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CHEM 103 > Empirical Formula > Flashcards

Empirical Formula Flashcards

1
Q

What is the first step/most common approach in determining a compound’s chemical formula?

A

First measure the masses of its constituent elements

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2
Q

What do chemical formulas actually represent?

A

The relative numbers of atoms in the substance, not the masses of such

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3
Q

True or False: An empirical formula of a compound will always be the same as its molecular formula

A

This may or not be the compound’s molecular formula as well; however, we would need additional information to make that determination

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4
Q

Consider and describe a basic process of determining empirical formulas?

A

To accomplish this, we can use molar masses to convert the mass of each element to number of moles. We then consider the moles of each element relative to each other, converting these numbers into a whole-number ratio that can be used to derive the empirical formula of the substance.

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5
Q

Describe the step-by-step process of determining empirical formula.

A

In summary, empirical formulas are derived from experimentally measured element masses by:

Deriving the number of moles of each element from its mass
Dividing each element’s molar amount by the smallest molar amount to yield subscripts for a tentative empirical formula
Multiplying all coefficients by an integer, if necessary, to ensure that the smallest whole-number ratio of subscripts is obtained

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6
Q

What should one do if the empirical formula subscript is a decimal, but we need it to be in a whole number ratio?

A

Multiply by two

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7
Q

True or False: We can derive empirical formulas from a compound’s percent composition

A

True

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8
Q

Can percent composition be used to calculate empirical formulas? If yes, describe how.

A

In such cases, the percent composition can be used to calculate the masses of elements present in any convenient mass of compound; these masses can then be used to derive the empirical formula in the usual fashion.

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9
Q

What is the empirical formula of a compound containing 40.0% C, 6.71% H, and 53.28% O?

A

CH2O

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10
Q

True or False: Determining the absolute numbers of atoms that compose a single molecule of a covalent compound requires knowledge of both its empirical formula and its molecular mass or molar mass.

A

True

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11
Q

Molecular formulas are derived by comparing the compound’s molecular or molar mas to its?

A

Empirical formula mass

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12
Q

The empirical formula mass is the sum of what?

A

The sum of the average atomic masses of all the atoms represented in an empirical formula

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13
Q

How do we calculate empirical formula mass?

A
  1. If we know the molecular (or molar) mass of the substance, we can divide this by the empirical formula mass in order to identify the number of empirical formula units per molecule, which we designate as n:
  2. The molecular formula is then obtained by multiplying each subscript in the empirical formula by n, as shown by the generic empirical formula AxBy:
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14
Q

Nicotine, an alkaloid in the nightshade family of plants that is mainly responsible for the addictive nature of cigarettes, contains 74.02% C, 8.710% H, and 17.27% N. If 40.57 g of nicotine contains 0.2500 mol nicotine, what is the molecular formula?

A
  1. Percent composition to derive compound’s empirical formula
  2. Calculate molar rations to the least abundant element
    Empirical= C5H7N
    Empirical Formula Mass= 81.13 g/mol
  3. Calculate nicotine molar mass and molar amount of compound
  4. Molar mass of compound/empirical mass formula units to receive amount of units/molecule
  5. Molecular formula by multiplying empirical by formula unit value received
    Molecular Formula= C10H14N2
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15
Q

What is the molecular formula of a compound with a percent composition of 49.47% C, 5.201% H, 28.84% N, and 16.48% O, and a molecular mass of 194.2 amu?

A

C8H10N4O2

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16
Q

True or False: Mass spectrometry can also provide experimental data on the empirical and molecular formula of a molecular substance.

A

True

17
Q

Atom-to-atoms bonds are broken in which ionization process?

A. Soft
B. Hard

A

B. Hard

18
Q

Atoms remain intact in which ionization process

A. Soft
B. Hard

A

Soft

19
Q

What can we calculate about the molecular substance from hard MS?

A

From the hard MS, we can calculate the molar mass of the lowest ratio of atoms, i.e., the empirical formula.

20
Q

What can we calculate about the molecular substance from soft MS?

A

Since the molecule is not broken into its constituent parts in soft MS, the soft MS spectrum tells us the molecular weight of the molecule and therefore its molecular formula.

21
Q

True or False: The empirical formula mass derived from hard MS is the same value as the major peak in the soft MS

A

True

22
Q

What is combustion analysis?

A

The elemental composition of hydrocarbons and related compounds may also be determined via this method

23
Q

What occurs in a combustion analysis?

A

In a combustion analysis, a weighed sample of the compound is heated to a high temperature under a stream of oxygen gas, resulting in its complete combustion to yield gaseous products of known identities

24
Q

The complete combustion of hydrocarbons will yield what products?

A

will yield carbon dioxide and water as the only products

25
Q

True or False: For combustion analysis, the mass increase of each device corresponds to the mass of the absorbed product and may be used in an appropriate stoichiometric calculation to derive the mass of the relevant element.

A

True

26
Q

Polyethylene is a hydrocarbon (with formula CxHy) polymer used to produce food-storage bags and many other flexible plastic items. A combustion analysis of a 0.00126-g sample of polyethylene yields 0.00394 g of CO2 and 0.00161 g of H2O. What is the empirical formula of polyethylene?

A
  1. The primary assumption in this exercise is that all the carbon in the sample combusted is converted to carbon dioxide, and all the hydrogen in the sample is converted to water
  2. To derive the empirical formula of the compound, only the subscripts x and y are needed.

First, calculate the molar amounts of carbon and hydrogen in the sample, using the provided masses of the carbon dioxide and water, respectively. With these molar amounts, the empirical formula for the compound may be written as described earlier.
3. The empirical formula for the compound is then derived by identifying the smallest whole-number multiples for these molar amounts.
Empirical Formula: CH2

27
Q

A 0.00215-g sample of polystyrene, a polymer composed of carbon and hydrogen, produced 0.00726 g of CO2 and 0.00148 g of H2O in a combustion analysis. What is the empirical formula for polystyrene?

A

CH

28
Q

What information do we need to determine the molecular formula of a compound from the empirical formula?

A

You need a given mass and the molar amount of the compound to get the molar mass so it can be compared to the empirical mass

CHEM 103 (16 decks)

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