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InkFlaskCambridge IGCSE

Pupil sheet · Relative masses

How heavy is a water molecule? — questions

Name ______________________ Date ____________

Try every question before you use a nudge. Write in full sentences where you are asked to explain.

1. describe

Which statement describes the relative atomic mass, Ar, of an element?

Circle one answer.

  • The average mass of its isotopes, compared to 1/12 of the mass of a carbon-12 atom
  • The total number of protons and neutrons in one atom of the element
  • The mass of its most common isotope, compared to 1/12 of the mass of a carbon-12 atom
  • The actual mass of one atom of the element, measured in grams

Nudge (only if you are stuck): Which statement would still work for chlorine, whose Ar is 35.5?

2. explain / deduce

The Periodic Table gives silver a relative atomic mass of 108, yet no silver atom has a mass of 108. Silver has two isotopes, silver-107 and silver-109. (a) Explain why the relative atomic mass is 108. (b) Deduce what 108 tells you about how common the two isotopes are.

(a) Why the relative atomic mass is 108

(b) How common the isotopes are

Nudge (only if you are stuck): Ar is worked out from many atoms, not one. Where does 108 lie between 107 and 109?

3. deduce

Gallium has two isotopes, gallium-69 and gallium-71. Its relative atomic mass is 69.7. Which statement is correct?

Circle one answer.

  • Gallium-69 atoms are more common, because 69.7 is nearer 69
  • The two isotopes are equally common, because 69.7 is between 69 and 71
  • You cannot tell anything about the isotopes from Ar
  • Gallium-71 atoms are more common, because they are heavier

Nudge (only if you are stuck): Where is 69.7: halfway between 69 and 71, or nearer one of them?

4. calculate / identify

Methane, CH4, is made of molecules. Potassium oxide, K2O, is made of ions. (Ar: H 1, C 12, O 16, K 39.) Which row is correct?

RowMr of CH4Mr of K2OName for the Mr of K2O
A1655relative formula mass
B1394relative formula mass
C1694relative formula mass
D1694relative molecular mass

Circle one answer.

  • A
  • B
  • C
  • D

Nudge (only if you are stuck): Work out both Mr values first. Then ask: does potassium oxide have molecules?

5. define

Complete the definitions. Relative molecular mass, Mr, is the ___ of the relative atomic masses of the atoms in a formula. For an ionic compound, such as potassium iodide, the same sum is called the relative ___ mass.

Gap 1

Gap 2

Nudge (only if you are stuck): What do you do with the Ar values, and what does an ionic compound have instead of molecules?

6. calculate

Calculate the relative formula mass of iron(III) hydroxide, Fe(OH)3, calcium nitrate, Ca(NO3)2, and aluminium sulfate, Al2(SO4)3. (Ar: H 1, N 14, O 16, Al 27, S 32, Ca 40, Fe 56.)

Mr of Fe(OH)3: ______________ unit ________

Mr of Ca(NO3)2: ______________ unit ________

Mr of Al2(SO4)3: ______________ unit ________

Nudge (only if you are stuck): The number after a bracket multiplies everything inside it.

7. calculate

Pink crystals of hydrated cobalt(II) chloride are CoCl2·6H2O. (a) Calculate the relative formula mass of CoCl2·6H2O. (b) Calculate how much of that relative formula mass is water. (Ar: H 1, O 16, Cl 35.5, Co 59.)

(a) Mr of CoCl2·6H2O: ______________ unit ________

(b) Water in it: ______________ unit ________

Nudge (only if you are stuck): What does the 6 in front of H2O count?

8. identify

Which name does the Mr of each substance have: relative molecular mass or relative formula mass?

Write the number of the matching option.

  1. Relative molecular mass
  2. Relative formula mass
  • Sulfur dioxide, SO2: ______
  • Potassium bromide, KBr: ______
  • Lithium fluoride, LiF: ______
  • Ammonia, NH3: ______
  • Calcium chloride, CaCl2: ______

Nudge (only if you are stuck): Is the substance made of molecules, or of ions in a giant lattice?

9. calculate

Almost every sulfur atom is one of three isotopes. In a sample of sulfur, 95 % of the atoms are sulfur-32, 1 % are sulfur-33 and 4 % are sulfur-34. Calculate the relative atomic mass of sulfur. Give your answer to one decimal place.

Ar of sulfur: ______________ unit ________

Nudge (only if you are stuck): Imagine 100 atoms of the sample: find their total mass, then share it between the 100 atoms.

10. calculate

High in the atmosphere, some oxygen exists as ozone, O3, a gas that absorbs ultraviolet light from the Sun. (Ar: O 16. A carbon-12 atom is 12 on this scale.) (a) Calculate the relative molecular mass of ozone. (b) Calculate how many times as heavy as an oxygen molecule, O2, an ozone molecule is. (c) Calculate how many times as heavy as an atom of carbon-12 an ozone molecule is.

(a) Mr of O3: ______________ unit ________

(b) Times as heavy as O2: ______________ unit ________

(c) Times as heavy as carbon-12: ______________ unit ________

Nudge (only if you are stuck): Relative masses are all on one scale, so you can divide one by another.

11. calculate / explain

Lithium has two isotopes, lithium-6 and lithium-7. In natural lithium there is 1 lithium-6 atom for every 12 lithium-7 atoms. A second sample has had some of its lithium-6 removed, and it now has 1 lithium-6 atom for every 30 lithium-7 atoms. (a) Calculate the relative atomic mass of natural lithium. (b) Calculate the relative atomic mass of the second sample. Give both answers to two decimal places. Then explain why the two values are different.

(a) Ar of natural lithium: ______________ unit ________

(b) Ar of the second sample: ______________ unit ________

Why the two values are different

Nudge (only if you are stuck): Imagine a small group of atoms that matches each ratio.