together, or when solids dissolve in water, can be solved directly. 19 If metals having volumes V1, V1, V ̧, &c. and densities 3 A1, A, A, are fused together to form an alloy having the volume V, and the density ▲, V1A, + VA, + VA,+ &c. = VA, Again, suppose that the masses of the metals mixed are M1, M2, M, and the mass of the alloy formed is M, if no permanent change of volume takes place, 3 Thus, the native alloy of mercury (A13.59) and gold (A19.3) (Hg, Au,) consists of 600 parts of mercury alloyed with 393-2 parts of gold, find its density if no contraction takes place. 600 393.2 993.2 х .. x= 15.4. The density is found by experiment to be 15-47, hence a small contraction takes place during the reaction. When solids dissolve in water the volume of the solution is usually less than the volume of the water and of the solid together. When 20 grams of nitre (A2·12) are dissolved in 80 grams of water the solution has the density 1.136, what would the volume have been if no contraction had taken place, and what is it? 20 80 2-12 1 = 9.56 +80= 89.56 c.c. if there had been no contraction. But 100 (10) FROM THE FORMULA OF A SUBSTANCE TO FIND ITS MOLECULAR WEIGHT AND PERCENTAGE COMPOSITION. The Atomic weight of an element is the number of times, which the smallest portion of that element, that can take part in a chemical change, is heavier than the smallest portion of hydrogen, that can take part in a chemical change. An atom of hydrogen is supposed to weigh 3.5 × 10-** grams. 24 These atomic weights are expressed by symbols, which are usually the first letters or first two letters of the English or Latin name of the element. Thus H represents the hydrogen-atom weighing 1 The molecular weight of an element or compound is the number of times which the smallest portion of that element or compound, that can exist in the free state, is heavier than the atom of hydrogen. Comparatively few of the molecular weights of the elements have yet been determined, they are usually multiples of the atomic weights by 2, but in some instances by 1, 4, or 6. K ̧=39·1 × 2 = 78.2 is the molecular weight of Potassium. The molecular weight of a compound is the sum of the atomic weights of all the atoms of the elements which compose it. Thus the molecular weight of hydrogen chloride HCl is 1+ 35.5 = 36.5. The molecular weight of water H2O is The molecular weight of hydrogen sulphate H,SO, is 1 x 2 + 32+ 16 x 4 = 98. 4 From the symbol, also, the percentage composition, or number of grams of each element in 100 grams of a compound, may be readily calculated. The name of each element present being written in a column, and opposite to each the multiple of its atomic weight which is present in the compound, on adding these numbers together the molecular weight of the compound is obtained. Since the numbers opposite to each element express the number of parts of that element present in the molecular weight of the compound, on multiplying each of these numbers by 100 and dividing by the molecular weight, parts in 100 or percentages are obtained. Thus, to find the percentage composition of carbon di (11) FROM AN EQUATION TO CALCULATE A MASS. A chemical equation expresses that certain substances present in definite proportions react upon one another to form certain other substances also in definite proportion. Since matter is indestructible, the sum of the masses or molecular weights of the substances before the change must be equal to the sum of the masses or molecular weights of the substances after the change. Thus Mg+0= MgO expresses that 24 parts of magnesium react with 16 parts of oxygen to form 24+ 16 or 40 parts of magnesium oxide. And if it be required to find how many grams of magnesium oxide are formed on burning 10 grams of magnesium, the equation shews that 24 grams of magnesium form 40 grams of magnesium oxide, The same result may be reached with still greater rapidity by the use of simple proportion. Find the multiples of the atomic or molecular weights of the substances given and asked in the equation, and make the proportion :— The molecular weight of substance given: the molecular weight of substance asked :: the real mass of substance given the real mass of substance asked. : Thus, to find how many grams of sodium sulphate can be obtained from 100 grams of sodium hydrate, 2HNaO+H,SO, 2H,O+ Na,SO. 2 × 40 2 × 40: 142 :: 100 : x 142 When the action of pressure upon fluids is investigated, it is found that some fluids, e. g. water and mercury, decrease in volume very slightly even under a greatly increased pressure; such fluids are roughly said to be incompressible : but other fluids, e.g. air and hydrogen, are found to decrease in volume considerably under increased pressure; such fluids are said to be compressible. |