General orrganic and biological chemistry structures off liffe 5th by karen timberlake08 worked examples

Sample Problem 8.1 Properties of Gases
Identify the property of a gas that is described by each of the following:
a. increase the kinetic energy of gas particles
b. the force of the gas particles hitting the walls of the container
c. the space that is occupied by a gas

Solution
a. temperature
b. pressure
c. volume

Study Check 8.1
As more helium gas is added to a balloon, the number of grams of helium increases.
What property of a gas is described?

Answer
The mass, in grams, gives the amount of gas.

General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake

© 2016 Pearson Education, Inc.


Sample Problem 8.2 Units of Pressure
The oxygen in a tank in the hospital respiratory unit has a pressure of 4820 mmHg. Calculate the pressure, in
atmospheres, of the oxygen gas (see Table 8.2).

Solution
The equality 1 atm = 760 mmHg can be written as two conversion factors:

General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake

© 2016 Pearson Education, Inc.


Sample Problem 8.2 Units of Pressure
Continued
Using the conversion factor that cancels mmHg and gives atm, we can set up the problem as

Study Check 8.2
A tank of nitrous oxide (N2O) used as an anesthetic has a pressure of 48 psi. What is that pressure in atmospheres (see
Table 8.2)?

Answer
3.3 atm

General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake

© 2016 Pearson Education, Inc.


Sample Problem 8.3 Calculating Volume When Pressure Changes
When Whitney had her asthma attack, oxygen was delivered through a face mask. The gauge on a 12-L tank of
compressed oxygen reads 3800 mmHg. How many liters would this same gas occupy at a pressure of 0.75 atm
when temperature and amount of gas do not change?

Solution
Step 1

Organize the data in a table of initial and final conditions. To match the units for the initial and final
pressures, we can convert either atm to mmHg or mmHg to atm.

General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake

© 2016 Pearson Education, Inc.


Sample Problem 8.3 Calculating Volume When Pressure Changes
Continued
We place the gas data using units of mmHg for pressure and liters for volume in a table. (We could have both
pressures in units of atm as well.) The properties that do not change, which are temperature and amount of
gas, are shown below the table. We know that pressure decreases, and can predict that the volume increases.

Factors that remain constant: T and n
Step 2

Rearrange the gas law equation to solve for the unknown quantity. For a PV relationship, we use Boyle’s law
and solve for V2 by dividing both sides by P2. According to Boyle’s law, a decrease in the pressure will cause
an increase in the volume when T and n remain constant.

General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake

© 2016 Pearson Education, Inc.


Sample Problem 8.3 Calculating Volume When Pressure Changes

Continued
Step 3

Substitute values into the gas law equation and calculate. When we substitute in the values with pressures in
units of mmHg or atm, the ratio of pressures (pressure factor) is greater than 1, which increases the volume as
predicted in Step 1.

Study Check 8.3
In an underground natural gas reserve, a bubble of methane gas, CH4, has a volume of 45.0 mL at 1.60 atm. What
volume, in milliliters, will the gas bubble occupy when it reaches the surface where the atmospheric
pressure is
744 mmHg, if there is no change in the temperature or amount of gas?

Answer
73.5 mL
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake

© 2016 Pearson Education, Inc.


Sample Problem 8.4 Calculating Volume When Temperature Changes
Helium gas is used to inflate the abdomen during laparoscopic surgery. A sample of helium gas has a volume of
5.40 L and a temperature of 15 °C. What is the final volume, in liters, of the gas if the temperature has been
increased to 42 °C at constant pressure and amount of gas?

Solution
Step 1

Organize the data in a table of initial and final conditions. The properties that change, which are the

temperature and volume, are listed in the following table. The properties that do not change, which are
pressure and amount of gas, are shown below the table. When the temperature is given in degrees Celsius, it
must be changed to kelvins. Because we know the initial and final temperatures of the gas, we know that the
temperature increases. Thus, we can predict that the volume increases.
T1 = 15 °C + 273 = 288 K
T2 = 42 °C + 273 = 315 K

Factors that remain constant: P and n
Step 2

Rearrange the gas law equation to solve for the unknown quantity. In this problem, we want to know the final
volume (V2) when the temperature increases. Using Charles’s law, we solve for V2 by multiplying both sides
by T2.

General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake

© 2016 Pearson Education, Inc.


Sample Problem 8.4 Calculating Volume When Temperature Changes
Continued

Step 3

Substitute values into the gas law equation and calculate. From the table, we see that the temperature has
increased. Because temperature is directly related to volume, the volume must increase. When we substitute
in the values, we see that the ratio of the temperatures 1temperature factor2 is greater than 1, which increases
the volume as predicted in Step 1.

Study Check 8.4
A mountain climber with a body temperature of 37 °C inhales 486 mL of air at a temperature of –8 °C. What volume,
in milliliters, will the air occupy in the lungs, if the pressure and amount of gas do not change?

Answer
569 mL
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake

© 2016 Pearson Education, Inc.


Sample Problem 8.5 Calculating Pressure When Temperature Changes
Home oxygen tanks, which provide an oxygen-rich environment, can be dangerous if they are heated, because
they can explode. Suppose an oxygen tank has a pressure of 120 atm at a room temperature of 25 °C. If a fire in
the room causes the temperature of the gas inside the oxygen tank to reach 402 °C, what will be its pressure in
atmospheres? The oxygen tank may rupture if the pressure inside exceeds 180 atm. Would you expect
it to rupture?

Solution
Step 1

Organize the data in a table of initial and final conditions. We list the properties that change, which are the
pressure and temperature, in a table. The properties that do not change, which are volume and amount of gas,
are shown below the table. The temperatures given in degrees Celsius must be changed to kelvins. Because
we know the initial and final temperatures of the gas, we know that the temperature increases. Thus, we can
predict that the pressure increases.
T1 = 25 °C + 273 = 298 K
T2 = 402 °C + 273 = 675 K

Factors that remain constant: V and n

General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake

© 2016 Pearson Education, Inc.


Sample Problem 8.5 Calculating Pressure When Temperature Changes
Continued
Step 2

Rearrange the gas law equation to solve for the unknown quantity. Using Gay-Lussac’s law, we can solve
for P2 by multiplying both sides by T2.

Step 3

Substitute values into the gas law equation and calculate. When we substitute in the values, we see that
the ratio of the temperatures (temperature factor), is greater than 1, which increases pressure as predicted
in Step 1.

Because the calculated pressure of 270 atm of the gas exceeds the limit of 180 atm, we would expect the
oxygen tank to rupture.

General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake

© 2016 Pearson Education, Inc.

Sample Problem 8.5 Calculating Pressure When Temperature Changes
Continued

Study Check 8.5
In a storage area of a hospital where the temperature has reached 55 °C, the pressure of oxygen gas in a 15.0-L steel
cylinder is 965 Torr. To what temperature, in degrees Celsius, would the gas have to be cooled to reduce the pressure to
850. Torr when the volume and amount of the gas do not change?

Answer
16 °C

General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake

© 2016 Pearson Education, Inc.


Sample Problem 8.6 Using the Combined Gas Law
A 25.0-mL bubble is released from a diver’s air tank at a pressure of 4.00 atm and a temperature of 11 °C. What is
the volume, in milliliters, of the bubble when it reaches the ocean surface, where the pressure is 1.00 atm and the
temperature is 18 °C? (Assume the amount of gas in the bubble does not change.)

Solution
Step 1

Organize the data in a table of initial and final conditions. We list the properties that change, which are the
pressure, volume, and temperature, in a table. The property that remains constant, which is the amount of gas,
is shown below the table. The temperatures in degrees Celsius must be changed to kelvins.
T1 = 11 °C + 273 = 284 K
T2 = 18 °C + 273 = 291 K

Factors that remain constant: n

General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake

© 2016 Pearson Education, Inc.


Sample Problem 8.6 Using the Combined Gas Law
Continued
Step 2

Rearrange the gas law equation to solve for the unknown quantity.

Step 3

Substitute values into the gas law equation and calculate. From the data table, we determine that both the
pressure decrease and the temperature increase will increase the volume.

However, when the unknown value is decreased by one change but increased by the second change, it is
difficult to predict the overall change for the unknown.

General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake

© 2016 Pearson Education, Inc.


Sample Problem 8.6 Using the Combined Gas Law

Continued

Study Check 8.6
A weather balloon is filled with 15.0 L of helium at a temperature of 25 °C and a pressure of 685 mmHg. What is
the pressure, in millimeters of mercury, of the helium in the balloon in the upper atmosphere when the temperature
is –35 °C and the final volume becomes 34.0 L, if the amount of He does not change?

Answer
241 mmHg

General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake

© 2016 Pearson Education, Inc.


Sample Problem 8.7 Calculating Volume for a Change in Moles
A weather balloon with a volume of 44 L is filled with 2.0 moles of helium. What is the final volume, in liters, if
3.0 moles of helium gas are added, to give a total of 5.0 moles of helium gas, if the pressure and temperature do
not change?

Solution
Step 1

Organize the data in a table of initial and final conditions. We list those properties that change, which are
volume and amount (moles) of gas, in a table. The properties that do not change, which are pressure and
temperature, are shown below the table. Because there is an increase in the number of moles of gas, we can
predict that the volume increases.

Factors that remain constant: P and T

Step 2

Rearrange the gas law equation to solve for the unknown quantity. Using Avogadro’s law, we can solve for V2
by multiplying both sides by n2.

General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake

© 2016 Pearson Education, Inc.


Sample Problem 8.7 Calculating Volume for a Change in Moles
Continued

Step 3

Substitute values into the gas law equation and calculate. When we substitute in the values, we see that the
ratio of the moles (mole factor) is greater than 1, which increases the volume as predicted in Step 1.

Study Check 8.7
A sample containing 8.00 g of oxygen gas has a volume of 5.00 L. What is the final volume, in liters, after 4.00 g of
oxygen gas is added to the 8.00 g of oxygen in the balloon, if the temperature and pressure do not change?

Answer
7.50 L

General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake

© 2016 Pearson Education, Inc.

Sample Problem 8.8 Using Molar Volume to Find Volume at STP
What is the volume, in liters, of 64.0 g of O2 gas at STP?

Solution
Step 1

State the given and needed quantities.

Step 2

Write a plan to calculate the needed quantity. The grams of O 2 are converted to moles using molar mass. Then a
molar volume conversion factor is used to convert the number of moles to volume (L).

Step 3

Write the equalities and conversion factors including 22.4 L/mole at STP.

General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake

© 2016 Pearson Education, Inc.


Sample Problem 8.8 Using Molar Volume to Find Volume at STP
Continued
Step 4

Set up the problem with factors to cancel units.

Study Check 8.8
How many grams of N2(g) are in 5.6 L of N2(g) at STP?

Answer
7.0 g of N2

General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake

© 2016 Pearson Education, Inc.


Sample Problem 8.9 Using the Ideal Gas Law
Dinitrogen oxide, N2O, which is used in dentistry, is an anesthetic also called laughing gas. What is the pressure, in
atmospheres, of 0.350 mole of N2O at 22 °C in a 5.00-L container?

Solution
Step 1

State the given and needed quantities. When three of the four quantities (P, V, n, and T) are known, we use the
ideal gas law equation to solve for the unknown quantity. It is helpful to organize the data in a table. The
temperature is converted from degrees Celsius to kelvins so that the units of V, n, and T match the unit of the
gas constant R.

General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake

© 2016 Pearson Education, Inc.

Sample Problem 8.9 Using the Ideal Gas Law
Continued
Step 2

Rearrange the ideal gas law equation to solve for the needed quantity. By dividing both sides of the ideal gas
law equation by V, we solve for pressure, P.

Step 3

Substitute the gas data into the equation and calculate the needed quantity.

Study Check 8.9
Chlorine gas, Cl2, is used to purify water. How many moles of chlorine gas are in a 7.00-L tank if the gas has a pressure
of 865 mmHg and a temperature of 24 °C?

Answer
0.327 mole of Cl2

General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake

© 2016 Pearson Education, Inc.


Sample Problem 8.10 Calculating Mass Using the Ideal Gas Law
Butane, C4H10, is used as a fuel for camping stoves. If you have 108 mL of butane gas at 715 mmHg and 25 °C ,
what is the mass, in grams, of butane?

Solution

Step 1

State the given and needed quantities. When three of the quantities (P, V, and T) are known, we use the
ideal gas law equation to solve for the unknown quantity, moles (n). Because the pressure is given in
mmHg, we will use R in mmHg. The volume given in milliliters (mL) is converted to a volume in liters
(L). The temperature is converted from degrees Celsius to kelvins.

General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake

© 2016 Pearson Education, Inc.


Sample Problem 8.10 Calculating Mass Using the Ideal Gas Law
Continued
Step 2

Rearrange the ideal gas law equation to solve for the needed quantity. By dividing both sides of the ideal gas
law equation by RT, we solve for moles, n.

Step 3

Substitute the gas data into the equation and calculate the needed quantity.

Now we can convert the moles of butane to grams using its molar mass of 58.12 g/mole:

General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake

© 2016 Pearson Education, Inc.

Sample Problem 8.10 Calculating Mass Using the Ideal Gas Law
Continued

Study Check 8.10
What is the volume, in liters, of 1.20 g of carbon monoxide at 8 °C if it has a pressure of 724 mmHg?

Answer
1.04 L

General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake

© 2016 Pearson Education, Inc.


Sample Problem 8.11 Gases in Chemical Reactions
Calcium carbonate, CaCO3, in antacids reacts with HCl in the stomach to reduce acid reflux.

How many liters of CO2 are produced at 752 mmHg and 24 °C from a 25.0-g sample of calcium carbonate?

Solution
Step 1

State the given and needed quantities.

Step 2

Write a plan to convert the given quantity to the needed moles.

General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake

© 2016 Pearson Education, Inc.


Sample Problem 8.11 Gases in Chemical Reactions
Continued
Step 3

Write the equalities and conversion factors for molar mass and mole–mole factors.

Step 4

Set up the problem to calculate moles of needed quantity.

Step 5

Convert the moles of needed quantity to mass or volume using the molar mass or the ideal gas law equation.

General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake

© 2016 Pearson Education, Inc.