## How to do these STP gas and mass stoichiometry problems in general.

All of the problems in this set are stoichiometry problems with at least one equation participant as a gas at STP. (a) Write and balance the chemical equation. (2) Do the math in DA style using 1 mole gas at STP = 22.4 liters as a factor. In the following problems ALL GASES ARE AT STP.

In this set of problems for the first time you will be using the idea of stoichiometry. These problems always refer to a chemical reaction. The chemical reaction must be first written and balanced. You will be given an amount of one of the materials and be expected to find out how much that corresponds to another one of the materials in that chemical reaction. Because we are beginning with an amount of a material and we are looking for another amount of (another) material, the DA method is standardly used for this type of problem.

- Begin each problem with what you know, the GIVEN material and the GIVEN amount in whatever units it is given.
- Change the GIVEN amount of material to units of mols. In the case of gases at STP, the conversion factor is: 1 mol of gas = 22.4 liters at STP. In the case of masses, you must use the formula weight of the material to change from mass to mols.
- Use the mol ratio to change from one material to another. The mol ratio is the name (or symbol) of the material and the coefficient of that material in the balanced chemical reaction.
- Change the mols of the new material to whatever units are asked for.

ANSWER AND DISCUSSION

PROBLEM #1, SET 2 - STP GAS AND MASS STOICHIOMETRY

1. How many moles of nitrogen gas is needed to react with 44.8 liters of hydrogen gas to produce ammonia gas?

3H_{2} +
N_{2}
===>
2NH_{3}

GIVEN: 44.8 L of H_{2} at STP.

FIND: mols of N_{2}.

Here the sequence is: GIVEN liters of H_{2} at STP,
CHANGE liters of H_{2} at STP to mols of
H_{2}, MOL RATIO to change from H_{2}
to N_{2}. There is no need to go any further
to change the N_{2} into mols, because the
mol ratio leaves the material in that unit anyway.

The MVG is the "molar volume of gas at STP." The MR is the mol ratio. Notice the numbers in blue in the mol ratio. Those numbers come from the coefficients in the balanced chemical equation.

The math is: 44.8 ÷ 22.4 ÷ 3 =

ANSWER AND DISCUSSION

PROBLEM #2, SET 2 - STP GAS AND MASS Stoichiometry

2. How many liters of ammonia are produced when 89.6 liters of hydrogen are used in the above reaction?

The "above reaction" from problem #1 is: N_{2} +
3H_{2}
===>
2NH_{3}

GIVEN: 89.6 L of H_{2} at STP.

FIND: Volume of ammonia (in liters at STP)

Take the GIVEN quantity, use the Molar Volume of Gas at STP (MVG) to change it to mols, change the material with the mol ratio (MR), and change the mols of new material to the requested liters at STP using the MVG again.

The math is: 89.6 ÷ 22.4 ÷ 3 x 2 x 22.4 =

or, if the MVG's cancel, 89.6 ÷ 3 x 2 =

ANSWER AND DISCUSSION

PROBLEM #3, SET 2 - STP GAS AND MASS STOICHIOMETRY

3. Ten grams of calcium carbonate was produced when carbon dioxide was added to lime water (calcium hydroxide in solution). What volume of carbon dioxide at STP was needed?

CO_{2} +
Ca(OH)_{2}
===>
CaCO_{3} +
H_{2}O

GIVEN: 10.0 g = mass of calcium carbonate

FIND: Volume of carbon dioxide (in liters at STP)

Take the GIVEN quantity, a mass, use the Formula Weight of the given quantity to change it to mols, change the material with the mol ratio (MR), and change the mols of new material to the requested liters at STP using the MVG.

The math is: 10 ÷ 100.1 x 22.4 =

ANSWER AND DISCUSSION

PROBLEM #4, SET 2 - STP GAS AND MASS STOICHIOMETRY

4. When 11.2 liters of hydrogen gas is made by adding zinc to sulfuric acid, what mass of zinc is needed?

Zn + H_{2}
SO_{4}
===>
H_{2} + ZnSO_{4}

GIVEN: 11.2 L of H_{2} at STP

FIND: mass of Zn

Take the GIVEN quantity, a volume at STP, use the MVG to change it to mols, change the material with the mol ratio (MR), and change the mols of new material to the mass using the formula weight of the new material. Find this pathway on the Stoichiometry Roadmap

The math is: 11.2 ÷ 22.4 x 65.4 =

ANSWER AND DISCUSSION

PROBLEM #5, SET 2 - STP GAS AND MASS STOICHIOMETRY

5. What volume of ammonia at STP is needed to add to water to produce 11 moles of ammonia water?

The balanced chemical equation is:

NH
_{3}
+ H_{2}
O
===> NH_{4}
OH

GIVEN: 11.0 mols of ammonia water (NH_{4}OH)

FIND: Volume of ammonia gas (NH_{3}) in liters at STP.

Take the GIVEN quantity, a number of mols, and directly use the mol ratio (MR) to change to the other material. The mols of FIND material can be changed to the requested liters at STP using the MVG.

The math is: 11 x 22.4 =

ANSWER AND DISCUSSION

PROBLEM #6, SET 2 - STP GAS AND MASS STOICHIOMETRY

6. How many grams of carbonic acid is produced when 55 liters of carbon dioxide is pressed into water?

There is no such thing as solid carbonic acid. It only exists in ionic form in solution, but we can consider this exercise anyway.

The balanced chemical equation is:

CO
_{3}
+ H_{2}
O ===> H_{2}
CO_{3}

GIVEN: 55.0 mols of carbon dioxide (CO_{2})

FIND: mass of carbonic acid (H_{2}CO_{3}).

Take the GIVEN quantity, a number of mols, and directly use the mol ratio (MR) to change to the other material. The mols of FIND material can be changed to the requested mass the formula weight of the FIND material.

The math is: 55 ÷ 22.4 x 62 =

ANSWER AND DISCUSSION

PROBLEM #7, SET 2 - STP GAS AND MASS STOICHIOMETRY

7. magnesium hydroxide + ammonium sulfate ===> magnesium sulfate + water + ammonia

How much (grams) magnesium hydroxide do you need to use in the above reaction to produce 500 liters of ammonia?

First change the word chemical equation to a balanced symbol equation:

Mg(OH)_{2}
+ (NH_{4}
)_{2}
SO_{4} ===>
2H_{2}
O + MgSO_{4}
+ 2NH_{3}

GIVEN: 500 liters of ammonia gas (NH_{3}) at STP.

FIND: mass of magnesium hydroxide (Mg(OH)_{2}).

Take the GIVEN quantity, a volume of gas at STP, and find the mols given with the MVG, the molar volume of gas. Use the mol ratio (MR) to change to the other material. The mols of FIND material can be changed to the requested mass the formula weight of the FIND material. Find this pathway on the Stoichiometry Roadmap

The math is: 500 ÷ 22.4 ÷ 2 x 58.3 =

ANSWER AND DISCUSSION

PROBLEM #8, SET 2 - STP GAS AND MASS STOICHIOMETRY

8. How much strontium bromide is needed to add to chlorine gas to produce 75 liters of bromine gas?

GIVEN: 75 liters of bromine gas (Br_{2}) at STP.

FIND: mass of strontium bromide (SrBr_{2}).

Take the GIVEN quantity, a volume of gas at STP, and find the mols given with the MVG, the molar volume of gas. Use the mol ratio (MR) to change to the other material. The mols of FIND material can be changed to the requested mass the formula weight of the FIND material. Find this pathway on the Stoichiometry Roadmap

The math is: 17 X 4 =

ANSWER AND DISCUSSION

PROBLEM #8, SET 2 - STP GAS AND MASS STOICHIOMETRY

9. What mass of ammonium chlorate is needed to decompose to give off 200 liters of oxygen?

10. Your car burns mostly octane, C_{8}H_{18}, as a fuel. How many
liters of oxygen is needed to burn a kilogram of octane?

11. copper + sulfuric acid ===> copper II sulfate + water + sulfur dioxide

How many moles of copper are needed to produce 1000 L of
SO_{2}?

12. What volume of oxygen is needed to burn a pound of magnesium?

13. How many grams of sodium do you have to put into water to make 30 liters of hydrogen at STP?

14. ammonia gas and hydrogen chloride gas combine to make ammonium chloride. What volume of ammonia at STP is needed to react with 47.7 liters of hydrogen chloride at STP?

15. How many liters of oxygen are needed to burn 10 liters of acetylene?

ANSWERS TO GAS STOICHIOMETRY PROBLEMS

1. 0.667 MOL | 2. 59.7 L | 3. 2.24 L | 4. 32.7 g |

5. 246 L | 6. 152 g | 7. 651 g | 8. 828 g |

9. 604 g | 10. 2.46 KL | 11. 44.6 MOL | 12. 210 L |

13. 61.6 g | 14. 47.7 L | 15. 25 L |