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Year 10 Science — How atoms combine, and what a molecule weighs

Use the laws of conservation of mass and of constant proportions, tell a covalent bond from an ionic one, write a formula by crossing over valencies, and add up a molecular or formula unit mass.

Name: ________________________

Before you start

A sodium atom beside a chlorine atom, with each shell named.

1234
  1. 1. Sodium's inner shells, holding 2 then 8 electrons
  2. 2. Sodium's valence shell, holding its single electron
  3. 3. Chlorine's inner shells, holding 2 then 8 electrons
  4. 4. Chlorine's valence shell, holding 7 electrons

Sodium has one electron more than a full inner arrangement; chlorine is one short of eight. Hand the electron across and both are satisfied — that is an ionic bond in one move.

  1. 1.Taking K as 39 u and Cl as 35.5 u, what is the formula unit mass of potassium chloride, KCl, in u? Answer with a number.

  2. 2.A reaction is carried out in a sealed flask on a balance. What does the Law of Conservation of Mass predict about the reading?

    • a) It falls, because some matter is used up
    • b) It rises, because new substances have been made
    • c) It is the same before and after the reaction
    • d) It cannot be predicted without knowing the products

  3. 3.Why is sodium chloride described by a formula unit mass rather than a molecular mass?

    • a) Its atoms share electrons instead of transferring them
    • b) Its formula has more than one kind of atom in it
    • c) Its ions form a crystal rather than separate molecules
    • d) It is too heavy for a molecular mass to be worked out

  4. 4.Hydrogen and oxygen combine to make water in the mass ratio 1 to 8. What mass of oxygen combines with 3 g of hydrogen, in g? Answer with a number.

  5. 5.Taking C as 12 u and O as 16 u, what is the molecular mass of carbon dioxide, CO2, in u? Answer with a number.

  6. 6.Taking H as 1 u, N as 14 u and O as 16 u, what is the molecular mass of nitric acid, HNO3, in u? Answer with a number.

  7. 7.Taking C as 12 u and H as 1 u, what is the molecular mass of methane, CH4, in u? Answer with a number.

  8. 8.Which of these describes a molecule, as the chapter defines it?

    • a) An electrically neutral group of more than one atom that can exist on its own
    • b) A charged particle formed when an atom gains an electron
    • c) The simplest whole-number ratio of ions in a crystal
    • d) A single atom of any element

Answer key — Year 10 Science — How atoms combine, and what a molecule weighs

  1. 1. 74.5
  2. 2. c) It is the same before and after the reaction
  3. 3. c) Its ions form a crystal rather than separate molecules
  4. 4. 24
  5. 5. 44
  6. 6. 63
  7. 7. 16
  8. 8. a) An electrically neutral group of more than one atom that can exist on its own