Chapter 9: Chemical Quantities

Question 1

What is the purpose of balancing equations?

Answer 1

To account for every atom. Since atoms are simply rearranged in a chemical reaction, atoms are neither created nor destroyed. Thus, all the participating elements of a reactant must also be present on the product side of the chemical equation.

Question 2

The __________ in a balanced equation give ____________.

Answer 2

The coefficients in a balanced equation give the relative numbers of molecules.

Question 3

An equation with noninteger coefficients makes sense only if the equation _______________________.

Answer 3

An equation with noninteger coefficients makes sense only if the equation means moles (of molecules) of the various reactants and products.

Question 4

Mole Ratio

Answer 4

The ratio of moles of one substances to moles of another substance in a balanced chemical equation.

Question 5

Steps for Calculating the Masses of Reactants and Products in Chemical Reactions

Answer 5

1. Balance the equation for the reaction.
2. Convert the masses of reactants or products to moles.
3. Use the balanced equation to set up the appropriate mole ratio(s).
4. Use the mole ratio(s) to calculate the number of moles of the desired reactant or product.
5. Convert from moles back to masses.

Question 6

Stoichiometry

Answer 6

The process of using a balanced chemical equation to determine the relative masses of reactants and products involved in a reaction.

Question 7

Chemists say that the balanced equation for a chemical reaction describes the _______________ of the reaction.

Answer 7

Chemists say that the balanced equation for a chemical reaction describes the stoichiometry of the reaction.

Question 8

Stoichiometric Mixture

Answer 8

A type of mixture containing the relative amounts of reactants that matches exactly the numbers in the balanced equation.

Question 9

Limiting Reagent

Answer 9

The reactant that is completely consumed when a reaction is run to completion. In other words, the reactant that runs out first and thus limits the amounts of products that can form.

(aka the limiting reactant)

Question 10

Two ways to determine the limiting reactant in a chemical reaction

Answer 10

1. Comparing the moles of reactants to see which runs out first.
2. Consider the amounts of products that can be formed by completely consuming each reactant. The reactant that produces the smallest amount of product must run out first and thus be limiting.

Question 11

Steps for Solving Stoichiometry Problems Involving Limiting Reactants

Answer 11

1. Write and balance the equation for the reaction.
2. Convert known masses of reactants to moles.

(OPTION 1)

3. Using the numbers of moles of reactants and the appropriate mole ratios, determine which reactant is limiting.
4. Using the amount of the limiting reactant and the appropriate mole ratio, compute the number of moles of the desired product.

PROCEED TO STEP 5.

(OPTION 2)

1. Using the approrpiate mole ratios, compute the numbers of moles of product formed if each reactant were consumed.
2. Choose the least number of moles of product formed from Step 3.
   * PROCEED TO STEP 5.*
3. Convert from moles of product to grams of product, using the molar mass (if required).

Question 12

Theoretical Yield

Answer 12

The maximum amount of a given product that can be formed when the limiting reactant is completely consumed.

Question 13

Percent Yield

Answer 13

The actual yield of a product as a percentage of the theoretical yield.

Actual Yield x 100% = percent yield
Theoretical Yield

Question 14

To convert between moles of reactants and moles of products, we use ________________ derived from the balanced chemical equation.

Answer 14

To convert between moles of reactants and moles of products, we use mole ratios derived from the balanced chemical equation.

Question 15

To calculate masses from the moles of reactants needed or products formed, we can use the _____________________ for finding the masses (g) needed or formed.

Answer 15

To calculate masses from the moles of reactants needed or products formed, we can use the molar masses of substances for finding the masses (g) needed or formed.

Question 16

Often, reactants in a chemical reaction are __________________ in stoichiometric quantities.

Answer 16

Often, reactants in a chemical reaction are not present in stoichiometric quantities (i.e., they do not “run out” at the same time).

In this case, we must determine which reactant runs out first and thus limits the amount of products that can form - this is called the limiting reactant.

Question 17

The actual yield (amount produced) of a reaction is usually _________ than the maximum expected (theoretical yield).

Answer 17

The actual yield (amount produced) of a reaction is usually less than the maximum expected (theoretical yield).

Question 18

The actual yield is often expressed as ________________ of the theoretical yield.

Answer 18

The actual yield is often expressed as a percentage of the theoretical yield.

Percent yield = actual yield (g)/theoretical yield (g) x !00%

Question 19

What does it mean to say that the balanced chemical equation for a reaction describes the stoichiometry of the reaction?

Answer 19

Stoichiometry is the process of using a chemical equation to calculate the relative masses of reactants and products involved in a reaction.