Calculating the Molecular Mass of a Compound

Problem Using the periodic table (Fig 1), calculate the molecular (or formula) mass of

(a) Tetraphosphorus trisulfide

(b) Ammonium nitrate

Question

Calculating the Molecular Mass of a Compound

Problem Using the periodic table (Fig 1), calculate the molecular (or formula) mass of

(a) Tetraphosphorus trisulfide

(b) Ammonium nitrate

Accepted Answer

Plan We first write the formula, then multiply the number of atoms (or ions) of each element by its atomic mass (from the periodic table (Fig 1)), and find the sum.

Solution
(a) The formula is [latex]P_4S_3[/latex].

[latex]               ext{Molecular mass = (4 × atomic mass of P) + (3 × atomic mass of S)}[/latex]
[latex]                             = (4 × 30.97 ext{amu}) + (3 × 32.06 ext{amu})[/latex]
[latex]                             = 220.06 ext{amu}[/latex]

(b) The formula is [latex]NH_4NO_3[/latex]. We count the total number of N atoms even though they belong to different ions:
Formula mass

[latex]               ext{= (2 × atomic mass of N) + (4 × atomic mass of H) + (3 × atomic mass of O)}[/latex]
[latex]               = (2 × 14.01 ext{amu}) + (4 × 1.008 ext{amu}) + (3 × 16.00 ext{amu})[/latex]
[latex]               = 80.05 ext{amu}[/latex]

Check You can often find large errors by rounding atomic masses to the nearest 5 and adding: (a) (4 × 30) + (3 × 30) = 210 ≈ 220.06. The sum has two decimal places because the atomic masses have two. (b) (2 × 15) + 4 + (3 × 15) = 79 ≈ 80.05.

Question 2.16: Calculating the Molecular Mass of a Compound Problem Using t......

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