Chemistry Ch. 4 DSM

The empirical formula is a chemical formula that provides the smallest, whole-number ratio of elements in a compound. Using the mass of each element or the mass percentage of each element in a compound, the empirical formula can be determined.

What is the empirical formula for a substance containing 42.05 grams of nitrogen, N, and 95.95 grams of oxygen, O?

NO₂

A mole is a counting number that means 6.022 × 10²³ just as a dozen means 12. We can create a conversion factor that can be used to determine the number of particles in a mole or vice versa.

(6.022×10²³ particles) / 1 mole OR 1 mole / (6.022×10²³ particles)

The chemical formula of a compound gives the ratio of the elements in that compound. The subscripts can be used as a ratio of atoms or moles. The ratio can be used to convert between atoms and molecules.

How many atoms of H are in 2.8 mol N₂H₄?

6.7 × 10²⁴ atoms

Molar mass is the mass of 1 mole, in grams, of a given substance. This results in the units of g/mol for molar mass. The number of grams is determined by adding up the atomic masses of the individual elements on the periodic table.

Molar mass is a conversion factor that can be used to determine the number of moles in a given number of grams or vice versa. The conversion factor for molar mass would be the following:

g / (1 mole) OR (1 mole) / g

How many moles of ethane, C₂H₆, are found in 81 grams of C₂H₆?

A mole is a counting number that means 6.022 × 10²³ just as a dozen means 12. We can create a conversion factor that can be used to determine the number of particles in a mole or vice versa.

(6.022×10²³ particles) / 1 mole OR 1 mole / (6.022×10²³ particles)

The molar mass has units of grams per mole and can be used as a conversion factor between the grams and moles of a substance. The numerical value for the relationship between grams and moles will vary depending on the identity of the substance.

grams / mole OR mole / grams

How many grams of H₂S are found in a sample with 5.03 × 10²⁴ molecules of H₂S?

• Conversion Steps: Convert molecules to moles using Avogadro’s number, then convert moles to grams using the molar mass of H2S.

• Molar Mass Calculation: The molar mass of H2S is 34.086 grams per mole, calculated as 2*(1.008) + 32.07.

• Number of Molecules: The sample has 5.03 × 10^24 molecules of H2S.

• Moles of H2S: 5.03 × 10^24 divided by 6.022 × 10^23 is approximately 8.35 moles.

• Conversion from Moles to Grams: Multiply by the molar mass (34.086 g/mol).

• Calculation Steps: 8.35 moles of H2S is converted to grams using the molar mass of 34.086 g/mol, resulting in 284.6181 grams.

• Significant Figures Rounding: Rounded to three significant figures, the final answer is 285 grams.

How many grams of Br contain 8.94 × 10²¹ atoms of bromine?

Convert atoms to moles using Avogadro’s number:

• (8.94 × 1021) atoms / (6.022 × 1023) atoms/mol = 0.01485 mol

• Multiply moles by the molar mass of Bromine:

• 0.01485 mol × 79.904 g/mol = 1.186 g

• Round to three significant figures:

• 1.19 g

The empirical formula is a chemical formula that provides the smallest, whole-number ratio of elements in a compound. Using the mass of each element or the mass percentage of each element in a compound, the empirical formula can be determined.

What is the empirical formula for a substance containing 37.5 grams of carbon, C, 12.5 grams of hydrogen, H, and 50.0 grams of oxygen, O?

To find the empirical formula, the masses of each element need to be converted into moles.

• The molar mass of carbon is 12.01 g/mol, so 37.5 g / 12.01 g/mol gives approximately 3.12 moles.

• The molar mass of hydrogen is 1.008 g/mol, so 12.5 g / 1.008 g/mol gives roughly 12.4 moles.

• The molar mass of oxygen is 16.00 g/mol, so 50.0 g / 16.00 g/mol gives about 3.125 moles.

• The smallest number here is approximately 3.12. Dividing each mole value by 3.12 should help.

• Carbon: 3.12 / 3.12 = 1

• Hydrogen: 12.4 / 3.12 ≈ 4

• Oxygen: 3.125 / 3.12 ≈ 1

• The ratio is C:H:O = 1:4:1.

• The empirical formula should be CH4O.

Mass percent composition involves calculating the percentage of mass for each of the individual elements in a given substance.

Mass percent composition is calculated according to the following equation:

% of an element = ((number of atoms of element)*(atomic mass of element) / atomic mass of chemical) * 100%

The percentages for each of the individual elements will then add up to 100%.

What is the mass percent composition of oxygen in ethylene glycol, C₂H₆O₂?

1. Determine the molar mass of ethylene glycol:

   • Carbon (C): 2 atoms × 12.01 g/mol = 24.02 g/mol

   • Hydrogen (H): 6 atoms × 1.008 g/mol = 6.048 g/mol

   • Oxygen (O): 2 atoms × 16.00 g/mol = 32.00 g/mol

   • Total molar mass = 24.02 + 6.048 + 32.00 = 62.068 g/mol

2. Calculate the mass contribution of oxygen:

   • Total oxygen mass = 2 × 16.00 g/mol = 32.00 g/mol

3. Apply the mass percent formula:

   • % Oxygen = (32.00 g/mol ÷ 62.068 g/mol) × 100% ≈ 51.55%

Final Answer:
The mass percent composition of oxygen in ethylene glycol is 51.55%.

The empirical formula is a chemical formula that provides the smallest, whole-number ratio of elements in a compound. Using the mass of each element or the mass percentage of each element in a compound, the empirical formula can be determined.

 

The molecular formula is a whole-number multiple of the empirical formula. The multiple is determined from the ratio of the molar mass to the empirical formula mass.

What is the molecular formula if the molar mass is 234.0 g/mol and the empirical formula is C₅O₂NH₁₁?