Book: Ch. 3

Question 1

Define mole

Answer 1

The mole (abbreviated mol) is the SI unit for amount of substance. It is defined as the amount of a substance that contains the same number of entities as the number of atoms in 12 g of carbon-12.

Question 2

What is Avogadro’s number?

Answer 2

One mole (1 mol) contains 6.022x10²³ entities (to four significant figures)

Question 3

How do you find 1 mol of a given element?

Answer 3

Elements. The mass in atomic mass units (amu) of one atom of an element is the same numerically as the mass in grams (g) of 1 mole of atoms of the element.

Question 4

How do you find 1 mol of a given compound?

Answer 4

Compounds. The mass in atomic mass units (amu) of one molecule (or formula unit) of a compound is the same numerically as the mass in grams (g) of 1 mole of the compound.

Question 5

The two key points to remember about the importance of the mole unit are _____

Answer 5

1. The mole lets us relate the number of entities to the mass of a sample of those entities.
2. The mole maintains the same numerical relationship between mass on the atomic scale (atomic mass units, amu) and mass on the macroscopic scale (grams, g).

Question 6

What is the molar mass of a substance?

Answer 6

The mass per mole of its entities (atoms, molecules, or formula units) and has units of grams per mole (g/mol).

Question 7

How do you find the molar mass for a given element?

Answer 7

To find the molar mass, look up the atomic mass and note whether the element is monatomic or molecular.

Question 8

How do you find the molar mass for a given monatomic element?

Answer 8

The molar mass is the periodic-table value in grams per mole.

Question 9

How do you find the molar mass for a given molecular element?

Answer 9

You must know the formula to determine the molar mass (see Figure 2.15). For example, in air, oxygen exists most commonly as diatomic molecules, so the molar mass of O2 is twice that of O

Question 10

How do you find the molar mass for a given compound?

Answer 10

The molar mass is the sum of the molar masses of the atoms in the formula. Thus, from the formula of sulfur dioxide, SO2, we know that 1 mol of SO2 molecules contains 1 mol of S atoms and 2 mol of O atoms

Question 11

What do subscripts in a formula refer to?

Answer 11

Subscripts in a formula refer to individual atoms (or ions) as well as to moles of atoms (or ions).

Question 12

Explain converting between amount and mass

Answer 12

1. From amount (mol) to mass (g), multiply by the molar mass to cancel the mole unit: mass (g) = amount (mol) x no. of grams / 1 mol.
2. From mass (g) to amount (mol), divide by the molar mass (multiply by 1/m) to cancel the mass unit: amount (mol) = mass (g) x 1 mol / no. of grams

Question 13

Explain converting between amount and number (of entities)

Answer 13

1. From amount (mol) to number of entities, multiply by Avogadro’s number to cancel the mole unit: no. of entities = amount (mol) x (6.022x10²³ entities / 1 mol)
2. From number of entities to amount (mol), divide by Avogadro’s number to cancel the number of entities: amount (mol) = no. of entities x (1 mol / 6.022x10²³ entities)

Question 14

For molecular elements, Avogadro’s number gives _____.

Answer 14

molecules per mole

Question 15

To convert between mass and number, first _____.

Answer 15

convert to amount

Question 16

Only one new step is needed to solve amount-mass-number problems involving compounds: _____

Answer 16

we need the chemical formula to find the molar mass and the amount of each element in the compound.

Question 17

How is mass percent for a molecule of a compound found?

Answer 17

Mass % of element X = ((atoms of X in formula) x (atomic mass of X (amu)) / (molecular mass of compound)

Question 18

How is mass percent for a mole of compound found?

Answer 18

Mass % of element X = ((moles of X in formula) x (molar mass of X (g/mol))) / (mass (g) of 1 mol of compound)

Question 19

An element always constitutes _____ fraction of the mass of a given compound

Answer 19

the same

Question 20

How do you find the mass of an element in a mass of a compound?

Answer 20

mass of element = mass of compound x (mass of element in 1 mol of compound / mass of 1 mol of compound)

Question 21

The empirical formula is derived from _____. It shows the _____, and thus the relative number of atoms, of each element in the compound. The empirical formula for hydrogen peroxide, for example is _____

Answer 21

1. mass analysis
2. lowest whole number of moles
3. HO

Question 22

The molecular formula shows ______: the molecular formula of hydrogen peroxide is _____, twice the empirical formula.

Answer 22

1. the actual number of atoms of each element in a molecule

2. H₂O₂

Question 23

The structural formula shows _____: the structural formula of hydrogen peroxide is _____

Answer 23

1. the relative placement and connections of atoms in the molecule
2. H-O-O-H

Question 24

A chemist studying an unknown compound goes through a three-step process to find the empirical formula: _____

Answer 24

1. Determine the mass (g) of each component element.
2. Convert each mass (g) to amount (mol), and write a preliminary formula.
3. Convert the amounts (mol) mathematically to whole-number (integer) subscripts. To accomplish this math conversion,
   • Divide each subscript by the smallest subscript, and
   • If necessary, multiply through by the smallest integer that turns all subscripts into integers.

Question 25

How do you find the whole number multiple for an empirical formula to derive its molecular formula?

Answer 25

Whole number multiple = (molar mass (g/mol)) / (empirical formula mass (g/mol))

Question 26

What is combustion analysis?

Answer 26

Used to measure the amounts of carbon and hydrogen in a combustible organic compound. The unknown compound is burned in an excess of pure O2; during the combustion, the compound’s carbon and hydrogen react with the oxygen to form CO2 and H2O, respectively, which are absorbed in separate containers (Figure 3.4). By weighing the absorbers before and after combustion, we find the masses of CO2 and H2O and use them to find the masses of C and H in the compound; from these results, we find the empirical formula.

Question 27

A formula represents _____.

Answer 27

a real, three-dimensional object

Question 28

Explain molecular structure for different compounds with the same empirical formula.

Answer 28

The empirical formula tells nothing about molecular structure because it is based solely on mass analysis. In fact, different compounds can have the same empirical formula. NO₂ and N₂O₄ are inorganic cases, and there are numerous organic ones. For example, many compounds have the empirical formula CH₂ (the general formula is C_(n)H_(2n), with n an integer greater than or equal to 2), such as ethylene (C₂H₄) and propylene (C₃H₆), starting materials for two common plastics. Table 3.2 shows some biological compounds with the same empirical formula, CH₂O.

Question 29

Explain molecular structure for isomers: different compounds with the same molecular formula.

Answer 29

A molecular formula also tells nothing about structure. Different compounds can have the same molecular formula because their atoms can bond in different arrangements to give more than one structural formula. Isomers are compounds with the same molecular formula, and thus molar mass, but different properties. Constitutional, or structural, isomers occur when the atoms link together in different arrangements. Table 3.3 shows two pairs of examples. The left pair, butane and 2-methylpropane, share the molecular formula C₄H₁₀. One has a four-C chain and the other a one-C branch off a three-C chain. Both are small alkanes, so their properties are similar, but not identical. The two compounds with the molecular formula C₂H₆O have very different properties; indeed, they are different classes of organic compound—one is an alcohol and the other an ether.

Question 30

What is a chemical equation?

Answer 30

A statement that uses formulas to express the identities and quantities of substances in a chemical or physical change.

Question 31

What does it mean for a chemical equation to be perfectly balanced as all things should be?

Answer 31

The same number of each type of atom must appear on both sides.

Question 32

What are the steps for balancing a chemical equation?

Answer 32

1. Translating the statement. We first translate the chemical statement into a “skeleton” equation: the substances present before the change, called reactants, are placed to the left of a yield arrow, which points to the substances produced during the change, called products: [for example, _Mg + _O₂ → _MgO]
2. Balancing the atoms. By shifting our attention back and forth, we match the numbers of each type of atom on the left and the right of the yield arrow. In each blank, we place a balancing (stoichiometric) coefficient, a numerical multiplier of all the atoms in the formula that follows it. In general, balancing is easiest when we
   • Start with the most complex substance, the one with the largest number of different types of atoms.
   • End with the least complex substance, such as an element by itself.
3. Adjusting the coefficients. There are several conventions about the final coefficients:
   • In most cases, the smallest whole-number coefficients are preferred.
   • We used the coefficient 1 to remind us to balance each substance.
4. Checking. After balancing and adjusting the coefficients, we always check that the equation is balanced:
5. Specifying the states of matter. The final equation also indicates the physical state of each substance or whether it is dissolved in water.

Question 33

balancing coefficients refer to _____

Answer 33

both individual chemical entities and moles of entities.

Question 34

if you know the number of moles of one substance, the balanced equation tells you _____.

Answer 34

the number of moles of the others

Question 35

In a balanced equation, the amounts (mol) of substances are _____ to each other

Answer 35

stoichiometrically equivalent

Question 36

The steps for solving any stoichiometry problem: _____

Answer 36

1. Write the balanced equation.
2. When necessary, convert the known mass (or number of entities) of one substance to amount (mol) using its molar mass (or Avogadro’s number).
3. Use the molar ratio to calculate the unknown amount (mol) of the other substance.
4. When necessary, convert the amount of that other substance to the desired mass (or number of entities) using its molar mass (or Avogadro’s number).

Question 37

The steps in writing the overall (net) equation for a reaction are _____.

Answer 37

1. Write the sequence of balanced equations.
2. Adjust the equations arithmetically to cancel the common substance(s).
3. Add the adjusted equations together to obtain the overall balanced equation.

Question 38

To determine which is the limiting reactant, we _____.

Answer 38

use the molar ratios in the balanced equation to perform a series of calculations to see which reactant forms less product

Question 39

A reaction table shows the following elements:

Answer 39

• In the Initial line, “product” has not yet formed, so the entry is “0 sundaes.”
• In the Change line, since the reactants (ice cream, cherries, and syrup) are used during the reaction, their quantities decrease, so the changes in their quantities have a negative sign. At the same time, the quantity of product (sundaes) increases, so the change in its quantity has a positive sign.
• For the Final line, we add the Change and Initial lines. Notice that some reactants (ice cream and cherries) are in excess, while the limiting reactant (syrup) is used up.

Question 40

the limiting reactant is the one that yields the _____ amount of product.

Answer 40

least

Question 41

theoretical yield is _____

Answer 41

the amount of product calculated from the molar ratio in the balanced equation

Question 42

What are the causes of theoretical yield being different to actual yield?

Answer 42

• Reactant mixtures often proceed through side reactions that form different products (Figure 3.12). In the rocket fuel reaction in Sample Problem 3.20, for example, the reactants might form some NO in the following side reaction: N2H4(l) 1 2N2O4(l) -£ 6NO(g) 1 2H2O(g) This reaction decreases the amounts of reactants available for N2 production.
• Even more important, many reactions seem to stop before they are complete, so some limiting reactant is unused. (We’ll see why in Chapter 4.)
• Physical losses occur in every step of a separation (see Tools of the Laboratory, Section 2.9): some solid clings to filter paper, some distillate evaporates, and so forth. With careful technique, you can minimize, but never eliminate, such losses.

Question 43

Explain actual yield.

Answer 43

Given these reasons for obtaining less than the theoretical yield, the amount of product actually obtained is the actual yield. Theoretical and actual yields are expressed in units of amount (moles) or mass (grams).

Question 44

Explain percent yield.

Answer 44

The percent yield (% yield) is the actual yield expressed as a percentage of the theoretical yield: % yield = (actual / theoretical) x 100

Question 45

Atom economy is found via the equation _____

Answer 45

% atom economy = ((no. of moles x molar mass of desired product)/(sum of (no. of moles x molar mass) for all products)) x 100