Few exam question queries

[mrb3n]

Hi I was working through last year's Exam 1, and I came across a few questions in the multiple choice which i couldnt nut out. Any help would be appreciate. I've attached a screenshot of the questions.

If you can help me, I would really appreciate an explanation which outlines the thinking processes and the different steps used to approach the questions.

[REBORN]

Q15

Molar mass of the whole compound is 853 g/mol.

Carbon contributs 47x12 = 564g/mol which is 564/853 = 66.11%

How much % is oxygen? 224/853 = 26.26%

Set up a ratio

0.157g = 66.11%
(unknown)g = 26.26%

0.157 x 26.26 = 66.11(unknown)

(unknown) = 0.06236 = B

NOTE: there are other methods

[FlorianK]

200GL are 200*10⁹L.
0.5 mg L^-1 is 5*10^-7kg L^-1

Multiply them by each other and you get 10⁵ so (B)



You find the number of moles of carbon first so you divide the weight by the molar mass:
0.157/12=0.01308moles

For 47 carbon atoms you have 14 oxygen atoms so you divide by 47 and multiply by 14

0.01308/47*14=0.003897moles

Now from the moles to the mass you just times by the molecular mass of oxygen.

0.003897*16=0.06235