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MCAT GC WORKING > Stoichiometry > Flashcards

Stoichiometry Flashcards

1
Q
A
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2
Q

Define and give the value for:

One mole

A

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.

One mole is equal to 6.02 x 1023 particles.

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3
Q

What is the mass of one mole of helium atoms?

(one helium atom has a mass of 4 AMU)

A

4 g

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4
Q

Define:

Atomic Weight

A

Atomic weight is the mass, in grams, of one mole of an 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 isotopes, 35Cl (75% abundance) and 37Cl (25% abundance). The atomic weight of Cl is therefore:

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

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5
Q

Define

Atomic Mass

A

Atomic mass is the mass, in Atomic Mass Units, 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 is 235 AMU.

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6
Q

Define:

Molecular Weight

A

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.

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7
Q

What is the molecular weight of glucose, C6H12O6?

A

180.20

[6 * 12.01] + [12 * 1.01] + [6 * 16.00]

Remember, on the MCAT you won’t have use of a calculator, so approximate this to 180!

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8
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9
Q

Define:

Molecular Formula

A

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

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10
Q

Define:

Empirical Formula

A

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

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11
Q

What is the molecular formula of this molecule?

A

C4H8

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

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12
Q

What is the empirical formula of this molecule?

A

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.

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13
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14
Q

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

A
  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.

Ex: 1 mole of H2O has a mass of 18 g.

(1 * 2) + (16 * 1) = 18

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15
Q

What is the weight of:

1 mole of carbon dioxide, CO2?

A

44 g

To calculate the weight of 1 mole of a substance, add the atomic weights of every atom in the molecular formula. Remember that you won’t have a calculator on the MCAT, so approximate!

Weight (CO2) =

12 + 2(16) = 44 g

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16
Q

How many moles are in:

32 g of SO2?

A

1/2 a mole

1 mole of SO2 has a mass of:

32 + 2(16) = 64 g

and, thus, 32 g is one half a mole.

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17
Q

Give the formula for:

Percent composition

A
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18
Q

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

A

53.3%

But remember - on the MCAT, you’re going to approximate, so 50% is close enough to get this question right on Test Day.

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19
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20
Q

Define and give the SI units for:

Density

A

Density is a characteristic property of a substance, describing the mass of a particular volume of the substance.

The SI units for density are kg/m3,though on the MCAT you may also see g/cm3.

21
Q

What is the density of water in:

  1. g/cc
  2. g/cm3
  3. g/mL
  4. kg/m3
  5. kg/L
A
  1. 1
  2. 1
  3. 1
  4. 1,000
  5. 1
22
Q

If the density of liquid sulfur is 1.819 g/cm3, what is the approximate mass of 500 mL of liquid sulfur?

A

900 g

Mass = Density * Volume

1.819 * 500 mL ≈ 900 g

Remember that 1 mL = 1 cm3.

23
Q
A
24
Q

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

Hg2Cl2 → HgCl2 + Hg

A

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.

25
Q

Define:

Oxidation

A

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-

26
Q

Define:

Reduction

A

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)

27
Q

Identify the oxidizing and reducing agent in the following reaction:

Na(s) + Co3+ ⇒ Na+ + Co2+

A

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.

28
Q

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

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

29
Q

Define:

Oxidation State

A

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.

30
Q

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

A

Zero.

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

31
Q

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

  1. oxygen
  2. hydrogen
A
  1. -2
  2. +1

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

32
Q

Knowing that the oxidation state of the oxygens in NO3- is -2, how do you calculate the oxidation state of the nitrogen?

A

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.

33
Q

What oxidation state is usually assigned to:

  1. Alkali metals
  2. Alkali earth metals
  3. Halogens
A
  1. +1
  2. +2
  3. -1

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

34
Q

What is the oxidation state of the oxygen atom(s) in:

  • O2
  • H2O
  • H2O2
A
  • 0

The oxidation number for every element in its standard state is zero.

  • -2

This is oxygen’s typical oxidation state in compounds.

  • -1

Peroxides are the one compound where oxygen has this oxidation state.

35
Q

What is the oxidation state of the nitrogen atom in:

  • NO
  • NO2
  • HNO3
A
  • +2

Oxygen’s oxidation state is -2, and the molecule is neutrally charged, so the nitrogen must be +2.

  • +4

Oxygen’s oxidation state is -2 (total of -4 for both), and the molecule is neutrally charged, so the nitrogen must be +4.

  • +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.

36
Q

What is the oxidation state of the sulfur atom in:

  • H2S
  • S8
  • SO<span>2</span>
A
  • -2

H2=2(+1)=2, S must be -2 to compensate.

  • 0

This is Sulfur’s standard state, all atoms are zero oxidation in their standard state.

  • +4

The two oxygens (-2 each) have a total charge of -4, S must be +4 to compensate.

37
Q
A
38
Q

Please balance this chemical equation:

H2SO4 + NaOH →
Na2SO4 + H2O

A

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.

39
Q

Please balance this chemical equation:

AgNO3 + Cu →
Cu(NO3)2 + Ag

A

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.

40
Q

Please balance this chemical equation:

CrO3 →
Cr2O3 + O2

A

4 CrO3 →
2 Cr2O3 + 3 O2

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 O2, violating the rules of equation balancing. Doubling every coefficient, however, allows you to balance the equation using whole numbers.

41
Q

Define:

Limiting Reactant

A

The reactant which is totally consumed when the chemical reaction is complete.

The amount of limiting reactant present defines the amount of product which can be created.

42
Q

What is the limiting reactant in:

AgNO3 + NaCl ⇒
AgCl + NaNO3

if there are 1 mole of AgNO3 and 2 moles of NaCl present?

A

AgNO3

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).

43
Q

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

A
  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.
44
Q

What is the limiting reactant in the following reaction:

HCl + NaOH ⇒
NaCl + H2O

if there are 37g HCl and 60g NaOH present?

A

HCl

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.

45
Q

What is the limiting reactant in the following reaction:

CH4 + 2 O2 →
CO2 + 2 H2O

if there are 24g CH4 and 64g O2 present?

A

O2

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 CH4remaining.

46
Q

Define:

Theoretical Yield (of a reaction)

A

Theoretical Yield (or Expected Product) is the maximum amount of product that the given amount of reactants are capable of producing.

This amount can be in number (moles) or weight (grams) and assumes ideal reactivity, with 100% efficiency and no experimental error.

47
Q

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?

A

1 mole of NaCl

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 lesser of these factors (1.0 moles due to HCl) is the maximum product that is possible from this system.

48
Q

What does the Δ sign indicate in the below reaction?

A

Heat added

The Δ sign indicates that heat is necessary for the reaction to occur. In the case of this reaction, 172.5 kJ per mole are required.

49
Q

What is the difference between the arrows in these two equations?

2 H2O ⇔
H3O+ + OH-

NaCl (s) ⇒
Na+(aq) + Cl-(aq)

A

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.

MCAT GC WORKING (11 decks)

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