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In a $\pu{45.9 g}$ sample of borane, a compound containing only boron and hydrogen, that's $85.63\%$ boron, what is the mass of hydrogen in this sample?

I was just going to take $\pu{45.9 g}$ and multiply by $0.1437$ to get the mass of hydrogen, but I don't think it works that way because they don't equally contribute to the mass of the compound. Then I thought, maybe I should consider the molar masses of both atoms, hydrogen being $\pu{1.008 g mol-1}$ and boron being $\pu{10.811 g mol-1}$. I am stuck at this point. How to solve the question.

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The problem does not need any difficult approach, not even relative atomic masses, if you trust the one who made this exercise. Since borane (whatever type it is)* consists (by exercise's hypothesis) only of hydrogen and boron and since boron is $85.63\%$ by mass, hydrogen's mass percentage would be the remaining $100\% - 85.63\% = 14.37\%$. Therefore, we have $\pu{6.60 g}$ of hydrogen in this sample.


The compound borane, $\ce{BH3}$, is about $78.1\%$ boron per mass. However, there is a class of compounds called boranes which also only consist of boron and hydrogen. From this class pentaborane(9), $\ce{B5H9}$, with about $85.7\%$ boron by mass would fit the exercise.

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