How is the number of moles of an element determined from a known mass? A. The known mass is multiplied by Avogadro's number to get moles.
B. The known mass is divided by Avogadro's number to get moles.
C. The atomic number is used as a conversion factor to convert
grams to moles.
D. The mass of 1 mole of the element is used as a conversion factor
to convert grams to moles.

D

Explanation:

most logical explanation for this question.

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.

answer: -

reaction:

na + cl ⇌ nacl

balanced chemical equation:

na + 2cl ⇌ nacl2

sodium + clorine goes to form sodium chloride (table salt).

the product is nacl2.

and the reactants are na and cl.

(note: if you don’t know what ⇌ means, read below: -

⇌(double head arrow) means reversible reaction).