What mass of ethane (ch3ch3) is contained in 50.0 ml of a 13.9% by mass solution of ethane in hexane?

the density of the solution is 0.611 g/ml.

Mass of ethane which is contained in 50 mL is 4.25 g

Explanation:

13.9% by mass, means that in 100 g of solution, we have 13.9 grams of solute.

Density of solution is 0.611 g/mL

Volume of solution is 50 mL

Density = mass of solution / 50mL

0.611 g/mL . 50 mL = mass of solution → 30.55 g

Now, the rule of three.

In 100 grams of solution we have 13.9 grams of solute

30.5 grams of solution we have (30.55 . 13.9) / 100 = 4.25 g  

answer is: c. the experiment shows that the red substance experienced a chemical change.

this is chemical change (chemical reaction), because two new substances are formed (metalig liquid and gas), the atoms are rearranged and the reaction is followed by an energy change.

this is example of chemical decomposition.

chemical decomposition (analysis) is the separation of a single chemical compound into its two or more elemental parts or to simpler compounds.  

chemical decomposition is opposite of chemical synthesis.

dynamic equilibrium is the answer.