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# Gas and Stoichiometry

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1.)(Ideal Gas Law) How many moles of chlorine atoms are there in a container that holds 1.529 kg of XCl5 gas (where X = unknown element) at 600.5°C at a pressure of 1213 torr? The density of the gas is 4.63 g/L.

2.) (Ideal Gas Law) A chemist carries out a reaction and isolates a gaseous product. Analysis shows that each molecule is composed of one phosphorus atom and an undetermined amount of chlorine atoms. Determine the molecular formula of the compound if a 0.515 g sample at a pressure of 923 mm Hg and a temperature of 95.3°C occupies 93.37 mL.

3.) (Gas Stoich.) How many liters of CO2 gas at 45.2°C and 716 torr are formed when 0.485 L of a sample of liquid decane (C10H22) undergoes complete combustion? The density of decane is 0.730 g/mL.

4.) (Dalton's Law Partial P) A sample of solid LiAlH4 reacts with water forming aqueous lithium hydroxide, solid aluminum hydroxide, and hydrogen gas.
a. Write the balanced chemical equation that describes this reaction. Use the correct phase labels for each reactant and product.
b. How many grams of LiAlH4 reacted with an excess of water to produce a sample of hydrogen gas collected over water in a gas collection apparatus at 26.0°C and 0.915 atm in a 1245 mL container?

https://brainmass.com/chemistry/gas-laws/gas-and-stoichiometry-87116

#### Solution Preview

1. How many moles of chlorine atoms are there in a container that holds 1.529 kg of XCl5 gas (where X = unknown element) at 600.5°C at a pressure of 1213 torr? The density of the gas is 4.63 g/L.

Response: We need to fill out the PV=nRT equation.

P = (1213 torr)(1 atm/760 torr) = 1.596 atm

V = (1 L/4.63 g)(1.529 Kg)(1000 g/Kg) = 330 L

R = 0.08206 L-atm/mol-K

T = 600.5 C + 273.15 C = 873.65 K

Now, we solve for n (moles).

n = PV/RT

n = (1.596 atm)(330 L)/(0.08206 L-atm/mol-K)(873.65 K) = 7.35 moles XCl5

Since there are 5 atoms of Cl for each molecule of XCl5, there are 7.35 x 5 = 36.8 moles Cl atoms.

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2. A chemist carries out a reaction and isolates a gaseous product. Analysis shows that each molecule is composed of one phosphorus atom and an undetermined amount of chlorine atoms. Determine the molecular formula of the compound if a 0.515 g sample at a pressure of 923 mm Hg and a temperature of 95.3°C occupies 93.37 mL.

Response: We need to fill out the PV=nRT equation.

P = (923 mm Hg)(1 atm/760 mm Hg) = 1.21 atm

V = (93.37 mL)(1 L/1000 mL) = 0.09337 L

R = 0.08206 L-atm/mol-K

T = 95.3 C + 273.15 C = 368.45 K

Now, we solve for n (moles).

n = PV/RT

n = (1.21 atm)(0.09337 L)/(0.08206 L-atm/mol-K)(368.45 K) = 0.00374 moles.

But since we know ...

#### Solution Summary

This solution shows step-by-step calculations to determine the moles of chlorine atoms and molecular formula of the unknown compound using the ideal gas equation. It also determines the liters formed when liquid decane is combusted and reaction of lithium hydroxide with water. All steps are shown with explanations.

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## Gas Laws, Stoichiometry and Enthalpy

Question 1
A sample of a gas occupies 1.40 x 103 mL at 25oC and 760 mmHg. What volume will it occupy at the same temperature and 380 mmHg?

2,800 mL
2,100 mL
1,400 mL
1,050 mL
700 mL

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Question 2
The gas pressure in an aerosol can is 1.8 atm at 25oC. If the gas is an ideal gas, what pressure would develop in the can if it were heated to 475oC?

0.095 atm
0.717 atm
3.26 atm
4.52 atm
34.2 atm

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Question 3
A small bubble rises from the bottom of a lake, where the temperature and pressure are 4oC and 3.0 atm, to the water's surface, where the temperature is 25oC and pressure is 0.95 atm. Calculate the final volume of the bubble if its initial volume was 2.1 mL.

0.72 mL
6.2 mL
7.1 mL
22.4 mL
41.4 mL

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Question 4
A sample of N2 gas occupies 2.40 L at 20oC. If the gas is in a container that can contract or expand at constant pressure, at what temperature will the N2 occupy 4.80 L?

10oC
40oC
146oC
313oC
685oC

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Question 5
A gas evolved during the fermentation of sugar was collected at 22.5oC and 702 mmHg. After purification its volume was found to be 25.0 L. How many moles of gas were collected?

0.95 mol
1.05 mol
12.5 mol
22.4 mol
724 mol

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Question 6
Which of the following gases will have the greatest density at the same specified temperature and pressure?

H2
CClF3
CO2
C2H6
CF4

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Question 7
A 0.271 g sample of an unknown vapor occupies 294 mL at 140oC and 847 mmHg. The empirical formula of the compound is CH2. What is the molecular formula of the compound?

CH2
C2H4
C3H6
C4H8
C6H12

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Question 8
What volume of oxygen gas at 320 K and 680 torr will react with 2.50 L of NO gas at the same temperature and pressure?

2NO(g) + O2(g) 2NO2(g)

1.25 L
2.50 L
3.00 L
1.00 L
5.00 L

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Question 9
What mass of KClO3 must be decomposed to produce 126 L of oxygen gas at 133oC and 0.880 atm?

2KClO3(s) 2KCl(s) + 3O2(g)

24.6 g
70.8 g
272 g
408 g
612 g

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Question 10
Which gas has molecules with the greatest average molecular speed at 25oC?

CH4
Kr
N2
CO2
Ar

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Question 11
An endothermic reaction causes the surroundings to ________.

warm up
become acidic
condense
decrease in temperature
release CO2

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Question 12
How many degrees of temperature rise will occur when a 25.0 g block of aluminum absorbs 10 kJ of heat? The specific heat of Al is 0.900 J/g · oC.

0.44oC
22.5oC
225oC
360oC
444oC

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Question 13
The specific heat of gold is 0.129 J/g · oC. What is the molar heat capacity of gold?

0.039 J/mol · oC
0.129 J/mol · oC
25.4 J/mol · oC
39.0 kJ/mol · oC
197.0 J/mol · oC

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Question 14
Which of the following processes is endothermic?

O2(g) + 2H2(g) 2H2O(g)
H2O(g) H2O(l)
3O2(g) + 2CH3OH(g) 2CO2(g) + 2H2O(g)
H2O(s) H2O(l)

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Question 15
When 0.56 g of Na(s) reacts with excess F2(g) to form NaF(s), 13.8 kJ of heat is evolved at standard-state conditions. What is the standard enthalpy of formation (&#916;H) of NaF(s)?

+24.8 kJ/mol
+570 kJ/mol
-24.8 kJ/mol
-7.8 kJ/mol
-570 kJ/mol

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Question 16
Find the standard enthalpy of formation of ethylene, C2H4(g), given the following data:

C2H4(g) + 3O2(g) 2CO2(g) + 2H2O(l)
&#916;H = -1411 kJ

C(s) + O2(g) CO2(g)
&#916;H = -393.5 kJ

H2(g) + ½O2(g) H2O(l)
&#916;H = -285.8 kJ

52 kJ
87 kJ
731 kJ
1.41 x 103 kJ
2.77 x 103 kJ

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Question 17
Given:

H2(g) + ½O2(g) H2O(l), &#916;H = -286 kJ

What is the enthalpy change for the following reaction?

2H2O(l) 2H2(g) + O2(g), &#916;Horxn = ?

&#916;Ho = -286 kJ
&#916;Ho = +286 kJ
&#916;Ho = -572 kJ
&#916;Ho = +572 kJ
&#916;Ho = -143 kJ

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Question 18
For the reaction:

C(graphite) + O2(g) CO2(g), &#916;H = -393 kJ

How many grams of C(graphite) must be burned to release 275 kJ of heat?

22.3 g
0.70 g
12.0 g
17.1 g
8.40 g

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Question 19
According to the first law of thermodynamics:

Energy is neither lost nor gained in any energy transformations.
Perpetual motion is possible.
Energy is conserved in quality but not in quantity.
Energy is being created as time passes. We have more energy in the universe now than when time began.

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Question 20
10.1 g CaO is dropped into a styrofoam coffee cup containing 157 g H2O at 18.0oC. If the following reaction occurs, then what temperature will the water reach, assuming that the cup is a perfect insulator and that the cup absorbs only a negligible amount of heat?

CaO(s) + H2O(l) Ca(OH)2(s), &#916;Horxn = -64.8 kJ/mol

18.02oC
35.8oC
311oC
42.2oC
117oC

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