The graph below shows the volume of hydrogen evolved against time for the reaction of a piece of solid magnesium metal with 1.00 mol dm-3 hydrochloric acid, HCl(aq). Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g) (a) Deduce the rate of the reaction at time t = 1.5 minutes. (i) Assuming magnesium is the limiting reagent and the gas was collected at STP (i) Calculate the mass of the piece of magnesium metal used. (ii) Describe how the graph would have looked if the magnesium metal had been in powdered form.
Answers
Answer:
Explanation:
Explanation:
This is a limiting reactant problem.
Mg(s)
+
2HCl(aq)
→
MgCl
2
(
aq
)
+ H
2
(
g
)
Determine Moles of Magnesium
Divide the given mass of magnesium by its molar mass (atomic weight on periodic table in g/mol).
4.86
g Mg
×
1
mol Mg
24.3050
g Mg
=
0.200 mol Mg
Determine Moles of 2M Hydrochloric Acid
Convert
100 cm
3
to
100 mL
and then to
0.1 L
.
1 dm
3
=
1 L
Convert
2.00 mol/dm
3
to
2.00 mol/L
Multiply
0.1
L
times
2.00 mol/L
.
100
cm
3
×
1
mL
1
cm
3
×
1
L
1000
mL
=
0.1 L HCl
2.00 mol/dm
3
=
2.00 mol/L
0.1
L
×
2.00
mol
1
L
=
0.200 mol HCl
Multiply the moles of each reactant times the appropriate mole ratio from the balanced equation. Then multiply times the molar mass of hydrogen gas,
2.01588 g/mol
0.200
mol Mg
×
1
mol H
2
1
mol Mg
×
2.01588
g H
2
1
mol H
2
=
0.403 g H
2
0.200
mol HCl
×
1
mol H
2
2
mol HCl
×
2.01588
g H
2
1
mol H
2
=
0.202 g H
2
The limiting reactant is
HCl
, which will produce
0.202 g H
2
under the stated conditions.
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Answer:
Explanation:
Each flask contains 0.1 mol of HCl. Magnesium is present in the following amounts:
Flask 1 0.0125 mol Mg 0.1 mol HCl excess HCl
Flask 2 0.0250 mol Mg 0.1 mol HCl excess HCl
Flask 3 0.0500 mol Mg 0.1 mol HCl stoichiometric HCl/Mg ratio
Flask 4 0.1000 mol Mg 0.1 mol HCl excess Mg
Flask 1 and 2 are limited by smaller quantities of Mg. Flask 3 will react to use both reagents evenly and completely. Flask 4 will produce only the same amount of hydrogen as Flask 3 and have excess Mg left over, since the reaction is limited by the HCl. Flask 4 will produce H2 faster than flask 3, but both balloons in the end will be nearly the same size.
Reaction Time: Perform demo at the beginning of lecture and leave for the rest of the class period to develop.
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