C3 - Calculating Percentage By Mass

What does percentage by mass tell us?

The proportion (in %) of a compound's mass that comes from a specific element.

What is the equation for percentage by mass of an element in a compound?

(Total Ar of the element ÷ Mr of the compound) × 100.

What does Ar stand for?

Relative atomic mass.

What does Mr stand for?

Relative formula mass.

Do relative formula masses (Mr) have units?

No, Mr has no units.

Step 1 in calculating percentage by mass

Find the relative formula mass (Mr) of the whole compound.

Step 2 in calculating percentage by mass

Find the total relative atomic mass (Ar) of the chosen element (taking into account how many atoms there are in the formula).

Step 3 in calculating percentage by mass

Divide the total Ar by the Mr, then multiply by 100.

Example: What is the formula of magnesium oxide?

MgO.

Example: What is the Mr of MgO?

40 (Mg = 24, O = 16).

Example: What is the percentage by mass of Mg in MgO?

(24 ÷ 40) × 100 = 60%.

Example: What is the formula of calcium chloride?

CaCl₂.

Example: What is the Mr of CaCl₂?

111 (Ca = 40, Cl = 35.5×2 = 71).

Example: What is the percentage by mass of Ca in CaCl₂?

(40 ÷ 111) × 100 = 36%.

Example: What is the formula of methane?

CH₄.

Example: What is the Mr of methane (CH₄)?

16 (C = 12, H = 1×4 = 4).

Example: What is the percentage by mass of H in CH₄?

(4 ÷ 16) × 100 = 25%.

Example: What is the formula of sodium sulfate?

Na₂SO₄.

Example: What is the Mr of Na₂SO₄?

142 (Na = 23×2 = 46, S = 32, O = 16×4 = 64).

Example: What is the percentage by mass of Na in Na₂SO₄?

(46 ÷ 142) × 100 = 32.4%.

Exam tip: What must you always do if the question asks for the percentage to a specific decimal place or whole number?

Round your final answer to the required number of decimal places or whole number.