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ASChemistryCIEExam Questions2. Atoms, Molecules & Stoichiometry2.1 Relative Masses of Atoms & Molecules

Relative Masses of Atoms & Molecules (CIE AS Chemistry)

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2.1 Relative Masses of Atoms & Molecules

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2.1 Relative Masses of Atoms & Molecules

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1a4 marks

Explain what is meant by the terms

Relative atomic mass

[2] 

Relative isotopic mass

 [2]

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1b3 marks
i)
State the symbols used for relative atomic mass and relative molecular mass.
[1]
 
ii)
Describe how to calculate the relative molecular mass of a compound.
[1]
 
iii)
State the difference between relative molecular mass and relative formula mass.
[1]

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1c1 mark

Ionic compounds such as sodium chloride are arranged in a giant three-dimensional lattice structure, with a regular and repeating pattern of positive and negative ions.

State the formula unit of sodium chloride.

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1d
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Complete Table 1.1

Table 1.1
 
Chemical Formula Relative molecular / formula mass
Magnesium oxide MgO  
Sulfuric acid H2SO4  
Ammonium phosphate (NH4)3PO4  
Copper(II) sulfate pentahydrate CuSO4•5H2O  
  

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1a
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The element chromium has several naturally occurring isotopes whose abundances are shown in Table 1.1

Table 1.1

Mass number

% abundance

50

4.345

52

83.789

53

9.501

54

2.365

 

Calculate the relative atomic mass of chromium to two decimal places.

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1b1 mark

Chromium can form a variety of compounds. Chromium(III) chloride hydrate is the dark green complex [CrCl2(H2O)4]Cl.

Write the full electronic configuration for the chromium ion in this compound.

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1c
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Calculate the relative molecular mass of [CrCl2(H2O)4]Cl.

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2a2 marks

Nitrous oxide is used as a sedative in dentistry and has the formula N2O.
Different sources of N2O contain different ratios of 14N and 15N.           

State the name of the instrument used to distinguish between 14N and 15N and outline two characteristic differences seen in the analysis of 14N and 15N.

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2b2 marks

A sample of nitrous oxide was enriched so that it contained 4% by mass of 15N. Calculate the relative molecular mass of the resulting nitrous oxide.

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2c2 marks

An atom has twice as many protons, and twice as many neutrons, as an atom of 15N.

Determine the chemical symbol for this atom, including the mass number, and deduce the number of electrons.

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3a2 marks

Ethanoic acid has a relative molecular mass of 60.0.

Explain what is meant by the term relative molecular mass.

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3b1 mark

State the empirical formula of ethanoic acid.

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3c2 marks

Ethanoic acid reacts with aqueous sodium carbonate to form a neutral salt solution and a gas.

Name the salt produced in this reaction and calculate its relative formula mass.

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1a
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Copper is an easily moulded base metal that is often added to precious metals to improve their elasticity, flexibility, hardness, colour, and resistance to corrosion.

Table 3.1 shows the number of protons, neutrons and electrons of different isotopes and ions of copper.

 
Table 3.1
 
Species Protons Neutrons Electrons
    34 27
65Cu+ 29    
 
i)
Complete Table 3.1.
 
[2]
 
ii)
Give the full electron configuration of the Cu+ ion. 
 
[1]

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1b
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Calculate the percentage abundance of 63Cu with a mass of 62.9296 and 65Cu with a mass of 64.9278, when the average mass of the Cu isotope is 63.546.

 

Express your answer to an appropriate number of significant figures.

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1c2 marks

Explain why the isotopes of copper exhibit the same chemical reactions but their densities differ.

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1d2 marks

Copper can form a salt with chlorine to form copper(II) chloride. This reacts with sodium phosphate to form two different salts only.

 

Write a balanced equation for this reaction.

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