Calculating Relative Formula Masses (CFU)

Match the following compounds with their relative formula masses (M_r).

• CO₂

• 20

• H₂O

• 18

• O₂

• 44

• Ne

• 32

• 58.5

• 60

• 55.5

• 57

Answer explanation

To find the relative formula mass of sodium chloride (NaCl), add the atomic masses of sodium (Na, 23) and chlorine (Cl, 35.5). Thus, 23 + 35.5 = 58.5, making the correct answer 58.5.

• 44
• 46

• 42

• 48

Answer explanation

The relative formula mass of carbon dioxide (CO₂) is calculated by adding the atomic masses of its components: carbon (12) and two oxygens (16 each). Thus, M_r = 12 + (2 × 16) = 44. Therefore, the correct answer is 44.

• 100

• 98

• 102

• 104

Answer explanation

To calculate the relative formula mass of calcium carbonate (CaCO₃), add the atomic masses: Ca (40) + C (12) + 3×O (16) = 40 + 12 + 48 = 100. Thus, the correct answer is 100.

• 160

• 150

• 170

• 180

Answer explanation

To find the relative formula mass of Fe2O3, calculate: (2 x 55.85 for Fe) + (3 x 16.00 for O) = 111.70 + 48.00 = 159.70, rounded to 160. Thus, the correct answer is 160.

• 98

• 96

• 100

• 102

Answer explanation

To find the relative formula mass of sulfuric acid (H₂SO₄), add the atomic masses: H (1) x 2 + S (32) + O (16) x 4 = 2 + 32 + 64 = 98. Thus, the correct answer is 98.

• 17

• 18

• 16

• 19

Answer explanation

To calculate the relative formula mass of ammonia (NH₃), add the atomic masses: Nitrogen (N) = 14 and Hydrogen (H) = 1 (3 H atoms). So, 14 + (3 x 1) = 17. Therefore, the correct answer is 17.

• 95

• 94

• 96

• 97

Answer explanation

To find the relative formula mass of magnesium chloride (MgCl₂), add the atomic masses: Mg (24) + 2 × Cl (35.5) = 24 + 71 = 95. Thus, the correct answer is 95.

• 101

• 100

• 102

• 103

Answer explanation

To find the relative formula mass of potassium nitrate (KNO3), add the atomic masses: K (39) + N (14) + 3*O (16) = 39 + 14 + 48 = 101. Thus, the correct answer is 101.

• 16

• 18

• 14

• 20

Answer explanation

To calculate the relative formula mass of methane (CH₄), add the atomic masses: Carbon (C) = 12, Hydrogen (H) = 1. Thus, M_r = 12 + (4 x 1) = 16. Therefore, the correct answer is 16.