8.29 Avogadro's number, moles, molar mass, and percent composition

A set of practice questions covering Avogadro's number, mole concepts, molar mass, and percent composition based on the lecture notes.

What is Avogadro's number and its value?

6.022 × 10^23 particles per mole; the conversion factor between moles and individual particles.

What does one mole of any substance contain?

6.022 × 10^23 representative particles (atoms for elements, molecules for most compounds).

How do you convert from moles to atoms or molecules?

Multiply by Avogadro's number (6.022 × 10^23) to obtain the number of particles.

What is molar mass?

Mass of one mole of a substance, expressed in grams per mole (g/mol).

What is the molar mass of chlorine (Cl)?

35.45 g/mol.

What is the molar mass of carbon dioxide (CO2)?

44.01 g/mol (12.01 for C plus 2×16.00 for O).

What does a subscript in a chemical formula represent?

The number of atoms of that element per molecule or formula unit, used to determine ratios and mass contributions.

In the formula C2Cl4F2, how many moles of chlorine are in one mole of the compound?

4 moles of Cl per mole of C2Cl4F2.

How do you compute mass percent composition?

Mass of the element in the formula (moles × molar mass per mole) divided by the total molar mass of the compound, times 100.

What is the molar mass of PbO2?

239.2 g/mol (Pb 207.2 + 2×16.00 from O).

If you have 57.8 g of sulfur (S), how many moles do you have?

57.8 g ÷ 32.07 g/mol ≈ 1.80 mol.

If you have 50 g of PbO2, how many moles do you have?

50 g ÷ 239.2 g/mol ≈ 0.209 mol.

What is the difference between grouping elements as atoms vs grouping compounds as molecules?

Elements are grouped by identical atoms (atoms); compounds are grouped as molecules composed of multiple atoms.

What does 'grams per mole' mean in practice?

It is the molar mass, the number of grams of a substance per one mole of that substance.

How many atoms are in one mole of any element?

6.022 × 10^23 atoms.

What is the starting and ending unit when converting from grams to moles?

Start with grams of the substance and end with moles; use molar mass as the conversion factor.

How do you compute the molar mass of a compound like CO2?

Add the molar masses of each element times its subscript: CO2 = 1×12.01 + 2×16.00 = 44.01 g/mol.

Why is percent composition useful and how is it checked for sense?

It shows how much of each element is in a compound by mass; check that results align with relative elemental masses and subscript weights (e.g., more of a heavier element should be reflected in the mass).