The mole — return questions key

1. LR1 · Return

Model answer. The mole, mol, is the unit of amount of substance. One mole contains 6.02 × 10²³ particles (atoms, ions or molecules), and this number is the Avogadro constant. Row B.

A full answer includes

• I chose row B.
• I did not choose row C: 6.02 × 10²³ counts particles, not grams.
• I did not choose row D: the molar mass is the mass of one mole, in g/mol.

Also accept

• Row B

Watch for

• “Row A.” — A concentration is an amount in each dm³, measured in mol/dm³. The mole itself is the unit of amount of substance.
• “Row C.” — 6.02 × 10²³ is a number of particles. The mass of one mole depends on the substance.
• “Row D.” — The molar mass is the mass of one mole, in g/mol. The number of particles in a mole is the Avogadro constant.

2. LR2 · Return

Model answer. M = m/n = 3.81 g ÷ 0.0300 mol = 127 g/mol for XCl₂. One mole of XCl₂ holds 2 mol of Cl atoms, 2 × 35.5 = 71 g, so the X in it has a mass of 127 − 71 = 56 g. The Ar of X is 56, with no unit, and the element with Ar 56 is iron: XCl₂ is iron(II) chloride, FeCl₂. Row D.

A full answer includes

• I chose row D.
• I can say why: M = 3.81 ÷ 0.0300 = 127 g/mol for XCl₂.
• I can say why: Ar of X = 127 − 2 × 35.5 = 56, which is iron.
• I did not choose row A: 127 is the molar mass of the whole XCl₂, not of X.
• I did not choose row B: a molar mass has the unit g/mol, and an Ar has no unit.

Also accept

• Row D

Watch for

• “Row A.” — 127 g/mol is the molar mass of the whole compound. Take away the mass of the two chlorine atoms to leave the Ar of X.
• “Row C.” — The 2 in XCl₂ means two chlorine atoms. Take away 2 × 35.5 = 71, not 35.5.
• “Row B.” — The numbers are right but the units are swapped. A molar mass is in g/mol; an Ar has no unit.

3. LR3 · Return

Model answer. V = 50.0 ÷ 1000 = 0.0500 dm³, so the concentration is 1.46 g ÷ 0.0500 dm³ = 29.2 g/dm³. HCl has M = 1 + 35.5 = 36.5 g/mol, so 29.2 g/dm³ ÷ 36.5 g/mol = 0.800 mol/dm³. (Or n = 1.46 ÷ 36.5 = 0.0400 mol, and 0.0400 ÷ 0.0500 = 0.800 mol/dm³.)

Checked automatically

• (a) Concentration in g/dm³: 29.2 g/dm³ (3 significant figures)
  • Common wrong answer 0.0292 : You divided by 50.0 cm³. Use the volume in dm³, 0.0500.
• (b) Concentration in mol/dm³: 0.800 mol/dm³ (3 significant figures)
  • Common wrong answer 1066 : You multiplied by M. From g/dm³ to mol/dm³, divide by M.
  • Common wrong answer 0.0400 : That is the amount of HCl. Divide it by the volume, 0.0500 dm³.

A full answer includes

• (a) I worked out 50.0 ÷ 1000 = 0.0500 dm³.
• (a) I worked out 1.46 ÷ 0.0500.
• (a) I got 29.2 g/dm³.
• (b) I found M = 1 + 35.5 = 36.5 g/mol.
• (b) I worked out 29.2 ÷ 36.5.
• (b) I got 0.800 mol/dm³.
• (b) I did not give the amount, 0.0400 mol, as the concentration.

Also accept

• 29.2 g/dm³; 0.800 mol/dm³

Watch for

• “Divide by the volume in cm³.” — Concentration is per dm³. Divide the cm³ by 1000 first: 50.0 cm³ is 0.0500 dm³.
• “g/dm³ and mol/dm³ have the same number.” — They describe the same solution with different numbers: g/dm³ = mol/dm³ × M.

4. LR4 · Return

Model answer. A concentration is a quantity of solute in each dm³: grams in each dm³, g/dm³, or moles in each dm³, mol/dm³. 8.30 g/dm³ is a concentration. 166 g/mol is the molar mass of KI, 0.0500 mol an amount and 500 cm³ a volume.

A full answer includes

• I chose the label 8.30 g/dm³.
• I did not choose 166 g/mol, which is the molar mass of potassium iodide.
• I did not choose 0.0500 mol, which is an amount.
• I did not choose 500 cm³, which is a volume.

Also accept

• 8.30 g/dm³

Watch for

• “166 g/mol.” — g/mol is the mass of one mole: a molar mass. It is the same for every bottle of KI.
• “0.0500 mol.” — mol is an amount. Without a volume it does not say how much is in each dm³.
• “500 cm³.” — A volume says how much solution there is, not how much solute is in each dm³ of it.