Stoichiometry Practice Problems

1. The attraction that nonpolar molecules have for each other is primarily caused by the presence of

(A)   gravity.
(B) hydrogen bonding.
(C) van der Waals' forces.
(D) permanent dipole-dipole interactions.

Ans: C

2. How many oxygen molecules are contained in a 3.25 L flask at 25.0 °C and a pressure of 725 torr (mm of Hg)? Assume ideal behavior.

(A)  7.63 x 1019
(B) 7.63 x 1022
(C) 9.90 x 1023
(D) 5.80 x 1025

Ans: B

3. A gas which will diffuse twice as fast as SO2 (Molar Mass = 64) is

(A)   He (mm = 4)
(B) CH4 (mm = 16)
(C) O2 (mm= 32)
(D) HI (mm = 128)

Ans: B

4. The density of an unknown gas at 65.0 °C and a pressure of 730 torr (mm of Hg) is 2.14 g L¯1. The density of oxygen gas at the same conditions is 1.11 g L¯1. What is the molar mass of the unknown gas?

(A)  16.6 g mol¯1
(B) 47.9 g mol¯1
(C) 61.7 g mol¯1
(D) 76.0 g mol¯1

Ans: C

5. Under proper conditions, ammonia, NH3, and oxygen, O2, react to form nitrogen and water. How many moles of oxygen would be consumed for each mole of nitrogen formed?

(A)  0.67

(B)    0.75

(C) 1.5
(D) 3.0

Ans: C

6. Ethane, C2H6, burns in oxygen to form carbon dioxide and water. How many grams of oxygen are required to burn 3.01 x 1023 ethane molecules?

(A) 1.04 g
(B) 12.0 g
(C) 56.0 g
(D) 104 g

Ans: C

7. For a given mass of an ideal gas, if the pressure of an ideal gas is tripled and its temperature (in kelvins) halved, its volume will be

(A) 3/2 of its original volume.
(B) 2/3 of its original volume.
(C) 1/6 of its original volume.
(D) 6 times its original volume

Ans: C

8. What is the volume of CO2 measured at 25 °C and 1.2 atm when 25 g of CaCO3 is totally decomposed?

CaCO3 -----> CaO + CO2

(A) 10.2 L
(B) 2.6 L
(C) 15.3 L
(D) 5.1 L

Ans: D

9. If 500 mL of methane, CH4, effuses through a small hole in 48 s, how much time is required for the same volume of helium to pass through the hole?

(A) 12 s
(B) 24 s
(C) 96 s
(D) 192 s

Ans: C

10. When the stocpcock is opened and equilibrium is established between the two bulbs, what is the total pressure in the system?

(A) 1.4 atm
(B) 1.8 atm
(C) 2.0 atm
(D) 3.6 atm

Ans: C

11. A sample of hydrogen gas was collected over water at 21°C and 685 mmHg.  The volume of the container was 7.80 L.  Calculate the mass of H2(g) collected.  The vapor pressure of water is 18.6 mmHg at 21°C.

1. 0.572 g
2. 0.993 g
3. 1.10 g
4. 0.889 g

Ans: A

12. 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. 2.50 L
2. 1.25 L
3. 2.00 L
4. 2.21 L

Ans: B

13. Chlorine gas can be prepared in the laboratory by the reaction of manganese dioxide with hydrochloric acid:

MnO2(s) + 4HCl(aq)  ®  MnCl2(aq) + 2H2O(l) + Cl2(g)

How much MnO2 should be added to excess HCl to obtain 275 mL of chlorine gas at 5.0°C and 650 mmHg?

Ans  0.896 g

14. Calculate the molecular mass of a gaseous substance if 0.125 g of the gas occupies 93.3 mL at STP.

30 g/mol

15. Ammonium nitrite undergoes decomposition to produce only gases as shown below.

NH4NO2  ®  N2 (g)    +   2H2O  (g)

How many liters of gas will be produced by the decomposition of 32.0  g of  NH4NO2 at 525°C and 1.5 atm?

66 L