Chemical Calculations and Gas Law Principles: Stoichiometry and Kinetic Theory

Stoichiometry and Fixed Ratios

Fixed Ratios in Building and Manufacturing

Describe an experience you’ve had making or building something where the amount of each ingredient or building block came in fixed ratios.

Building a model airplane is an example. For each model, there must be two wings and three wheels. In order for the model to look correct or work properly, there must be a certain, unvarying number of wings and wheels.

Direct Arithmetic Units in Chemical Problems

A chemical problem may be presented to you in units of moles, mass, or volume. Which one of these can be directly used in your arithmetic no matter what the conditions are?

Moles can be directly used. The number of molecules or moles is the basic unit used in solving chemical problems. When the number of moles are known, the mass and volume can be determined.

Importance of Coefficients in Balanced Reactions

What is the importance of the coefficients in a balanced chemical reaction?

The coefficients in a balanced chemical equation indicate the relative number of moles of reactants and products. From this information, the amounts of reactants and products can be calculated. The number of moles may be converted to:

• Mass
• Volume
• Number of representative particles

General Procedure for Stoichiometric Problems

What is the general procedure for solving a stoichiometric problem?

The general procedure involves using the balanced equation to relate quantities:

1. The coefficients from the balanced equation are used to write mole ratios.
2. The mole ratios relate the moles of reactants to the moles of product.
3. By multiplying the number of moles of the reactant by the mole ratio, you can determine the number of moles of the product.

Omitting Mole Conversions in Stoichiometry

In which kind of stoichiometric calculation can the steps involving conversion to and from moles be omitted? Explain why it is possible to do so.

Volume-volume conversions between gases do not require mole conversions. This is possible because the molar volumes of all gases at Standard Temperature and Pressure (STP) are the same. The coefficients in a balanced equation indicate both the relative number of moles and the relative volumes of interacting gases.

Can Percent Yield Exceed 100%?

Assuming no errors were made in measuring the yield, can the percent yield of a chemical reaction be greater than 100%?

No. The formula for percent yield is:

$$ \text{Percent Yield} = \frac{\text{Actual Yield}}{\text{Theoretical Yield}} \times 100\% $$

The theoretical yield is the maximum amount of product that could be formed from given amounts of reactants. Therefore, the actual yield must always be less than or equal to the theoretical yield.

Gas Laws and Kinetic Theory

Effect of Heating on Gas Pressure in Rigid Containers

How does the pressure of an enclosed gas in a rigid container change when the gas is heated? Explain why this change occurs.

The pressure increases when the gas is heated because increasing the temperature of the gas increases the average kinetic energy of the particles in the gas. This increase in kinetic energy contributes to pressure in two ways:

• There is an increase in the frequency of collisions between the particles and the container walls.
• The collisions occur with greater force because the particles are moving faster, on average.

Both factors contribute to the increase in pressure.

Pressure Increase When Pumping a Bicycle Tire

Explain how pumping air into a bicycle tire increases the pressure within the tire.

Adding air increases the number of gas particles in the tire. Collisions of particles with the inside walls of the tire cause the pressure exerted by the enclosed gas. Therefore, increasing the number of air particles increases the number of collisions, which in turn increases the pressure within the tire.

Differences Between Real Gases and Ideal Gases

What are some of the differences between a real gas and an ideal gas?

The behavior of a real gas deviates from the behavior of an ideal gas, particularly at low temperatures and high pressures. Key differences include:

• An ideal gas follows the gas laws at all conditions. A real gas does not.
• Kinetic theory assumes that ideal gas particles have no volume and are not attracted to each other. This is not true for real gases.
• Real gases can be liquefied and sometimes solidified by cooling and applying pressure, but ideal gases cannot.