴ 6.94 =

∴ x = 8.0 ∴ proportion of the lighter isotope = 8.0% Q3.

( x × 6.02) + ((100 − x) × 7.02) 100

Calculate the relative molecular masses of: a sulfuric acid (H2SO4) b ammonia (NH3) c ethane (C2H6)

A3. a

b

c

Mr(H2SO4) = 2 × Ar(H) + Ar(S) + 4 × Ar(O) = 2 × 1.008 + 32.06 + 4 × 16.00 = 98.1 Mr(NH3) = Ar(N) + 3 × Ar(H) = 14.007 + 3 × 1.008 = 17.0 Mr(C2H6) = 2 × Ar(C) + 6...