Question

Un amestec de benzen, xilen si naftalina contine 92,9% (procent de masa) carbon. Stiind ca raportul molar benzen:xilen este 1:2, procentul de naftalina din amestec este:
a)25,31%
b)54,4%
c)68,69%
d)70,33%

Answer 1

M benzen=78g/mol; M xilen=106g/mol; M naftalina=128g/mol
Notam 78x, 106*2x, 128y, masele de benzen, xilen respectiv naftalina
m amestec=290x + 128y
290x + 128y...........100%
264x + 120y..........92,9%
(264x + 120y)*100=(290x + 128y)*92,9
26400x + 12000y =26941x + 11891,2y
26941x - 26400x = 12000y - 11891,2y
541x = 108,8y => 5x = y => 1 mol de x la 5 moli de y
m de amestec devine: 290*1+128*5=290+640=930g amestec
930g.........100%
640g.............z%
z=68.81%, o mica eroare, deoarece erau defapt 4,97 moli si nu 5. => c)