基础化学精品教学（广西医科大学）Experiment 1 Determination of the Atomic Mass of Magnesium by Replacement Reaction.docVIP

Determination of the Atomic Mass of Magnesium by Replacement Reaction 【Objective】 (1)rasp the operation method of analytical balance (2)To learn to collect gas and determine volume of gas (3)To understand and apply the ideal gas equation (4)To learn the principle and methods in determination of atomic mass of Mg 【Pre-lab Assignment】(1) What is the principle for determining the atomic mass of Mg by displacement method? (2) What factors may lead to the errors in this experiment? (3) What other methods may use to determine atomic weight or molecular weight? And what are their principles? How can the molecular weight of glucose or macro-moleular such as polysaccharide and protein be determined? Write down at least one method and present its designing route referring to the references in the pre-lab assignment. 【Principles】 Magnesium is an active metal, which can replace hydrogen of dilute sulphuric acid in dilute solution to form H2 and magnesium sulphate. The reaction is given following: H2SO4 + Mg ＝ H2 ↑ + MgSO4 (14.1) According to the stoichiometic relationship, when the magnesium ribbon is used up, the amount of substance of Mg is equal to that of H2. n (Mg) ＝ n (H2) ollecting and determining volume of gas produced in the reaction, observing pressure and temperature of gas, then the amount of substance of H2 can be calculated according to the ideal gas equation. (14.2) where p (H2) / kPa is the partial pressure of hydrogen gas，V（H2） is the volume of gas collected，R is gas constant with 8.314 J·mol-1·K-1，and T is absolute temperature（273+t）。 If magnesium ribbon is weighed before reaction and the mass is m(Mg) in gram, the atomic(molar) mass of Magnesium can be determined by the following equation: (14.3) 【Apparatus , Reagents and Materials】 Apparatus: Analytical Balance, thermometer, barometer, base burette, big test tube, long-stem funnel, stand and clamp; pipe with stopper, rubber pi