Final answer:
The percent composition by mass of ammonia (NH₃) is found by calculating the atomic mass of nitrogen and hydrogen and then dividing by the total formula mass of ammonia. Nitrogen makes up 82.23%, while hydrogen accounts for 17.75% of the mass of ammonia.
Step-by-step explanation:
Percent Composition by Mass of a Compound
The percent composition by mass of a compound is the percentage by mass of each element within that compound. For ammonia (NH₃), you calculate the percent composition by dividing the atomic mass of each element by the formula mass of the entire compound, then multiplying by 100 to get a percentage.
To find the formula mass of ammonia, we sum up the atomic masses of nitrogen (N) and hydrogen (H). Since a molecule of ammonia consists of one nitrogen atom (14.01 amu) and three hydrogen atoms (3 x 1.008 amu), the total formula mass is (14.01 + 3.024) amu, which equals 17.034 amu.
Finally, the percent composition of nitrogen in ammonia can be calculated by dividing the mass of nitrogen (14.01 amu) by the total formula mass of ammonia (17.034 amu), then multiplying the result by 100%:
% composition of N in NH₃ = (14.01 amu / 17.034 amu) x 100% = 82.23%
Similarly, the percent composition of hydrogen is calculated using the combined mass of the three hydrogen atoms, leading to:
% composition of H in NH₃ = (3.024 amu / 17.034 amu) x 100% = 17.75%
Thus, in a molecule of ammonia, nitrogen accounts for 82.23% of the mass, while hydrogen accounts for 17.75% of the mass.