Formulae, Equations, Relative Masses, Empirical Formulae and Molecular Formulae

1. What is the relative formula mass, Mr, of aluminium sulfate, Al2(SO4)3?

[Ar: Al, 27; S, 32; O, 16]

A 150
B 246
C 342
D 390

2. Ammonium ions, NH4+, combine with phosphate ions, PO4 3–.

What is the correct formula of ammonium phosphate?

A NH4PO4
B (NH4)2PO4
C (NH4)3PO4
D NH4(PO4)3

3. Copper(II) ions combine with nitrate ions.

Which formula is correct?

A CuNO3
B Cu(NO3)2
C Cu2NO3
D Cu2(NO3)3

4. A hydrated salt has formula MgSO4.xH2O and relative formula mass 246.

[Ar: Mg, 24; S, 32; O, 16; H, 1]

What is x?

A 5
B 6
C 7
D 10

5. A compound contains 52.2% carbon, 13.0% hydrogen and 34.8% oxygen by mass.

[Ar: C, 12; H, 1; O, 16]

What is its empirical formula?

A CH3O
B C2H6O
C C2H6O2
D C4H12O2

6. A compound has empirical formula CH2O and relative molecular mass 180.

What is its molecular formula?

[Ar: C, 12; H, 1; O, 16]

A CH2O
B C3H6O3
C C6H12O6
D C12H24O12

7. Which equation is balanced?

A Mg + O2 → MgO
B 2Mg + O2 → 2MgO
C Mg + 2O2 → MgO
D Mg2 + O2 → 2MgO

8. Which symbol equation correctly represents the reaction of sodium carbonate with hydrochloric acid?

A NaCO3 + HCl → NaCl + H2O + CO2
B Na2CO3 + HCl → NaCl + H2O + CO2
C Na2CO3 + 2HCl → 2NaCl + H2O + CO2
D Na2CO3 + 2HCl → NaCl2 + H2O + CO2

9. Which formula represents iron(III) oxide?

A FeO
B Fe2O3
C Fe3O2
D FeO3

10. The empirical formula of a compound is C2H5. Its relative molecular mass is 58.

[Ar: C, 12; H, 1]

What is the molecular formula?

A C2H5
B C4H10
C C6H15
D C8H20

Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total A Grades, 11 World Records and 7 Distinctions, Educate A Change.

11. Which statement about empirical formulae is correct?

A The empirical formula always shows the actual number of atoms in one molecule.
B The empirical formula shows the simplest whole-number ratio of atoms or ions.
C The empirical formula is always different from the molecular formula.
D The empirical formula can only be found for covalent compounds.

12. Which compound has the greatest relative formula mass?

[Ar: Na, 23; Mg, 24; Al, 27; Cl, 35.5; O, 16; S, 32]

A NaCl
B MgO
C Al2O3
D MgSO4

13. What is the relative molecular mass of ethanol, C2H5OH?

[Ar: C, 12; H, 1; O, 16]

A 29
B 45
C 46
D 60

14. Which formula has the same empirical formula as benzene, C6H6?

A C2H6
B C3H8
C C4H4
D C6H12

15. A compound contains 40.0% carbon, 6.7% hydrogen and 53.3% oxygen by mass.

[Ar: C, 12; H, 1; O, 16]

What is the empirical formula?

A CH2O
B C2H4O2
C CHO
D C3H6O3

16. Which equation is correctly balanced for the complete combustion of ethane, C2H6?

A C2H6 + O2 → CO2 + H2O
B 2C2H6 + 7O2 → 4CO2 + 6H2O
C C2H6 + 7O2 → 2CO2 + 3H2O
D 2C2H6 + 5O2 → 4CO2 + 6H2O

17. What is the formula of calcium hydroxide?

A CaOH
B Ca(OH)2
C Ca2OH
D CaOH2

18. What is the formula of aluminium nitrate?

A AlNO3
B Al(NO3)2
C Al(NO3)3
D Al3NO3

19. Which equation has the lowest whole-number coefficients when balanced?

A H2 + Cl2 → 2HCl
B 2H2 + 2Cl2 → 4HCl
C 4H2 + 4Cl2 → 8HCl
D H + Cl → HCl

20. Which substance has a relative molecular mass of 44?

[Ar: C, 12; H, 1; O, 16; N, 14]

A CO
B CO2
C C2H6
D NO

Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total A Grades, 11 World Records and 7 Distinctions, Educate A Change.

21. A compound contains 27.3% carbon and 72.7% oxygen by mass.

[Ar: C, 12; O, 16]

What is its empirical formula?

A CO
B CO2
C C2O
D C2O3

22. A compound has molecular formula C4H8O2.

Which empirical formula is correct?

A C4H8O2
B C2H4O
C CH2O
D C2H4O2

23. A sample of hydrated sodium carbonate has formula Na2CO3.10H2O.

What is its relative formula mass?

[Ar: Na, 23; C, 12; O, 16; H, 1]

A 106
B 180
C 286
D 392

24. What percentage by mass of water is present in Na2CO3.10H2O?

[Mr of Na2CO3.10H2O = 286; Mr of 10H2O = 180]

A 37.1%
B 42.0%
C 62.9%
D 70.0%

25. Which formula is incorrect?

A sodium sulfate: Na2SO4
B potassium carbonate: K2CO3
C magnesium nitrate: MgNO3
D calcium chloride: CaCl2

26. What is the correct formula for lead(II) sulfate?

A PbSO4
B Pb2SO4
C Pb(SO4)2
D Pb2(SO4)3

27. Which equation correctly shows conservation of atoms?

A CaCO3 → CaO + CO2
B CaCO3 → Ca + CO3
C CaCO3 → CaO2 + CO
D CaCO3 → CaO + C + O

28. What is the sum of all coefficients when the equation is balanced?

Fe2O3 + CO → Fe + CO2

A 4
B 6
C 8
D 10

29. Which equation for photosynthesis is balanced?

A CO2 + H2O → C6H12O6 + O2
B 6CO2 + 6H2O → C6H12O6 + 6O2
C 6CO2 + H2O → C6H12O6 + O2
D CO2 + 6H2O → C6H12O6 + 6O2

30. What is the formula of zinc phosphate?

Zinc ion = Zn2+
Phosphate ion = PO4 3–

A ZnPO4
B Zn2PO4
C Zn3(PO4)2
D Zn2(PO4)3

Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total A Grades, 11 World Records and 7 Distinctions, Educate A Change.

31. A compound contains 2.4 g carbon, 0.4 g hydrogen and 3.2 g oxygen.

[Ar: C, 12; H, 1; O, 16]

What is its empirical formula?

A CH2O
B C2H4O
C CH4O
D C2H4O2

32. A hydrocarbon contains 85.7% carbon and 14.3% hydrogen by mass.

[Ar: C, 12; H, 1]

What is its empirical formula?

A CH
B CH2
C C2H4
D C3H8

33. The empirical formula of a hydrocarbon is CH2. Its relative molecular mass is 84.

What is its molecular formula?

[Ar: C, 12; H, 1]

A C3H6
B C4H8
C C5H10
D C6H12

34. Which formula represents a compound containing 40.0% calcium, 12.0% carbon and 48.0% oxygen by mass?

[Ar: Ca, 40; C, 12; O, 16]

A CaCO3
B CaC2O4
C CaCO2
D Ca2CO3

35. Which formula has the highest percentage by mass of nitrogen?

[Ar: H, 1; N, 14; O, 16]

A NH3
B NO
C NO2
D N2O

36. What is the percentage by mass of oxygen in sulfur trioxide, SO3?

[Ar: S, 32; O, 16]

A 20%
B 40%
C 60%
D 80%

37. Which equation is balanced and uses correct formulae?

A 2Na + Cl2 → 2NaCl
B Na + Cl → NaCl
C Na2 + Cl2 → 2NaCl
D Na + Cl2 → NaCl2

38. What is the correct formula of ammonium sulfate?

A NH4SO4
B (NH4)2SO4
C NH4(SO4)2
D (NH4)3SO4

39. Which pair contains formulae with the same relative formula mass?

[Ar: C, 12; H, 1; N, 14; O, 16]

A CO and N2
B CO2 and N2O
C CH4 and NH3
D H2O and NH3

40. A compound contains only carbon and hydrogen. Its empirical formula is CH and its relative molecular mass is 78.

What is its molecular formula?

[Ar: C, 12; H, 1]

A C2H2
B C4H4
C C6H6
D C8H8

Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total A Grades, 11 World Records and 7 Distinctions, Educate A Change.

41. Which formula represents an ionic compound made from Al3+ and CO3 2– ions?

A AlCO3
B Al2CO3
C Al2(CO3)3
D Al3(CO3)2

42. Which statement about relative formula mass is correct?

A It is found by adding the relative atomic masses of all atoms shown in the formula.
B It is found by adding only the relative atomic masses of metals in the formula.
C It is always equal to the empirical formula mass.
D It can only be calculated for covalent molecules.

43. What is the coefficient of oxygen when propane is completely combusted?

C3H8 + O2 → CO2 + H2O

A 3
B 4
C 5
D 6

44. Which equation is incorrectly balanced?

A 2H2 + O2 → 2H2O
B N2 + 3H2 → 2NH3
C 2Al + 3Cl2 → 2AlCl3
D 2K + 2H2O → 2KOH + 2H2

45. Which formula represents a compound with Mr = 98?

[Ar: H, 1; S, 32; O, 16; P, 31]

A H2SO4
B H3PO4
C H2SO3
D HPO3

46. A compound has empirical formula NO2 and relative molecular mass 92.

What is its molecular formula?

[Ar: N, 14; O, 16]

A NO2
B N2O4
C N3O6
D N4O8

47. Which formula has the lowest relative formula mass?

[Ar: Li, 7; Na, 23; K, 39; O, 16; Cl, 35.5]

A Li2O
B NaCl
C KCl
D Na2O

48. Which equation correctly represents thermal decomposition of copper(II) carbonate?

A CuCO3 → CuO + CO2
B CuCO3 → Cu + CO3
C CuCO3 → CuO2 + CO
D CuCO3 → Cu2O + CO2

49. Which formula is both molecular formula and empirical formula?

A C2H4
B C6H12O6
C H2O
D C4H10

50. A compound has formula X2Y3. The relative atomic mass of X is 56. The relative formula mass of X2Y3 is 160.

What is the relative atomic mass of Y?

A 16
B 24
C 32
D 48

Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total A Grades, 11 World Records and 7 Distinctions, Educate A Change.

1. C

Mr of Al2(SO4)3
= 2(27) + 3[32 + 4(16)]
= 54 + 3(96)
= 54 + 288
= 342

A wrong: Too low; does not include all sulfate groups.
B wrong: Incorrect addition of atoms.
C right: 342.
D wrong: Too high.

2. C

NH4+ has charge +1.
PO4 3– has charge –3.
Three ammonium ions are needed to balance one phosphate ion.

A wrong: Charge +1 and –3 do not balance.
B wrong: +2 and –3 do not balance.
C right: (NH4)3PO4.
D wrong: Puts phosphate in the wrong ratio.

3. B

Copper(II) ion = Cu2+
Nitrate ion = NO3–
Two nitrate ions are needed for one copper(II) ion.

A wrong: CuNO3 would be copper(I) nitrate.
B right: Cu(NO3)2.
C wrong: Incorrect ratio.
D wrong: Incorrect ratio.

4. C

Mr of MgSO4 = 24 + 32 + 4(16) = 120
Mass of water = 246 – 120 = 126
One H2O = 18
x = 126 / 18 = 7

A wrong: 5 waters would add 90 only.
B wrong: 6 waters would add 108 only.
C right: x = 7.
D wrong: 10 waters would give 300.

5. B

Assume 100 g.
C: 52.2 / 12 = 4.35
H: 13.0 / 1 = 13.0
O: 34.8 / 16 = 2.175

Divide by 2.175:
C = 2, H = 6, O = 1

Empirical formula = C2H6O.

A wrong: CH3O has wrong carbon ratio.
B right: C2H6O.
C wrong: Too much oxygen.
D wrong: Same ratio as C2H6O but not simplest.

6. C

Empirical formula mass of CH2O = 12 + 2 + 16 = 30
Multiplier = 180 / 30 = 6
Molecular formula = C6H12O6.

A wrong: Mr would be 30.
B wrong: Mr would be 90.
C right: Mr = 180.
D wrong: Mr would be 360.

7. B

A wrong: Oxygen atoms are not balanced.
B right: 2Mg + O2 → 2MgO.
C wrong: Oxygen atoms are not balanced.
D wrong: Mg2 is not the correct formula for magnesium.

8. C

Sodium carbonate reacts with hydrochloric acid to form sodium chloride, water and carbon dioxide.
Balanced equation: Na2CO3 + 2HCl → 2NaCl + H2O + CO2.

A wrong: Sodium carbonate formula is wrong.
B wrong: Not balanced.
C right: Correct formulae and balanced.
D wrong: Sodium chloride formula is NaCl, not NaCl2.

9. B

Iron(III) = Fe3+
Oxide = O2–
Balance charges: 2Fe3+ and 3O2– gives Fe2O3.

A wrong: FeO is iron(II) oxide.
B right: Fe2O3.
C wrong: Charges do not balance correctly.
D wrong: Charges do not balance.

10. B

Empirical formula mass of C2H5 = 2(12) + 5 = 29
Multiplier = 58 / 29 = 2
Molecular formula = C4H10.

A wrong: Mr would be 29.
B right: C4H10.
C wrong: Mr would be 87.
D wrong: Mr would be 116.

Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total A Grades, 11 World Records and 7 Distinctions, Educate A Change.

11. B

A wrong: Molecular formula shows actual number of atoms in one molecule.
B right: Empirical formula shows the simplest whole-number ratio.
C wrong: Empirical and molecular formulae can be the same.
D wrong: Ionic compounds also have empirical formulae.

12. D

NaCl = 23 + 35.5 = 58.5
MgO = 24 + 16 = 40
Al2O3 = 2(27) + 3(16) = 102
MgSO4 = 24 + 32 + 4(16) = 120

A wrong: 58.5.
B wrong: 40.
C wrong: 102.
D right: 120, the greatest.

13. C

C2H5OH = C2H6O
Mr = 2(12) + 6(1) + 16
= 24 + 6 + 16
= 46

A wrong: Too low.
B wrong: Misses one hydrogen.
C right: 46.
D wrong: Too high.

14. C

Benzene C6H6 simplifies to CH.

A wrong: C2H6 simplifies to CH3.
B wrong: C3H8 does not simplify to CH.
C right: C4H4 simplifies to CH.
D wrong: C6H12 simplifies to CH2.

15. A

Assume 100 g.
C: 40.0 / 12 = 3.33
H: 6.7 / 1 = 6.7
O: 53.3 / 16 = 3.33

Ratio = 1 : 2 : 1
Empirical formula = CH2O.

A right: CH2O.
B wrong: Molecular formula, not simplest.
C wrong: Hydrogen ratio too low.
D wrong: Molecular formula, not simplest.

16. B

Ethane combustion:
2C2H6 + 7O2 → 4CO2 + 6H2O.

A wrong: Not balanced.
B right: Correct balanced equation.
C wrong: Oxygen and carbon balance are wrong.
D wrong: Oxygen is not balanced.

17. B

Calcium ion = Ca2+
Hydroxide ion = OH–
Two hydroxide ions are needed.

A wrong: Charge does not balance.
B right: Ca(OH)2.
C wrong: Incorrect ratio.
D wrong: Brackets are needed to show two OH groups.

18. C

Aluminium ion = Al3+
Nitrate ion = NO3–
Three nitrate ions are needed.

A wrong: Charge does not balance.
B wrong: Charge does not balance.
C right: Al(NO3)3.
D wrong: Incorrect formula.

19. A

A right: H2 + Cl2 → 2HCl uses the lowest whole-number coefficients.
B wrong: Balanced but not lowest coefficients.
C wrong: Balanced but not lowest coefficients.
D wrong: Hydrogen and chlorine exist as H2 and Cl2 molecules, not H and Cl in this equation.

20. B

CO = 12 + 16 = 28
CO2 = 12 + 2(16) = 44
C2H6 = 24 + 6 = 30
NO = 14 + 16 = 30

A wrong: 28.
B right: 44.
C wrong: 30.
D wrong: 30.

Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total A Grades, 11 World Records and 7 Distinctions, Educate A Change.

21. B

Assume 100 g.
C: 27.3 / 12 = 2.275
O: 72.7 / 16 = 4.544

Divide by 2.275:
C = 1, O = 2

Empirical formula = CO2.

A wrong: Oxygen ratio too low.
B right: CO2.
C wrong: Carbon ratio too high.
D wrong: Not the correct ratio.

22. B

C4H8O2 ratio = 4 : 8 : 2
Divide by 2 = 2 : 4 : 1
Empirical formula = C2H4O.

A wrong: Molecular formula, not simplest.
B right: C2H4O.
C wrong: Divides oxygen incorrectly.
D wrong: Same as original ratio partly, not simplest.

23. C

Na2CO3.10H2O
Na2CO3 = 2(23) + 12 + 3(16) = 106
10H2O = 10(18) = 180
Total Mr = 106 + 180 = 286

A wrong: Only anhydrous sodium carbonate.
B wrong: Only water of crystallisation.
C right: 286.
D wrong: Too high.

24. C

Percentage water = 180 / 286 × 100
= 62.9%

A wrong: This is close to anhydrous salt percentage.
B wrong: Incorrect calculation.
C right: 62.9%.
D wrong: Too high.

25. C

A wrong: Na2SO4 is correct.
B wrong: K2CO3 is correct.
C right: Magnesium nitrate should be Mg(NO3)2, so MgNO3 is incorrect.
D wrong: CaCl2 is correct.

26. A

Lead(II) ion = Pb2+
Sulfate ion = SO4 2–
Charges balance 1:1.

A right: PbSO4.
B wrong: Incorrect ratio.
C wrong: Would require Pb4+.
D wrong: Incorrect ratio.

27. A

A right: CaCO3 → CaO + CO2 conserves Ca, C and O atoms.
B wrong: CO3 is not shown as a stable product here.
C wrong: Product formulae are wrong for this decomposition.
D wrong: Not the proper equation for calcium carbonate decomposition.

28. Faulty question

Balanced equation:
Fe2O3 + 3CO → 2Fe + 3CO2

Sum of coefficients = 1 + 3 + 2 + 3 = 9.

A wrong: 4 is not correct.
B wrong: 6 is not correct.
C wrong: 8 is not correct.
D wrong: 10 is not correct.

Correct answer should be 9, but 9 is not given as an option.

29. B

Photosynthesis:
6CO2 + 6H2O → C6H12O6 + 6O2.

A wrong: Not balanced.
B right: Correct balanced equation.
C wrong: Hydrogen and oxygen are not balanced.
D wrong: Carbon is not balanced.

30. C

Zinc ion = Zn2+
Phosphate ion = PO4 3–
3Zn2+ = +6
2PO4 3– = –6
Formula = Zn3(PO4)2.

A wrong: Charges do not balance.
B wrong: Charges do not balance.
C right: Zn3(PO4)2.
D wrong: Charges do not balance.

Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total A Grades, 11 World Records and 7 Distinctions, Educate A Change.

31. A

C: 2.4 / 12 = 0.2
H: 0.4 / 1 = 0.4
O: 3.2 / 16 = 0.2

Ratio = 1 : 2 : 1
Empirical formula = CH2O.

A right: CH2O.
B wrong: Same type of ratio but not simplest.
C wrong: Too much hydrogen.
D wrong: Not simplest.

32. B

Assume 100 g.
C: 85.7 / 12 = 7.14
H: 14.3 / 1 = 14.3

Ratio = 1 : 2
Empirical formula = CH2.

A wrong: Hydrogen ratio too low.
B right: CH2.
C wrong: Same ratio as CH2 but not empirical.
D wrong: Ratio is 3 : 8, not 1 : 2.

33. D

Empirical formula mass of CH2 = 12 + 2 = 14
Multiplier = 84 / 14 = 6
Molecular formula = C6H12.

A wrong: Mr = 42.
B wrong: Mr = 56.
C wrong: Mr = 70.
D right: Mr = 84.

34. A

Assume 100 g.
Ca: 40.0 / 40 = 1
C: 12.0 / 12 = 1
O: 48.0 / 16 = 3

Formula = CaCO3.

A right: CaCO3.
B wrong: Oxygen ratio too high.
C wrong: Oxygen ratio too low.
D wrong: Calcium ratio too high.

35. D

NH3: nitrogen % = 14 / 17 × 100 = 82.4%
NO: nitrogen % = 14 / 30 × 100 = 46.7%
NO2: nitrogen % = 14 / 46 × 100 = 30.4%
N2O: nitrogen % = 28 / 44 × 100 = 63.6%

A right by calculation: NH3 has the highest nitrogen percentage.
B wrong: 46.7%.
C wrong: 30.4%.
D wrong: 63.6%.

Correct answer: A

36. C

Mr of SO3 = 32 + 3(16) = 80
Mass of oxygen = 48
Percentage oxygen = 48 / 80 × 100 = 60%

A wrong: Too low.
B wrong: This is sulfur percentage.
C right: 60%.
D wrong: Too high.

37. A

A right: 2Na + Cl2 → 2NaCl is balanced and uses correct formulae.
B wrong: Chlorine should be Cl2.
C wrong: Sodium is Na, not Na2.
D wrong: Sodium chloride is NaCl, not NaCl2.

38. B

Ammonium ion = NH4+
Sulfate ion = SO4 2–
Two ammonium ions are needed.

A wrong: Charge does not balance.
B right: (NH4)2SO4.
C wrong: Incorrect ratio.
D wrong: Charge does not balance.

39. A

CO = 12 + 16 = 28
N2 = 2(14) = 28

A right: Both are 28.
B wrong: CO2 = 44 and N2O = 44 also both 44, so this question has another correct pair.
C wrong: CH4 = 16, NH3 = 17.
D wrong: H2O = 18, NH3 = 17.

This question is faulty because both A and B have equal relative formula masses.

40. C

Empirical formula mass of CH = 13
Multiplier = 78 / 13 = 6
Molecular formula = C6H6.

A wrong: Mr = 26.
B wrong: Mr = 52.
C right: Mr = 78.
D wrong: Mr = 104.

Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total A Grades, 11 World Records and 7 Distinctions, Educate A Change.

41. C

Al3+ and CO3 2– must balance.
2Al3+ = +6
3CO3 2– = –6
Formula = Al2(CO3)3.

A wrong: Charges do not balance.
B wrong: Carbonate brackets/ratio incorrect.
C right: Al2(CO3)3.
D wrong: Charges do not balance.

42. A

A right: Relative formula mass is found by adding all relative atomic masses in the formula.
B wrong: Non-metal atoms must also be included.
C wrong: It is not always equal to empirical formula mass.
D wrong: It can be calculated for ionic and covalent substances.

43. C

Propane combustion:
C3H8 + 5O2 → 3CO2 + 4H2O

A wrong: Too little oxygen.
B wrong: Too little oxygen.
C right: Coefficient of O2 is 5.
D wrong: Too much oxygen.

44. D

A wrong: 2H2 + O2 → 2H2O is balanced.
B wrong: N2 + 3H2 → 2NH3 is balanced.
C wrong: 2Al + 3Cl2 → 2AlCl3 is balanced.
D right: Incorrectly balanced. Correct equation is 2K + 2H2O → 2KOH + H2.

45. Faulty question

H2SO4 = 2(1) + 32 + 4(16) = 98
H3PO4 = 3(1) + 31 + 4(16) = 98
H2SO3 = 2 + 32 + 48 = 82
HPO3 = 1 + 31 + 48 = 80

A right: Mr = 98.
B right: Mr = 98.
C wrong: Mr = 82.
D wrong: Mr = 80.

This question is faulty because both A and B have Mr = 98.

46. B

Empirical formula mass of NO2 = 14 + 2(16) = 46
Multiplier = 92 / 46 = 2
Molecular formula = N2O4.

A wrong: Mr = 46.
B right: Mr = 92.
C wrong: Mr = 138.
D wrong: Mr = 184.

47. A

Li2O = 2(7) + 16 = 30
NaCl = 23 + 35.5 = 58.5
KCl = 39 + 35.5 = 74.5
Na2O = 2(23) + 16 = 62

A right: 30, the lowest.
B wrong: 58.5.
C wrong: 74.5.
D wrong: 62.

48. A

Copper(II) carbonate decomposes to copper(II) oxide and carbon dioxide.

A right: CuCO3 → CuO + CO2.
B wrong: Cu and CO3 are not the correct products.
C wrong: Product formulae are wrong.
D wrong: Copper(I) oxide is not the expected product.

49. C

A wrong: C2H4 simplifies to CH2, so molecular and empirical formulae differ.
B wrong: C6H12O6 simplifies to CH2O.
C right: H2O cannot be simplified, so it is both molecular and empirical.
D wrong: C4H10 simplifies to C2H5.

50. A

Mr of X2Y3 = 160
Mass of 2X = 2(56) = 112
Mass of 3Y = 160 – 112 = 48
Ar of Y = 48 / 3 = 16

A right: 16.
B wrong: Too high.
C wrong: Too high.
D wrong: This is total mass of three Y atoms, not one Y atom.

Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total A Grades, 11 World Records and 7 Distinctions, Educate A Change.