Chemistry Chapter on Atomic Mass and Moles

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Questions and Answers

How do you calculate the relative atomic mass of an element with multiple isotopes?

  • By multiplying the mass of each isotope by its percentage abundance and dividing by the total percentage. (correct)
  • By taking the most abundant isotope's mass as the relative atomic mass.
  • By adding the atomic masses of all isotopes regardless of abundance.
  • By averaging the atomic masses of the isotopes directly.

What is the relative formula mass of aluminium sulfate, Al2(SO4)3?

  • 98 g/mol
  • 342 g/mol (correct)
  • 196 g/mol
  • 54 g/mol

What is the definition of one mole of a substance?

  • The number of grams of a substance equal to its relative formula mass.
  • One mole contains as many particles as the atomic number of the element.
  • The number of particles in a mole is always 6.02 × 10^23. (correct)
  • A mole is defined as the total mass of an element in grams.

How would you determine the mass of a substance from its number of moles?

<p>Multiply the number of moles by the molar mass of the substance. (D)</p> Signup and view all the answers

What does the term 'percentage by mass' refer to in a compound?

<p>The proportion of each element's mass in relation to the total mass of the compound. (C)</p> Signup and view all the answers

What is the value of the Avogadro constant?

<p>6.02 × 10^23 (C)</p> Signup and view all the answers

If you have 19.6g of sulfuric acid (H2SO4), how many moles do you have?

<p>0.20 mol (C)</p> Signup and view all the answers

Which of the following correctly represents the calculation for the percentage mass of carbon in carbon dioxide (CO2)?

<p>$ rac{12}{44}$ × 100$ (B)</p> Signup and view all the answers

What would be the number of moles contained in a 342g sample of aluminium sulfate (Al2(SO4)3)?

<p>1 mol (B)</p> Signup and view all the answers

What is the first step in calculating the relative formula mass of a compound like CO2?

<p>Identify the atomic masses of the constituent elements. (D)</p> Signup and view all the answers

Flashcards

Relative Atomic Mass

The average mass of an atom of an element relative to one-twelfth the mass of a carbon-12 atom.

Relative Formula Mass

The sum of the relative atomic masses of all atoms in a formula unit of a compound.

Mole

The amount of substance containing the same number of entities as there are atoms in 12 grams of carbon-12.

Avogadro's Constant

The number of atoms, molecules, or ions in one mole of a substance (approximately 6.02 x 10^23).

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Moles Calculation

Number of moles = mass / molar mass

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Mass Calculation

Mass = number of moles x molar mass

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Percentage by mass

The percentage of an element's contribution to the total mass of a compound

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Isotope

Atoms of the same element with the same number of protons but different numbers of neutrons.

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Molar mass

The mass of one mole of a substance.

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Calculate Mr

Sum the atomic masses (AR) of each element present in the molecule based on the quantity of each element specified in the molecule.

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Study Notes

Relative Atomic Masses and Moles

  • Relative atomic mass (Ar) represents the average mass of an atom compared to 1/12th the mass of a carbon-12 atom
  • Relative atomic mass is not the actual mass of an atom
  • Ar values are used in calculations to determine the proportionate mass of elements in a compound
  • A standard reference point is needed for comparison, typically carbon-12

Calculating Relative Atomic Mass

  • To calculate, use the % abundance of isotopes
  • Assume 100 atoms; calculate the sum of the product of the mass of each isotope and its percentage abundance, then divide by 100.

Relative Formula Mass (Mr)

  • Calculated from the relative atomic masses (Ar) of the elements in a compound
  • Determine the number of each atom in the compound and multiply its Ar value by this number
  • Sum the results to get the relative formula mass

The Mole

  • Shorthand way to describe a large number of particles (atoms, molecules, or ions)
  • 1 mole = 6.02 x 1023 particles (Avogadro's constant)
  • Relative atomic/formula mass in grams is equal to 1 mole
  • Moles allow for efficient handling of extremely large numbers of particles

Moles from Masses

  • Number of moles (n) = mass (g) / relative atomic mass (Ar) or relative formula mass (Mr)

Masses from Moles

  • Mass (g) = number of moles (n) x relative atomic mass (Ar) or relative formula mass (Mr)

Percentage by Mass

  • Calculate percentage of each element in a compound by dividing the relative atomic mass of the element multiplied by its subscript in the compound, by the relative formula mass of the compound, then multiply by 100.

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