DATA AND OBSERVATIONS:
a) Length of magnesium ribbon:
b) Mass of 1.000 meter of magnesium:
c) Volume of hydrogen gas in tube:
d) Room temperature:
e) Water temperature:
f) Barometric pressure:
g) Water vapor pressure at water temperature:
Attempt #1
3.12 cm
1.1898 g
35.6 mL
23.9 °C
23.0 °C
762.6 mmHg
21.1 mmHg
CALCULATIONS:
1. 2HCl (aq) + Mg (s) => H2 (g) + MgCl2 (aq)
2. Mass of magnesium reacted:
Attempt #1
3.12 cm 1.000 m 1.1898 g = 0.0371 g Mg
100 cm 1.000 m
Attempt #2
2.89 cm 1.000 m 1.1898 g = 0.0344 g Mg
100 cm 1.000m
3. Moles of magnesium reacted:
Attempt #1
0.0371 g Mg 1 mole Mg = 0.00153 moles Mg
24.3 g Mg
Attempt #2
0.0344 g Mg 1 mole Mg = 0.00142 moles Mg
24.3 g Mg
4. Moles of hydrogen gas produced:
Attempt #1
0.00153 moles Mg 1 mole H2 = 0.00153 moles H2
1 mole Mg
Attempt #2
0.00142 moles Mg 1 mole H2 = 0.00142 moles H2
1 mole Mg
5. Partial pressure exerted by hydrogen gas:
Attempt #1
762.6 mmHg - 21.1 mmHg = 741.5 mmHg
Attempt #2
768.9 mmHg - 18.9 mmHg = 750.0 mmHg
Attempt #2
2.89 cm
1.1898 g
38.8 mL
21.9 °C
21.2 °C
768.9 mmHg
18.9 mmHg
6. Volume of hydrogen gas at STP:
Attempt #1
35.6 mL 741.5 mmHg 273.0 K = 31.9 mL
760.0 mmHg 296.9 K
Attempt #2
38.8 mL 750.0 mmHg 273.0 K = 35.4 mL
760.0 mmHg 294.9 K
7. Volume of one mole of hydrogen gas at STP:
Attempt #1
31.9 mL 1.000 L = 0.0319 L
= 20.8 L/mole
1000 mL 0.00153 moles
Attempt #2
35.4 mL 1.000 L = 0.0354 L
= 24.9 L/mole
1000 mL 0.00142 moles
8. Percent error:
Attempt #1
(20.8 - 22.4) x 100 = -7.14 % error
22.4
Attempt #2
(24.9 - 22.4) x 100 = +11.2 % error
22.4
QUESTIONS:
1. While the true molar volume of a gas will not change no matter how much magnesium
reacts, it can be miscalculated quite easily if not all the magnesium reacts. For example if
not all the magnesium reacted the incorrect mass value would cause and incorrect
magnesium mole value which would in turn cause an incorrect hydrogen mole value.
This would then cause an incorrect final molar volume.
2. We made several assumptions in this experiment. One of the most dangerous yet still
reasonably safe assumptions is that our calculated mass of magnesium was correct. Since
we did not directly measure the mass we had to take on faith that our method of
calculating the mass using its “linear density” would yield a reasonable correct value. If
this value was incorrect, it could snowball and result in fairly substantial error. The
number of moles of magnesium would be wrong, therefore the number of moles of
hydrogen would be wrong, therefore the final calculation of molar volume would be
wrong. Another assumption we made is that the temperature of both the water and the air
remained constant throughout the experiment. With the air temperature this is a fairly
safe assumption but the water temperature probably fluctuated a little bit (since the
reaction produced heat), albeit a very small amount. Another assumption that was made
is that we allowed the water to evaporate to its fullest extent, which was a reasonably safe
assumption. Since we had to wait not only for the reaction to complete but also an
additional 5 minutes afterward, it is unlikely that the water vapor pressure had not
reached its full value. However, by using the maximum value instead of trying to guess at
what it might be we were able to isolate this source of error to give us only positive error.
4.
0.0518 g Ca 1 mole Ca = 0.00129 moles Ca
40.1 g Ca
Ca (s) + 2HCl (aq) => H2 (g) + CaCl2 (aq)
0.00129 moles Ca 1 mole Ca = 0.00129 moles H2
1 mole H2
743.6 mmHg - 19.8 mmHg = 723.8 mmHg
28.68 mL 723.8 mmHg 273.0 K = 25.28 mL (at STP)
760.0 mmHg 295.0 K
25.28 mL 1.000 L = 0.02528 L
= 19.60 L/mole
1000 mL 0.00129 moles
(19.60 - 22.4) x 100 = -12.5 % error
22.4
CONCLUSION:
2. There were a variety of possible sources of error in this experiment. The first that
probably had the most impact was the inherently incorrect measurement of the mass of
the magnesium. While this calculated mass was probably reasonably close to the true
mass the likelihood of it actually being the true mass in slim. This affects not only the
number of moles of magnesium but also the number of moles of hydrogen and ultimately
the molar volume calculation. Another source of error would be the assumption that all of
the water that could evaporate actually had evaporated. We minimized the impact of this
error by allotting a reasonably amount of time for the evaporation to take place. However,
this probably had a significant affect on our second set of results because we did the
second experiment during lunch and perhaps rushed through it a bit to fast to allow the
water vapor pressure to reach its maximum value. Since we made sure that un-evaporated
water accounted for positive error, this could explain at least a large chunk of our +11.2%
error. Some error was also inevitably caused by the fluctuation in water temperature since
it probably increased slightly during the reaction due to the heat produced. However, I
believe the largest source of error in the experiment was the first that I stated, not directly
measuring the mass of the magnesium strip. Due to the “snowball” effect of this
miscalculation a small margin of error could become quite large indeed by the
completion of the experiment.