Chemistry 11 The Law of Multiple Proportions

Chemistry 11 - Discussion Session 2 Fall 2010 The Law of Multiple Proportions 1. Copper (I) nitrite has 58.00 g Cu, 12.79 g N, and 29.21 g O. Ratio: 4.54 : 1 : 2.29 Copper (II) nitrate has 33.88 g Cu, 14.94 g N and 51.18 g O. Ratio: 2.27 : 1 : 3.43 Cu/N(1)/Cu/N(2) = 4.54/1 : 2.27/1 = 2 : 1 O/N(1) / O/N(2) = 2.29/1 : 3.43/1 = 2 : 3 Yes, they obey the law of multiple proportions. 2. Copper (I) nitrite: CuNO2 Copper (II) nitrate: Cu(NO3)2 3. There are twice as many atoms of nitrogen per atom of copper in compound 2 than in compound 1. This is the inverse of the Cu/N(1)/Cu/N(2) ratio. There are two atoms of oxygen per atom of nitrogen in compound 1 and three atoms of oxygen per atom of nitrogen in compound 2. The ratio of these numbers is the O/N(1)/O/N(2) ratio. Chemistry 11 - Discussion Session 2 Fall 2010 Evaluating Avogadro’s Hypothesis of Diatomic Elemental Gases - Solutions 1. In the given reaction, 2x particles of hydrogen react with 1x particles of oxygen to form 2x particles of water. On the smallest possible scale, 2 particles of hydrogen react with 1 particle of oxygen to form 2 particles of water. 2. If 2 particles of water are formed, each of which contains a minimum of 1 atom of oxygen, then there must be 2 atoms (minimum) of oxygen on the left side of the equation. The only way to have 2 atoms of oxygen but only 1 particle is to have that 1 particle be a molecule of O2. Now, since we do not know the formula of water, we cannot necessarily draw the analogous conclusion about hydrogen at this point. 3. For water, this question is actually a bit ill-posed (it was originally designed for the ammonia problem we did in lecture. When we changed it, we neglected to realize this part of the question should be modified as well. Apologies!), in fact, we cannot conclude that hydrogen is diatomic from the data given. Since each of the 2 water molecules (particles) needs to have the same number of hydrogen atoms, the total number of hydrogen atoms in the reaction must be an even number. But, since two volumes of hydrogen react, this can be the case whether or not hydrogen is diatomic. If hydrogen is not diatomic, we draw the conclusion that water has the formula OH. If water is diatomic, it leads to the conclusion that water is H2O. We would need more information than what is given to distinguish between these possibilities. 4. Due to conservation of mass, each of the 2 water particles has the formula H2O. Some of Avogadro's original discussion (a bit tough to get through) is given at http://webserver.lemoyne.edu/faculty/giunta/avogadro.html. The key point is that by making this assumption, it gave rise to a self-consistent interpretation of the experimental data, which then suggested that the assumption was indeed correct.