Calculating With Chemical Equations Page 3
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1
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4Name
Period
Date
Pre-AP Chemistry
Homework: Calculating with Chemical Equations
[A] Stoichiometry Problems
1. ______________ Calculate the mass of magnesium oxide formed when 0.52 g of magnesium is burned according to the
following equation:
2 Mg
+ O
→ 2 MgO
(s)
2(g)
(s)
1mole Mg
40.31 g
2 MgO
0.52g Mg ×
= 0.0214 mole Mg
×
=0.0214 mole MgO ×
= 0.8626 = 0.86 grams of MgO
24.31g
1moleMgO
2 Mg
2. ______________ Calculate the mass of hydrogen gas that will be released when 3.0 g of sodium reacts with water.
2 Na
+ 2 H
O
→ 2 NaOH
+ H
(s)
2
(l)
(aq)
2(g)
1mole Na
2.016 g
1H
3g Na ×
= 0.1305 mole Na ×
=0.0652 mole H
×
= 0.1315 grams H
= 0.13 grams of H
2
2
2
2
22.99g
1 mole H
2Na
2
3. ______________ What mass of silver will be precipitated when 7.32g of zinc reacts completely with silver nitrate?
Zn
+ 2 AgNO
→ Zn(NO
)
+ 2 Ag
(s)
3(aq)
3
2(aq)
(s)
1moleZn
107.9 g
2 Ag
7.32 g Zn ×
= 0.1119 mole Zn
×
=0.2238 mole Ag
×
=24.1501 = 24.2 grams of Ag
65.41g
1mole Ag
1Zn
4. ______________ Potassium metal reacts with hydrochloric acid to produce aqueous potassium chloride and hydrogen gas.
How many grams of potassium are required to produce 5.00 g of hydrogen gas?
2K + 2HCl à 2KCl + H
2
1mole H
39.1 g
2K
5.00g H
×
=2.4802 mole H
×
=4.9603 mole K
×
= 193.4524 = 193 grams K
2
2
2
1 mole K
2.016g
1H
2
5. ______________ Ethanol burns according to the following equation. If 655 g of water is produced, what mass of ethanol is
burned?
C
H
OH
+ 3O
→ 2CO
+ 3H
O
2
5
(aq)
2(g)
2(g)
2
(g)
1mole H
46.068 g
1C
H
OH
O
655 g H
O ×
= 36.3566 mole H
O ×
=12.1189 mole C
H
OH ×
= 558.2915 = 558 g of ethanol
2
5
2
2
2
2
5
18.016g
1moleC
H
OH
3H
O
2
5
2
6. ______________ How many grams of potassium chlorate, KClO
, must be decomposed to produce oxygen and 71.8 g of
3
potassium chloride?
à 2KCl + 3O
2KClO
3
2
1moleKCl
122.55 g
2KCl
71.8g KCl ×
= 0.9631 mole KCl
×
=0.9631 mole KClO
×
= 118.0294 = 118 grams KClO
3
3
74.55g
1 mole KClO
2KClO
3
3
[B] Limiting Reactant Problems
1. Calcium hydroxide, used to neutralize acid spills, reacts with hydrochloric acid according to the following equation:
+ 2HCl àCaCl
Ca(OH)
+ 2H
O
2
2
2
a. ______________ If you have spilled 6.3 mol of HCl and put 2.8 mol of Ca(OH)
on it, which substance is the limiting
2
reactant?
1Ca(OH)
6.3 mole HCl ×
= 3.15 mole of Ca(OH)
needed. You have only 2.8 mole which isn’t enough. Ca(OH)
is the limiting reactant.
2
2
2
2HCl
b. ______________ How many moles of the excess reactant remain?
Use the limiting reactant to do the calculations
2HCl
2.8 mole Ca(OH)
×
=5.6 mole of HCl needed. You have 6.3 (this also confirms that the HCl is in excess)…6.3 – 5.8 actually used
2
1Ca(OH)
2
= 0.7 mole of HCl remaining
Homework: Calculating with Chemical Equations
3
1/12/15
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