There are two main isotopes of chlorine. 75% of chlorine atoms have a mass number of 35, while 25% have a mass number of 37. Use this information to work out the relative atomic mass of chlorine.

Relative atomic mass = 35.5

Explanation:

Given data:

Abundance of Cl -35 = 75%

Abundance of Cl -25 = 25%

Average atomic mass = ?

Solution:  

Relative atomic mass = (abundance of 1st isotope × its atomic mass) +(abundance of 2nd isotope × its atomic mass)  / 100

Relative atomic mass = (75×35)+(25×37) /100

Relative atomic mass =  2625 + 925 / 100

Relative atomic mass = 3550 / 100

Relative atomic mass = 35.5

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.

the correct answer would be d. hope this !