Question

What is the percent of carbon in 50.0 g of cyclohexane?

Answer 1

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$\huge\bold\pink{Answer}$

The % carbon in C6H12 is 85.7% irrespective of the mass of C6H12 you have. It will be 85.7% for 1g C6H12, 50g C6H12 or 1000g C6H5

If the mass% is 14.3% for H, then in 150g C2H4 there will be 14.3/100*150 = 21.45g hydrogen

You must not relate % composition to a mass. % composition of any compound is the same irrespective of the mass of compound.