Percent Composition

[Duby3L]

I was browsing through Fundamentals F and I came across this question:

(a) Write the formula for nitric acid. (b) Without doing a calculation, estimate which element in nitric acid occurs with the greatest mass percentage.
So for part b, do we just assume that nitric acid (HNO3 I believe is the formula) oxygen would have the greatest composition because it has the most atoms. If so, how do we estimate the percentage?

[aisteles1G]

Yes, I think so because the subscript gives either the exact number of that atom (molecular formula) or a ratio (empirical). So depending on which formula it is, oxygen would be the most either way. I'd say you could simply do 3/5 (total atoms) and get about 60 %

[Daniel_Frees_1L]

Knowing that oxygen has more atomic mass than nitrogen and hydrogen combined, you can estimate closer to 75% or greater. It just says don't use a calculation, not that you can't estimate using the atomic masses.

[Duby3L]

Okay, thank you. Also, for part b why is the mass percentage of oxygen 4 sig figs? Since the smallest number of sig figs throughout the calculation I believe is 5.

[Andie Jian 1D]

I totally agree with your answer, but I think it is also really important to take into account the molar mass along with the number of atoms. For example, H2 would still account for less percent than O in water. I don't think actual calculation was necessary for this question, but maybe it had 4 sig figs because moalr mass is often given with 4 sig figs?

[Sam Kelly 1K]

I agree with Andie on this one, in that just comparing the number of atoms in a compound is an unreliable way of determining mass composition. Rather, you can estimate the percent mass by comparing rounded values of the 3 elements in nitric acid (H ~ 1g, N ~ 14g, O3 ~ 48g), and see that oxygen has the greatest mass comp