What is the molecular formula of a substance that decomposes into #1.33# #g# of #H# and #21.3# #g# of #O#, and was found to have a molar mass of #34.1# #gmol^-1#?

1 Answer
May 21, 2017

Answer:

The molecular formula of the substance is #H_2O_2#.

Explanation:

#H# has a molar mass of #1# #gmol^-1#, so #1.33# #g# = #1.33# mol.

(I have assumed that we are talking about individual #H# atoms, not #H_2# molecules, and similarly for #O# versus #O_2#.

Oxygen has a molar mass of #16# #gmol^-1#, so using #n=m/M#, #n = 21.3/16=1.33# #gmol^-1#.

We see that there are the same number of moles of each element in the substance, so we can think of the molecular formula as #H_xO_x#. We need to find the value of #x#.

If #x# was equal to #1#, the molar mass would be #1+16 = 17# #gmol^-1#.

Since the given molar mass is almost exactly 2x that, at #34.1# #gmol^-1#, the value of #x# must be #2#, and the molecular formula must be #H_2O_2#.

(incidentally, this is the formula of the chemical substance hydrogen peroxide)