Stoichiometry Flashcards Preview

AP Chemistry > Stoichiometry > Flashcards

Flashcards in Stoichiometry Deck (40)
Loading flashcards...
1
Define and give the value for:

one mole

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.

2

What is the mass of one mole of helium atoms?

(one helium atom has a mass of 4 AMU)

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.

3
Define:

atomic weight

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

4
Define

atomic mass

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.

5
Define:

molecular weight

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.

6

What is the molecular weight of glucose, C6H12O6?

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.

7
Define:

molecular formula

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

8
Define:

empirical formula

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

9

What is the molecular formula of this molecule?

C4H8

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

10

What is the empirical formula of this molecule?

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.

11

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

  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

12
What is the weight of:

1 mole of carbon dioxide, CO2?

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

13
How many moles are in:

32 g of SO2?

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.

14
Give the formula for:

percent composition

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

15

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

53.3%

 

16

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

Hg2Cl2 → HgCl2 + Hg

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.

17
Define:

oxidation

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-

18
Define:

reduction

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)

19

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

Na(s) + Co3+ ⇒ Na+ + Co2+

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.

20

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

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

  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.

21
Define:

oxidation state

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.

22

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

The oxidation state is zero.

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

23

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

  1. oxygen
  2. hydrogen

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

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

24

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

The oxidation state of the nitrogen is +5

1) Add up the total oxidation for all the molecule's known atoms:
(-2) * 3 = -6

2) Subtract that amount from the molecule's net charge:
(-1) - (-6) = +5

The remaining amount is the unknown atom's oxidation state.

25
What oxidation state is usually assigned to the following?

  1. Alkali metals
  2. Alkali Earth Metals
  3. Halogens

  1. Alkali Metals = +1
  2. Alkali Earth Metals = +2
  3. Halogens = -1

Elements in these groups will typically give up or accept their standard number of electrons when making bonds in compounds.

26
What is the oxidation state of the oxygen atom(s) in the following?

  • O2
  • H2O
  • H2O2

  • 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.

27
What is the oxidation state of the nitrogen atom in the following?

  • NO
  • NO2
  • HNO3

  • 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.

28
What is the oxidation state of the sulfur atom in the following?

  • H2S
  • S8
  • SO2

  • 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.

29
Please balance this chemical equation:

H2SO4 + NaOH →
Na2SO4 + H2O

H2SO4 + 2 NaOH →
Na2SO4 + 2 H2O

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.

30
Please balance this chemical equation:

AgNO3 + Cu →
Cu(NO3)2 + Ag

2 AgNO3 + Cu →
Cu(NO3)2 + 2 Ag

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.