Determine the mass and mass fraction of carbon in 50 g of pentane.

Given:
m (C5H12) = 50 g

To find:
m (C in C5H12) -?
ω (C in C5H12) -?

Decision:
1) Find the amount of substance C5H12:
n (C5H12) = m (C5H12) / Mr (C5H12) = 50/72 = 0.7 mol;

2) Find the amount of substance C in C5H12 (by definition):
n (C) = n (C5H12) * 5 = 0.7 * 5 = 3.5 mol;

3) Find the mass C in C5H12:
m (C in C5H12) = n (C in C5H12) * Mr (C) = 3.5 * 12 = 42 g;

4) Find the mass fraction of C in C5H12:
ω (C in C5H12) = m (C in C5H12) * 100% / m (C5H12) = 42 * 100% / 50 = 84%.

Answer: The mass of C in C5H12 is 42 g; mass fraction of C in C5H12 – 84%.