We are investigating the reaction of magnesium and hydrochloric acid:
Mg + 2HCl MgCl2 + H2
The dependent variable will be the amount of hydrogen collected, the gas in the reaction.
There are many independent variables I could use for this experiment:
But the variable I will choose is length of Mg. I think this will definitely be the easiest to vary, as it is simply a case of cutting different lengths. It is a good one to choose also because it is almost certain that the reaction will be different at different lengths.
Whereas with variables such as ‘Temperature of room’, ‘Atmospheric pressure’ etc, it could be difficult to do the experiment and also know that it is changing in order to form a sensible conclusion
If I double the mass of Mg, it will double the volume of gas produced. The equation above shows us the molar ratio between Mg : H2 is 1 : 1. If the mass doubles, the number of moles of Mg doubles, the number of moles of H2 doubles so the volume doubles also.
Independent variable values
Maximum H2 we can collect = 50 cm3
Moles = 0.050 = 0.00208
Moles ratio H2 : Mg = 1:1
Maximum moles of Mg = 0.00208
Maximum mass of Mg = 2
0.00208 x 24 = 0.0499
1 metre of Mg = 1g 0.05g = 5cm
So I will use: 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5
Temperature of room – room temp – approx. 25ï¿½C
Type of Acid – HCl
Concentration of acid – 2.00M
Volume of HCl – 10 cm?/ml
Light intensity – all blinds shut
Atmospheric pressure – normal
> Graduated gas collection tube
> Water bath
> Beaker – 250 ml
> Test tube
> Ignition tube
> Delivery tubing
> Clamp on stand
1. Fill graduated gas collection tube with water, then, avoiding water loss, place upside down in water bath and clamp upright.
2. Fill test tube with 10 ml HCl and place in water full beaker.
3. Place ignition tube containing measured Mg piece inside test tube, avoiding Mg contacting HCl.
4. Set up rest of experiment as shown on diagram.
5. When experiment ready, tip test tube to side quickly, letting HCl to flow into the ignition tube, reacting with the Mg piece, then put back in water.
6. When fuzzing finished, record how much gas has collected at the top of the graduated gas collection tube. You can tell by how much water has been removed.
7. Repeat with different lengths
Length of Mg (cm)
Volume of H2 collected (cm3)
On separate graph sheet
The best-fit line shows that as the independent variable (length of Mg) increases, the dependent variable, gas collected, also increases giving a straight line graph.
This is because:
If I double the mass of Mg, it will double the volume of gas produced. The equation:
Mg + 2HCl MgCl2 + H2
….shows us the molar ratio between Mg : H2 is 1 : 1. If the mass doubles, the number of moles of Mg doubles, the number of moles of H2 doubles so the volume doubles also.
My conclusion supports my original hypothesis, as shown on the graph also.
> Measured HCl to 0.1cm?
> Measured Mg length to good degree of accuracy – using ruler 0.1 cm. We drew a line with a felt tip (which could have been inaccurate), then cut it using scissors.
> Gas collected could be read to 0.1cm?, but very small bubbles were still coming out long after the experiment appeared to have finished, which could have made a minute difference to my results.
> All points fitted the best-fit line.
> We repeated the experiment twice for accuracy, and fund the Expt generated the same results. The repeated measurements were just as reliable as the first set.
> The data is very reliable – all points lie on the line.
> We have 7 readings, more than required to form a firm conclusion.
> No other line could fit the points.
Suitability of procedure
> Range and spacing was suitable at o.5cm intervals.
> We repeated the Expt and took averages.
> The apparatus and method we used gave good results, coinciding with my prediction.
> We could control all variables to a reasonable degree of accuracy without difficulty or setbacks.
> We could have used a different set of apparatus, but using the same things, to make sure all the apparatus we used was fine.
> We could have used more measurements, but as we couldn’t use more than 4cm of Mg, and we generated good results twice, I don’t think it would be necessary.
> We could have been more precise in out measurement-taking in the experiment, but again the results we had were fine.
> Our method was good and efficient.
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