Stoichiometry

Question 1

Define and give the value for:

one mole

Answer 1

A mole is the number of particles of a substance that must be present in a sample such that the sample’s mass in grams is equal to the substance’s atomic weight in AMU.

One mole is equal to 6.02 x 10²³ particles.

Question 2

What is the mass of one mole of helium atoms?

(one helium atom has a mass of 4 AMU)

Answer 2

4 g

One mole of atoms or molecules is the exact number such that the entire sample will have a mass (in g) of the individual atom of molecule’s mass in AMU.

Question 3

Define:

atomic weight

Answer 3

Atomic weight is the mass, in grams, of one mole of a naturally occurring element.

To calculate atomic weight, one must take into account both the weight of all the naturally-occurring isotopes of that element, and their proportional abundance.

Ex: natural Cl appears as two significant isotopes, ³⁵Cl (75% abundance) and ³⁷Cl (25% abundance). The atomic weight of Cl is therefore:

(0.75 * 35) + (0.25 * 37) = 35.5

Question 4

Define

atomic mass

Answer 4

Atomic mass is the mass, in Atomic Mass Units (AMU), of one atom of a particular isotope of an element.

It also represent the mass in grams of one mole of the isotopes, and can be found by adding together the number of protons and neutrons in that particular isotope.

Ex: the atomic mass of ²³⁵U, with 92 protons and 143 neutrons, is 235 AMU.

Question 5

Define:

molecular weight

Answer 5

Molecular weight is the weight of one mole of molecules of a substance.

It can also be calculated by adding together the atomic weights of all the atoms in the molecule.

Question 6

What is the molecular weight of glucose, C₆H₁₂O₆?

Answer 6

180.20 AMU

[6 * C] + [12 * H] + [6 * 16] =
[6 * 12.01] + [12 * 1.01] + [6 * 16.00]

These 3 masses are worth memorizing. Remember, approximation is almost always good enough, so let C = 12, H = 1, and O = 16, for a total of 180 AMU.

Question 7

Define:

molecular formula

Answer 7

A molecular formula shows the total number and type of atoms in each molecule. This is the full, unsimplified formula.

Ex: The molecular formula of glucose, with 6 carbon atoms, 12 hydrogen atoms, and 6 oxygen atoms, is: C₆H₁₂O₆

Question 8

Define:

empirical formula

Answer 8

An empirical formula is a simplified ratio of whole numbers for the different elements in a compound.

Ex: The empirical formula of glucose, with 6 carbon atoms, 12 hydrogen atoms, and 6 oxygen atoms, is: CH₂O

Question 9

What is the molecular formula of this molecule?

Answer 9

C₄H₈

The molecular formula is the total of all the atoms present in a single molecule of the substance.

Question 10

What is the empirical formula of this molecule?

Answer 10

CH₂

The empirical formula is the ratio of the number of atoms in a substance, expressed as the lowest common denominator.

In this case, take the molecular formula, C₄H₈, and divide both subscripts by 4 to get to the final answer.

Question 11

How do you calculate the mass of one mole of a substance?

Answer 11

1. Find the atomic weight of each element in the molecular formula.
2. Multiply that weight by the subscript (number of that atom present).
3. Add these numbers together to get the total sum.

Ex: 1 mole of H₂O has a mass of 18 g.
(1 * 2) + (16 * 1) = 18

Question 12

What is the weight of:

1 mole of carbon dioxide, CO₂?

Answer 12

44 g

To calculate the weight of 1 mole of a substance, add the atomic weights of every atom in the molecular formula. Remember approximation is almost always good enough on most chemistry tests, especially on the AP Chemistry Exam.

Weight (CO₂) = 12 + 2(16) = 44 g

Question 13

How many moles are in:

32 g of SO₂?

Answer 13

1/2 a mole

1 mole of SO₂ has a mass of: 32 + 2(16) = 64 g
Thus, 32 g is one half a mole.

Question 14

Give the formula for:

percent composition

Answer 14

Percent composition is a calculation of the proportion of a substance that a particular element makes up, by weight.

Question 15

What is the percent composition of oxygen in glucose, C₆H₁₂O₆?

Answer 15

53.30%

Question 16

This reaction is a disproportionation reaction. What does that mean?

Hg₂Cl₂ → HgCl₂ + Hg

Answer 16

Disproportionation reactions are a subset of redox reactions, in which one species (in this case, the Hg atom) acts as both the oxidizing agent and the reducing agent.

Question 17

Define:

oxidation

Answer 17

Oxidation is the process of a chemical species losing electrons.

When a species is oxidized, its oxidation state increases.
Ex: Co²⁺ (aq) ⇒ Co³⁺ (aq) + e^-

Question 18

Define:

reduction

Answer 18

Reduction is the process of a chemical species gaining electrons.

When a species is reduced, its oxidation state decreases.
Ex: Cu²⁺ (aq) + e^- ⇒ Cu⁺(aq)

Question 19

Identify the oxidizing agent and reducing agent in the following reaction:

Na(s) + Co³⁺ ⇒ Na⁺ + Co²⁺

Answer 19

Co is the oxidizing agent, Na is the reducing agent.

In a redox reaction, the oxidizing agent is the species which receives electrons and is reduced, while the reducing agent is the species which donates electrons and is oxidized.

Question 20

Is each of the following an oxidizing agent, or a reducing agent?

1. MnO₄^-
2. NaBH₄
3. Cr₂O₇
4. O₂
5. LiAlH₄

Answer 20

1. MnO₄^-; Oxidizing
2. NaBH₄; Reducing
3. Cr₂O₇; Oxidizing
4. O₂; Oxidizing
5. LiAlH₄; Reducing

Most common oxidizing agents contain oxygen atoms, particularly in the presence of metal atoms

Most common reducing agents contain hydrogen atoms in the presence of metal atoms.

Question 21

Define:

oxidation state

Answer 21

Oxidation state is the formal charge left on an atom if it is assumed that every bond in a molecule is perfectly ionic.

Ex: in H₂O, assume that the more electronegative O atom takes all the electrons in both bonds, giving it an oxidation state of -2, and leaving each H with a +1 oxidation state.

Question 22

What is the oxidation state assigned to any element in its standard state?

Answer 22

The oxidation state is zero.

Ex: in O₂ the oxygen atoms share electrons perfectly, and so each has an oxidation state of zero.

Question 23

What oxidation states are commonly assigned to the following when not in their standard states?

1. oxygen
2. hydrogen

Answer 23

1. oxygen = -2
2. hydrogen = +1

Notable exceptions: oxygen is -1 in peroxides (such as H₂O₂), hydrogen is -1 in hydrides (such as NaH).

Question 24

Knowing that the oxidation state of the oxygen atoms in NO₃^- is -2, what is the oxidation state of the nitrogen?

Answer 24

The oxidation state of the nitrogen is +5

Add up the total oxidation for all the molecule’s known atoms: (-2) * 3 = -6
Subtract that amount from the molecule’s net charge: (-1) - (-6) = +5
The remaining amount is the unknown atom’s oxidation state.

Question 25

# What oxidation state is usually assigned to the following? 1. Alkali metals 2. Alkali Earth Metals 3. Halogens

Answer 25

1. Alkali Metals = +1 2. Alkali Earth Metals = +2 3. Halogens = -1 ## Footnote Elements in these groups will typically give up or accept their standard number of electrons when making bonds in compounds.

Question 26

# What is the oxidation state of the oxygen atom(s) in the following? * O₂ * H₂O * H₂O₂

Answer 26

* In O₂, O = 0 The oxidation number for every element in its standard state is zero. * In H₂O, O = -2 This is oxygen's typical oxidation state in compounds. * In H₂O₂, O = -1 Peroxides are the one compound where oxygen has this oxidation state.

Question 27

# What is the oxidation state of the nitrogen atom in the following? * NO * NO₂ * HNO₃

Answer 27

* In NO, N = +2 Oxygen's oxidation state is -2, and the molecule is neutrally charged, so the nitrogen must be +2. * In NO₂, N = +4 Oxygen's oxidation state is -2 (total of -4 for both), and the molecule is neutrally charged, so the nitrogen must be +4. * In HNO₃, N = +5 Oxygen's oxidation state is -2 (total of -6 for all 3), hydrogen's oxidation state is +1, and the molecule is neutrally charged, so the nitrogen must be +5.

Question 28

# What is the oxidation state of the sulfur atom in the following? * H₂S * S₈ * SO₂

Answer 28

* In H₂S, S = -2 H₂=2(+1)=2, S must be -2 to compensate. * In S₈, S = 0 This is sulfur's standard state, all atoms are zero oxidation in their standard state. * In SO₂, S = +4 The two oxygens (-2 each) have a total charge of -4, S must be +4 to compensate.

Question 29

# Please balance this chemical equation: H₂SO₄ + NaOH → Na₂SO₄ + H₂O

Answer 29

H₂SO₄ + 2 NaOH → Na₂SO₄ + 2 H₂O ## Footnote The first step in balancing any reaction is finding an atom which exists in a single molecule on each side (Na, in this case), and changing coefficients to have equal numbers on each side. H₂O also needed to be doubled, to equate the total number of H's and O's on both sides.

Question 30

# Please balance this chemical equation: AgNO₃ + Cu → Cu(NO₃)₂ + Ag

Answer 30

2 AgNO₃ + Cu → Cu(NO₃)₂ + 2 Ag ## Footnote The first step to balancing this reaction was to balance the NO₃ groups by doubling AgNO₃ on the left. But doing so unbalanced the equation in Ag, making it necessary to double Ag on the right to restore balance.

Question 31

# Please balance this chemical equation: CrO₃ → Cr₂O₃ + O₂

Answer 31

4 CrO₃ → 2 Cr₂O₃ + 3 O₂ ## Footnote The first step in this reaction was to balance Cr atoms by doubling the CrO₃. To balance O atoms at that point would have required a coefficient of 3/2 in front of the O_2. The rules of equation balancing require all coefficients to be whole numbers though. Doubling every coefficient, however, allows you to balance the equation using whole numbers.

Question 32

# Define: **limiting reactant**

Answer 32

The **limiting reactant** is the reactant which is totally consumed when a chemical reaction is complete. ## Footnote The amount of limiting reactant present defines the amount of product which can be created.

Question 33

What is the limiting reactant in the reaction AgNO₃ + NaCl ⇒ AgCl + NaNO₃ if there is 1 mole of AgNO₃ and 2 moles of NaCl present?

Answer 33

AgNO₃ is the limiting reactant. ## Footnote 1 mole of AgNO₃ will react completely with 1 mole of NaCl, since the equation shows that they react in a 1:1 ratio. Thus, there will be 1 mole of NaCl left over (in excess).

Question 34

What is the process for finding the limiting reactant of a reaction?

Answer 34

1. If necessary, balance the reaction. 2. If given amounts of molecules in grams, convert to moles. 3. Take the number of moles of the substance, and divide into that the coefficient in front of that substance in the balanced equation. 4. Do this for all reactants. 5. Whichever reactant has the lowest final value from that calculation is the limiting reactant.

Question 35

What is the limiting reactant in the reaction HCl + NaOH ⇒ NaCl + H₂O if there are 37g HCl and 60g NaOH present?

Answer 35

HCl is the limiting reactant. ## Footnote After converting grams to moles: there is one mole of HCl and 1.5 moles of NaOH. Since these combine in the ratio 1:1, when all the HCl has been consumed, there will be 0.5 moles of excess NaOH remaining.

Question 36

What is the limiting reactant in the reaction CH₄ + 2 O₂ → CO₂ + 2 H₂O if there are 24g CH₄ and 64g O₂ present?

Answer 36

O₂ is the limiting reactant. ## Footnote Converting grams to moles: there are 2 moles of O₂ and 1.5 moles of CH₄. Due to the 2:1 ratio in the equation, two moles of O₂ are needed for every mole of CH₄ consumed. The 2 moles of O₂ will react completely with 1 mole CH₄, leaving 0.5 moles excess CH₄ remaining.

Question 37

# Define: **theoretical yield** (of a reaction)

Answer 37

**Theoretical yield** (or expected product) is the maximum amount of product that the given amount of reactants are capable of producing. ## Footnote This amount can be in number (moles) or weight (grams) and assumes ideal reactivity, with 100% efficiency and no experimental error.

Question 38

What is the theoretical yield of NaCl for the equation NaOH + HCl ⇒ H₂0 + NaCl if there are 1.5 moles NaOH and 1 mole HCl present?

Answer 38

The theoretical yield is 1 mole of NaCl. ## Footnote The 1.5 moles NaOH could support the formation of 1.5 moles NaCl. The 1 mole HCl could support the formation of 1.0 moles NaCl. The limiting reagent is HCl though, hence 1 mole is the maximum product that is possible from this system.

Question 39

What does the Δ sign indicate in the below reaction?

Answer 39

Heat added ## Footnote The Δ sign indicates that heat is necessary for the reaction to occur. In the case of this reaction, one could find in a table that 172.5 kJ per mole are required.

Question 40

# What is the difference between the arrows in these two equations? 2 H₂O ⇔ H₃O⁺ + OH^-  NaCl (s) ⇒ Na⁺(aq) + Cl^-(aq)

Answer 40

The two-sided arrow ⇔ implies a reaction which reaches an equlibrium between reactants and products. The single-sided arrow ⇒ indicates a reaction which goes to completion, leaving only products.