posted by Confused .
What alkane effuses at 1/5 the rate of He
rate He = 5L/sec
rate x = 1 L/sec
5/1 = sqrt(Mx/4)
Mx = ?
CH3 = 15 and 15*2 = 30 for the two ends.
That leaves Mx-30 for the (CH2)y, solve for y to obtain the formula of the alkane.