Law of Constant Composition Lab: Magnesium and Oxygen

2. The second source of error in the lab is when opening the lid of the crucible which allowed smoke of magnesium oxide to escape. During the lab, we removed the lid a couple of times to allow oxygen to enter the crucible so the magnesium reacts with the air to form magnesium oxide. However, the smoke could have easily escaped from the crucible because of the strong force of heat from the laboratory burner. This could have affected the lab results by decreasing the final mass when some of the product have escaped.

Law of definite proportion, sometimes called Proust’s law, states that a compound always has exactly the same proportion of elements by mass. This means that contents of a compound does not depend on its source or its method of preparation.

The first experiment is about the combustion of magnesium after which the ash is formed.

There are several sources of error to this experiment due to random and systematic errors. The only source of random error was the measurement that we took through the graduated cylinder which was only accurate to the nearest 1%. We took the largest error from this one percent, which was +/- 3. The largest relative error this yielded was only 3%, so this did not affect how precise this experiment was too much. We can still make this more precise by making the masses of the water larger. For example if we started the masses at 300mL and went up by 50mL, the largest error this would yield would be 2% due to the largest error being +/- 5. This would cause smaller errors in the amount of water.

I found out that both of my experiments were off by 0.46 and 0.13 but were remotely

e) As stated in the law of definite proportion the percentage composition of Magnesium oxide was approximately same for our classmates. The following table shows the results of two other groups.

    In this lab, a calorimeter was used to find the enthalpy of reaction for two reactions, the first was between magnesium and 1 molar hydrochloric acid, and the second was between magnesium oxide and 1 molar hydrochloric acid. After the enthalpy for both of these were found, Hess’ law was used to find the molar enthalpy of combustion of magnesium, using the enthalpies for the two previous reactions and the enthalpy of formation for water. The enthalpy of reaction for the magnesium + hydrochloric acid reaction was found to be -812.76 kJ. The enthalpy of reaction for the magnesium oxide + hydrochloric acid reaction was found to be -111.06 kJ. These two enthalpies and the enthalpy of formation for water were manipulated and added together using Hess’s law to get the molar enthalpy of combustion of magnesium. It was found that the molar enthalpy of combustion of magnesium was -987.5 kJ/mol. The accepted enthalpy was -601.6 kJ/mol, which means that there is a percent difference of 64%. This percent difference is very high which indicates that this type of experiment is very inefficient for finding the molar enthalpy of combustion of magnesium. Most likely, a there are many errors in this simple calorimeter experiment that make it inefficient for finding the molar enthalpy of combustion of magnesium.

About 80 mL of HCl was obtained and mixed with phenolphthalein. Using a LabQuest unit and Gas Pressure Sensor kit, the HCl mixture was added to the flask with the magnesium ribbon and allowed to react. When reaction was complete, the change of temperature and gas was recorded. This procedure was repeated for different masses of magnesium ribbon (masses found on page 89 of the lab manual). After the completed procedure, moles of H₂ produced in each trial were calculated. (The actual procedure can be found on pages 87-89 of the lab manual)

Using elemental analysis to determine the percent mass composition of each element in a compound is the first step in creating an empirical formula. There are many different types of elemental analysis, but in this experiment gravitational analysis and Beer’s Law are used. Elemental analysis is first used to find the moles of each element, then converted to mass, and then the percent mass of the element in the product is found (2).

1. Label each substance on seven pieces of paper. Put two pieces of magnesium ribbon on the paper labeled “Magnesium”.

Because it is dangerous to burn magnesium, it is not possible to directly record heat change. Our lab team suggests an indirect way of determining the heat of combustion for magnesium. To accomplish this, we need to perform two separate trials. One uses a solid (powder) version of MgO, while the other uses Mg ribbon. With the results from these, we can use Hess’ Law to determine q=∆H. This provides both a safe and successful way of indirectly determining the heat of combustion for magnesium.

It is to tell how the reaction between magnesium and hydrochloric acid will be effected if we change the concentration of hydrochloric acid.

Because of the Law of Definite Proportions, when the elements of magnesium and oxygen chemically react to form a compound they do so in definite proportions by weight. The Law of Definite Proportions states that if elements form a compound, they must combine in definite proportions by weight. This means that all atoms form chemical compounds by combining in whole number ratios. Take for example the elements of hydrogen and oxygen: they can combine to make water (H2O) but in other conditions can combine to make hydrogen peroxide (H2O2). In both examples the ratio stays the same. Likewise, in this experiment the elements magnesium and oxygen combine in whole-number ratios. We have hypothesized that magnesium and oxygen

5.3 mL of bromobenzne and 15 mL of anhydrous ether was then placed into the separatory funnel and was shaken and vented in order to mix the solution. Half of the bromobenzene solution was added first into the round bottom flask and as soon as a color change was observed, the remaining half of the bromobenzene was added drop wise into the round bottom flask. The mixture was then refluxed on a heating mantle for 10 minutes until most of the magnesium has been consumed.

In order to draw a clear conclusion, advantages and problems of applying Magnesium will be analyzed thoroughly and supported by specific examples. In the meanwhile, discussion of metal Aluminium will be included as the properties of Aluminium and Magnesium are similar. By comparison, information can be understood more easily.