A substance containing 2.04% H, 32.6% S, and 65.3% O was found to have a molecular weight of 98. In these 98 parts of the substance there are 98 x 2.04% = 2 parts of H, 98 x 32.6% = 32 parts of S, and 98 x 65.3% = 64 parts of O. If the molecule weighs 98, the hydrogen atoms present must together weigh 2, the sulphur atoms 32, and the oxygen atoms 64. Dividing these figures by the respective atomic weights of the three elements, we have, for H, 2 / 1 = 2 atoms; for S, 32 / 32 = 1 atom; for O, 64 / 16 = 4 atoms. Hence the formula is H_{2}SO_{4}. We have, then, this general procedure: Find the percentage composition of the substance and also its molecular weight. Multiply the molecular weight successively by the percentage of each element present, to find the amount of the element in the molecular weight of the compound. The figures so obtained will be the respective parts of the molecular weight due to the several atoms. Divide by the atomic weights of the respective elements, and the quotient will be the number of atoms present. ~Avogadro's hypothesis and chemical calculations.~ This law simplifies many chemical calculations. 1. _Application to volume relations in gaseous reactions._ Since equal volumes of gases contain an equal number of molecules, it follows that when an equal number of gaseous molecules of two or more gases take part in a reaction, the reaction will involve equal volumes of the gases. In the equation C_{2}H_{2}O_{4} = H_{2}O + CO_{2} + CO, since 1 molecule of each of the gases CO_{2} and CO is set free from each molecule of oxalic acid, the two substances must always be set free in equal volumes. Acetylene burns in accordance with the equation 2C_{2}H_{2} + 5O_{2} = 4CO_{2} + 2H_{2}O. Hence 2 volumes of acetylene will react with 5 volumes of oxygen to form 4 volumes of carbon dioxide and 2 volumes of steam. That the volume relations may be correct a gaseous element must be given its molecular formula. Thus oxygen must be written O_{2} and not 2O. 2. _Application to weights of gases._ It will be recalled that the molecular weight of a gas is determined by ascertaining the weight of 22.4 l. of the gas. This weight in grams is called the _gram-molecular weight_ of a gas. If the molecular weight of any gas is known, the weight of a liter of the gas under standard conditions may be determined by dividing its gram-molecular weight by 22.4. Thus the gram-molecular weight of a hydrochloric acid gas is 36.458. A liter of the gas will therefore weigh 36.458 / 22.4 = 1.627 g. EXERCISES 1. From the following data calculate the atomic weight of sulphur. The equivalent, as obtained by an analysis of sulphur dioxide, is 16.03. The densities and compositions of a number of compounds containing sulphur are as follows: