# 45.9 gram sample of borane (a compound containing only boron and hydrogen) and is 85.63% boron. What's the mass of hydrogen in this sample?

45.9 gram sample of borane (a compound containing only boron and hydrogen) that's 85.63% boron. What's the mass of hydrogen in this sample?

I was just going to take 45.9 grams and multiply by .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? So then I thought, maybe I should consider the molar masses of both atoms, hydrogen being 1.008g and boron being 10.811g. I am stuck at this point, please help.

• Homework questions should show the work or thinking the poster has already done in an initial attempt to answer the question. For help asking a good homework question, see: How do I ask homework questions on Chemistry Stack Exchange? meta.chemistry.stackexchange.com/questions/141/… – ron Sep 14 '14 at 17:25
• Okay, I added my thought process and what I have done so far. – user3138766 Sep 14 '14 at 17:29
• Borane, $\ce{BH3}$, contains 78.1% boron by mass, so I'm confused by your 85.63% number. Had you said 78.1%, then I would have agreed with your approach of mass hydrogen = (1-.781)*45.9 Let's see if someone else chimes in. – ron Sep 14 '14 at 17:45
• I get that but can we just say that the remaining 14.37% of 45.9 grams of the compound is the mass of the hydrogen when they do not equally contribute to the mass? – user3138766 Sep 14 '14 at 17:51
• @user3138766 Yes you can. – LDC3 Sep 14 '14 at 18:26

Well, 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 $100\%-85.63\%=14.37\%$. Therefore, we have 6,59583 g of hydrogen in this sample.