# Stoichiometry/Limiting Reagents

1. Feb 25, 2009

### wvlaxxer81

Could someone please check my work on some of these problems so I can fix whatever I need to. I need to know if I'm doing this right because our test is tomorrow on this unit.

STOICHIOMETRY

1. 150 grams of potassium chlorate is decomposed

A. Balanced equation = 2KClO3 --> 2KCl + 3O2

B. How many moles of potassium chlorate are there? 150g KClO3 x 1 mol / 122.5g KClO3 = 1.22 mol KClO3

C. How many moles of potassium chlorate are formed? 1.22 mol KClO3 x 2 KCl / 2KClO3 = 1.22 mol KCl

D. How many moles of oxygen are formed? 1.22 mol KClO3 x 3 O2 / 2KClO3 = 1.83 mol O2

E. What is the mass of the potassium chloride? 74.5g x 1.22 mol = 90.89g KCl

F. What is the mass of the oxygen? 32g x 1.83 mol = 58.56g O2

G. What is the volume of the oxygen? 1.83 mol O2 x 22.4L / 1 mol = 40.992 L O2

2. Iron (III) reacts with hydrochloric acid. If 25 liters of hydrogen gas are formed at STP.

A. Balanced Equation. 2Fe3 + 18 HCl --> 6FeCl3 + 9H2

B. What is the mass of the Iron (III)? 25/22.4 = 1.12 mol H2

1.12 mol H2 x 2 mol Fe3/9 mol H2 = 0.25 mol Fe3

0.25 mol Fe3 x 56g Fe3 = 14g Fe3

C. What is the mass of the hydrochloric acid? 1.12 mol H2 x 18 mol HCl/9 mol H2 = 2.24 mol HCl

2.24 mol HCl x 36.5g HCl/1 mol HCl = 81.76g HCl

D. What is the mass of the Iron (III) Chloride? 1.12 mol H2 x 6 mol FeCl3/9 mol H2 = 0.75 mol FeCl3

0.75 mol x 91.5g FeCl3/1 mol FeCl3 = 68.63g FeCl3

E. What is the mass of the Hydrogen gas? 1.12 mol x 2g H2/1 mol H2 = 2.24g H2

3. Phosphorous reacts with Oxygen to yield Diphosphorous Pentaoxide (gas). If 50 liters of Oxygen is used:

A. Balanced Equation. 4P +5O2 --> 2(P2O5)

B. What is the volume of the Diphosphorous Pentaoxide? 50 L/22.4L = 2.23 mol O2

2.23 mol O2 x 2 mol P2O5/5 mol O2 = 0.89 mol P2O5

0.89 mol P2O5 x 22.4 L/1 mol P2O5 = 19.94 L P2O5

C. What is the mass of the Diphosphorous Pentaoxide? 0.89 mol P2O5 x 142g P2O5/1 mol P2O5 = 126.38g P2O5

D. What is the mass of the Phosphorous? 2.23 mol O2 x 4 mol P/5 mol O2 = 1.78 mol P

1.78 mol P x 31g P/1 mol P = 55.18g P

E. What is the mass of the Oxygen? 2.23 mol O2 x 32g O2/1 mol O2 = 71.36g O2

F. How many molecules of oxygen was used? 2.23 mol O2 x 6.02x10^23 mc O2 = 13.42x10^23 mc O2

G. How many molecules of Diphosphorous Pentaoxide were formed? 0.89 mol P2O5 x 6.02x10^23 mc P2O5 = 5.36x10^23 mc P2O5

H. How many atoms of Phosphorous reacted? 1.78 mol P x 6.02x10^23 atoms P = 10.72x10^23 at P

LIMITING REAGENTS

1. 2.8 moles of H2O reacts with 3.1 moles of Cl2O7

A. Balanced equation. Cl2O7 + H2O --> 2HClO4

B. What is the limiting reagent? H2O

C. How many moles of the product are produced? 2.8 mol H2O x 2 mol HClO4/1 mol H2O = 5.6 mol HClO4

D. How many moles of the excess reagent remains unreacted? 3.1 mol Cl2O7 - 2.8 mol H2O = 0.3 mol Cl2O7

2. 9.8 moles of Al reacts with 12.4 moles of CuSO4

A. Balanced equation. 2Al + 3CuSO4 --> 3Cu + Al2(SO4)3

B. What is the limiting reagent? Aluminum

C. How many moles of Al2(SO4)3 are produced? 9.8 mol Al x 1 mol Al2(SO4)3/2 mol Al = 4.9 mol Al2(SO4)3

D. How many moles of the excess reagent remain? 12.4 mol CuSO4 - 9.8 mol Al = 2.6 mol CuSO4

3. 2.4 grams of Na reacts with 36 grams of H2O

A. Balanced Equation. 2Na + H2O --> Na2O + H2

B. What is the limiting reagent? H2O

C. How many moles of H2 are formed? 36g H2O x 1 mol H2O/18g H2O = 2 mol H2O

2 mol H2O x 1 mol H2/1 mol H2O = 2 mol H2

D. How many grams of H2 are formed? 2 mol H2 x 2g H2/1 mol H2 = 4g H2

E. How many grams of the excess reagent are left? 2.4g Na - 1.2g = 1.2g Na

F. How many liters of H2 are formed? 2 mol H2 x 22.4 L H2/1 mole H2 = 44.8 L H2

4. 123g of Fe reacts with 56g of O2

A. Balanced Equation. 4Fe + 3O2 --> 2Fe2O3

B. What is the limiting reagent? O2

C. How many grams of Fe2O3 are formed? 56g O2 x 2 mol Fe2O3/3 mol O2 = 37.3g Fe2O3

D. How many grams of the excess reagent are left? 123g Fe x 3 mol O2/4 mol Fe = 92.25g Fe

123g Fe - 92.25g Fe = 30.75g Fe

All help is greatly appreciated. Thank you.

2. Feb 26, 2009

### Staff: Mentor

Correct.

Now you know answers to two questions ;)