Composition by Percent Mass – MCAT General Chemistry | MedSchoolCoach

Composition by Percent Mass

MCAT General Chemistry Chapter 1 - Section 1.4 - Stoichiometry

Sample MCAT Question - Composition by Percent Mass

What is the approximate percent mass of nitrogen in nitric acid?

a) 22%

b) 32%

c) 45%

d) 58%

A is correct. 22%. The percent mass of nitrogen in nitric acid (HNO3) can be found by dividing the molar mass of nitrogen (14 g/mol) over the total molecular weight of the molecule, HNO3 (1 + 14 + 3 x 16 = 63 g/mol). If we simplify the math to 15/60, we will get 0.25, or 25%. The closest answer choice is A.

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Composition by Percent Mass

Composition by percent mass is the relative amount of the mass of a molecule represented by each constituent element within the molecule. Let’s consider as an example the percent mass of perchloric acid, HClO4. From our previous review of molecular formula, we can say that perchloric acid will consist of one hydrogen (with a molar mass of 1 g/mol), one chlorine (35 g/mol), and four oxygens (each with a molar mass of 16 g/mol, or 64 g/mol altogether). Therefore, the total molar mass of perchloric acid will be approximately 100 g/mol, as demonstrated below:

Composition by Percent Mass

To calculate the percent mass of each substituent element, we simply divide its molar mass by the molar mass of the compound. Hydrogen, therefore, would have a percent mass of 1% (1 g/mol divided by 100 g/mol), chlorine would have a percent mass of 35% (35 g/mol divided by 100 g/mol), and oxygen a percent mass of 64% (64 g/mol divided by 100 g/mol). This gives us the relative amount of each element present in a given sample. For example, in a 10 gram sample of perchloric acid, we would find 0.1 gram of hydrogen, 3.5 grams of chlorine, and 6.4 grams of oxygen. This is especially useful in decomposition reactions, and can be used to determine the original molecular formula of an unknown molecule based on the relative mass of the decomposed elements that are measured

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