Determination of the Molar Mass of Magnesium
Introduction
The aim of this investigation is to find the molar mass of magnesium by making magnesium and hydrochloric acid react and then apply the ideal gas equation.
The independent variable of this experiment is the amount used of each substance. However, these were not not be varied. The dependent variable is the amount of hydrogen gas which is formed. The controlled variables are the pressure and temperature in the room, and the width (and thereby weight) of the magnesium ribbon.
Materials and Methods
Materials used:
- Magnesium (Mg) ribbon
- Hydrochloric acid (HCl), 35% concentrated
- Copper (Cu) wire
- Eudiometer
- Thermometer
- Barometer
- Measuring cylinder (2000 cm3)
Method:
- 50 mm of the Mg ribbon was weighed.
- 10 cm3 of HCl was poured into the eudiometer.
- The rest of the eudiometer was filled with water.
- The Mg ribbon was wrapped around the Cu wire and hung in the eudiometer cork.
- The measuring cylinder was filled with water.
- The eudiometer was put upside-down in the measuring cylinder.
- After the reaction between HCl and Mg was complete, the eudiometer was allowed to cool down to reach room temperature.
- The eudiometer was arranged so that the gas inside had the same pressure as the air pressure outside the room.
- The molar mass of magnesium was calculated using
In order to ensure that the investigation resulted in valid results, the experiment was conducted four times.
Results
These were the measured values:
| Table 1: Controlled Variables | |
|---|---|
| Variable | Value |
| Pressure of the room | 100.3 * 103 Pa |
| Temperature of the room | 292.65 K |
| Weight of the magnesium ribbon | 0.06 g |
| Table 2: Dependent Variables | |
|---|---|
| Iteration | Volume of H2 [ml] |
| 1 | 65 |
| 2 | 64 |
| 3 | 68 |
| 4 | 62 |
The mean volume is then 64.75 ml.
The ideal gas equation will be used to calculate the number of moles:
n = (PV)/(RT)
n = (9.81 * 64.75) / (8.31 * 293 ) = 2.61
Thus, we have 2.61 moles of H2. The reaction carried out should be:
Mg + 2HCl => MgCl2 + H2
For every mole of Mg, we will have one mole of H2. Therefore, we know that we will also have 2.61 moles of Mg. This is the formula for molar mass:
M = m/n = 64.75 / 2.61 = 24.8
The molar mass of magnesium must then be 24.8 g.
Conclusion
From this, we can conclude that the average magnesium atom has 24.8 protons. According to Wikipedia, the molar mass of magnesium is 24.3 g, which is close to our result.
Evaluation
This investigation could have been performed better in an environment where we could control the pressure more accurately, such as in a pressure chamber.
Använder FireStats
in n = (9.81 * 64.75) / (8.31 * 293 ) = 2.61 where did 9.81 come from?
Comment by Mona — March 4, 2008 @ 2:09 am