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Queen's University
CHEM 499
Ingrid Johnsrude

Law of Conservation of Mass In 1774, Antoine Lavoisier (1774-1794) performed an experiment in which he heated a sealed glass vessel containing a sample of tin and some air. He found that the glass before heating and the glass after heating were the same. Through further investigation, he proved that the product of the reaction, tin calx, consisted of the original tin together with a portion of the air. This proved that oxygen from air is essential to combustion and it led to the formulation of the Law of Conservation of Mass: the total mass of substance present after a chemical reaction is the same as the total mass of substances before the reaction. Problem: A 0.455g sample of Mg is allowed to burn in 2.315 g of O g2s. The sole produce is MgO. After the reaction, no magnesium remains and the mass of unreacted oxygen is 2.015g. What mass of magnesium oxide is produced? Solution Mass before reaction: 0.455g magnesium + 2.315g oxygen = 2.770 g 2.770g mass after reaction =? g magnesium oxide after reaction + 2.015g oxygen after reaction ? g magnesium oxide after reaction = 2.770 g mass after reaction – 2.015 g oxygen after reaction = 0.755 g magnesium oxide after reaction. Law of Constant Composition In 1799, Joseph Proust: Law of constant composition or law of definite proportions: All samples of a compound have the same composition – the same proportions by mass of the constituent elements. Consider H 2 at 10.000g: 1.119 g H = 11.19 % Consider H 2 at 27.000g: 3.021 g H = 11.19% 8.881 g O = 88.81% 23.979 g O= 88.81% Problem: 0.455g of Mg reacted with an excess of oxygen to produce 0.755g of magnesium oxide. According to the law of constant composition, the mast ratio: 0.455 g magnesium/0.755 g magnesium oxide should exist in all samples of magnesium oxide. Thus, in 0.500 g of magnesium oxide, the mass of magnesium is: Solution: ? g magnesium = 0.500 g magnesium oxide x (0.455 g magnesium/0.755 g magnesium oxide) = 0.301 g magnesium Dalton’s Atomic Theory 1. Each chemical element is composed of minute, indivisible particles called atoms. Atoms can be neither created nor destroyed during chemical change. 2. All atoms of an element are alike in mass (weight) and other properties, but the atoms of one element are different from those of all other elements. 3. In each of their compounds, different elements combine in a simple numerical ratio: for example, one atom of A to one of B (AB), or one atom of A to two of B (A2 ). Daltons 1 assumption explained the law of conservation of mass and his 2 ndand 3rd assumption explained the law of constant composition. This theory led to the prediction: the law of multiple proportions. If two elements form more than a single compound, the masses of one element combined with the fixed ratio of the - second are in the ration of small whole numbers (e.g. C2 , 2 O,2H O, N3 ) Chemical Elements # of A ±? # of Z E protons – # protons + of electrons # of # of protons A= mass number and Z= atomic number ***Z is often not specified when writing*** Example: An element with two stable isotopes has 35 protons. The separate isotopes contain 44 and 46 neutrons. What is the element
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