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MCAT > MCAT Gen. Chemistry > Stoichiometry > Flashcards

Stoichiometry Flashcards

Empirical formulas, percent composition by mass, limiting reagents…these topics appear often on the MCAT, and many students haven’t seen them in years! Use these cards to master stoichiometry as it appears on the Chemical & Physical Foundations section.

1
Q

Define:

mole

A

It 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, which is Avogadro’s number.

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2
Q

What is the mass of one mole of helium atoms?

One helium atom has a mass of 4 AMU.

A

4 g

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

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3
Q

Define:

atomic weight

A

It 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.

For example, 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

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4
Q

Define:

atomic mass

A

It is the mass, in atomic mass units (AMU), of one atom of a particular isotope of an element.

It also represents 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.

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

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5
Q

Define:

molecular weight

A

It 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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6
Q

What is the molecular weight of glucose, C6H12O6?

A

180.20 AMU

(6 * C) + (12 * H) + (6 * O) =
(6 * 12.01) + (12 * 1.01) + (6 * 16.00)

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

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7
Q

Define:

molecular formula

A

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

For example, the molecular formula of glucose, with 6 carbon atoms, 12 hydrogen atoms, and 6 oxygen atoms, is C6H12O6.

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8
Q

Define:

empirical formula

A

It is the simplified ratio of whole numbers for the different elements in a compound.

For example, the empirical formula of glucose, with 6 carbon atoms, 12 hydrogen atoms, and 6 oxygen atoms, is CH2O.

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9
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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10
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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11
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 to get the total sum.

For example, 1 mole of H2O has a mass of 18 g.

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

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12
Q

What is the approximate 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, approximation is almost always good enough on most chemistry tests, especially on the MCAT.

Weight (CO2) = 12 + 2(16) = 44 g

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13
Q

How many moles are in 32 g of SO2?

A

1/2 of a mole

1 mole of SO2 has a mass of:

32 + 2(16) = 64 g

Thus, 32 g is one half a mole.

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14
Q

Define:

percent composition

A

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

The formula for percent composition is:

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15
Q

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

A

53.3%

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16
Q

What is density, and in what units is it typically measured?

A

It is a characteristic property of a substance. It describes the mass of a particular volume of that substance.

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

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17
Q

What is the density of water in:

  1. g/cm3
  2. g/mL
  3. kg/m3
  4. kg/L
A
  1. 1 g/cm3
  2. 1 g/mL
  3. 1,000 kg/m3
  4. 1 kg/L
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18
Q

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

A

About 900 g

Mass = Density * Volume

1.819 g/cm3 * 500 mL ≈ 900 g

Remember that 1 mL = 1 cm3. Also, remember that you won’t have a calculator on the MCAT, so approximations will be necessary.

19
Q

Define:

oxidation state

A

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

For example, 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.

20
Q

Define:

oxidation

A

It is the process in which a chemical species loses electrons.

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

21
Q

Define:

reduction

A

It is the process in which a chemical species gains electrons.

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

22
Q

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

Na(s) + Co3+ ⇒ Na+ + Co2+

A
  • Co3+ 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.

23
Q

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

  1. MnO4-
  2. NaBH4
  3. Cr2O7
  4. O2
  5. LiAlH4
A
  1. MnO4- is an oxidizing agent
  2. NaBH4 is a reducing agent
  3. Cr2O7 is an oxidizing agent
  4. O2 is an oxidizing agent
  5. LiAlH4 is a reducing agent

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.

24
Q

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

Hg2Cl2 → HgCl2 + Hg

A

They 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

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

A

zero

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

26
Q

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

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

Remember these notable exceptions: oxygen is -1 in peroxides (such as H2O2), while hydrogen is -1 in hydrides (such as NaH).

27
Q

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

A

+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.

28
Q

What oxidation state is usually assigned to the following?

  1. alkali metals
  2. alkaline earth metals
  3. halogens
A
  1. alkali metals = +1
  2. alkaline earth metals = +2
  3. halogens = -1

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

29
Q

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

  • O2
  • H2O
  • H2O2
A
  • 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.

30
Q

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

  • NO
  • NO2
  • HNO3
A
  • 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.

31
Q

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

  • H2S
  • S8
  • SO2
A
  • 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.

32
Q

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 both sides. H2O also needs to be doubled, to equate the total number of H’s and O’s on both sides.

33
Q

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 is to balance the NO3 groups by doubling AgNO3 on the left. But doing so unbalances the equation in Ag, making it necessary to double Ag on the right to restore balance.

34
Q

Balance this chemical equation:

CrO3 → Cr2O3 + O2

A

4 CrO3 → 2 Cr2O3 + 3 O2

The first step in this reaction is to balance Cr atoms by doubling the CrO3. To balance O atoms at this point requires a coefficient of 3/2 in front of the O2. However, the rules of equation-balancing require all coefficients to be whole numbers. Doubling every coefficient, however, allows you to balance the equation using whole numbers.

35
Q

Define:

limiting reactant

A

It is the reactant which is totally consumed when a chemical reaction is complete.

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

36
Q

What process can be used to find 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.
37
Q

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?

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).

38
Q

What is the limiting reactant in the reaction

HCl + NaOH ⇒ NaCl + H2O

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

A

HCl

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

39
Q

What is the limiting reactant in the reaction

CH4 + 2 O2 ⇒ CO2 + 2 H2O

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

A

O2

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 of excess CH4 remaining.

40
Q

What is the theoretical yield of a reaction?

A

It is the maximum amount of product that the given amount of reactants are capable of producing.

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

41
Q

What is the theoretical yield of NaCl for the equation

NaOH + HCl ⇒ H2O + NaCl

if there are 1.5 moles NaOH and 1 mole HCl present?

A

1 mole of NaCl

1.5 moles of NaOH could support the formation of 1.5 moles of NaCl. In contrast, 1 mole of HCl could support the formation of 1.0 moles of NaCl. However, the limiting reagent is HCl, so 1 mole is the maximum product that is possible from this system.

42
Q

What does the Δ sign indicate in the below reaction?

A

heat was added

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.

43
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 Gen. Chemistry (10 decks)

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