18.    The law of multiple proportions says: When two elements form a series of compounds, the ratios of the masses of the element A that combines with 1 gram of  element B can always be reduced to small whole numbers.

Compound I has 1.188g of F for each 1.000g of S

Compound II has 2.375g of F for each 1.000g of S

Compound III has 3.563g of F for each 1.000g of S

So:  I/II = 1.188g/2.235g = 1/2  or a ratio of 1:2.  Compound II has twice as many F atoms per S atom compared with compound I

       II/III = 2.375g/3.563g = 2/3 or a ratio of 2:3

       I/III = 1.188g/3.563g = 1/3 or a ratio of 1:3    Compound III has three times as many F atoms per S atom compared with I
 

Two possibilities (there are many others) for the formulae are:
 
 

Compound I	FS		F2S
Compound II	F2S		F4S
Compound III	F3S		F6S

20(a).    The law of definite proportions says: A given compound always contains the same proportion of elements by mass. (Proust's law)

So it doesn't matter if we start with excess of  H₂ or Cl₂, they will still combine to form the same product - there will simply be some H₂ or Cl₂ left over at the end.

20(b).

            The reaction forming HCl is as follows:

                        H₂ + Cl₂      —>      2HCl

            Remember that equal volumes of gases at the same temperature and pressure contain the same number of particles. (Avogadro's hypothesis) If a volume contains 100 particles of H₂ and 100 particles of Cl₂ (200 particles in total) and then they react, they will produce 200 particles of HCl.  Thus the number of particles in the vessel will remain the same, and so the volume of the product will be exactly the same as the starting material.

22.        The particles in the cathode ray tube were repelled by the negative plate of an electric field and hence must be negatively charged.

24.    Mass of electron = 9.11 x 10^-31 kg = 9.11 x 10^-28 g    (page 28)
         Mass of proton = 1.67 x 10^-27 kg = 1.67 x 10^-24 g    (page 28)
         Diameter of nucleus = 1 x 10^-13 cm so radius = 5 x 10^-14 cm    (page 27)
         Diameter of atom = 1 x 10^-8 cm    cm so radius = 5 x 10^-9 cm (page 27)

            We will assume that the nucleus and the atom are both spherical in shape.  The volume of a sphere is given by:

                                                            V = 4/3 r³

            So the volume of the nucleus is       4/3 x (5x10^-14 )³cm x = 5.24x10^-40  cm³

            And the volume of the atom is        4/3 x (5 x 10^-9)³cm x = 5.24x10^-25  cm³

Density is given by D = Mass/Volume                    The SI units are kilograms per cubic metre, but chemists usually use grams per cubic centimetre.
 

            So Density of the nucleus is 1.67 x 10^-24 g / 5.24x10^-40 cm³ = 3x10¹⁵ g/cm³

            Density of the hydrogen atom is 1.67 x 10^-27 g / 5.24x10^-25 = 3x10^-3 g/cm³
 

Note:

• It is good to keep extra significant figures during intermediate calculations, and round off ONCE at the end.  Why only one sig. fig. in the answers?  Because the diameter and radius given had only one s.f.
• The density of water at 20 deg.C is about 1.0